Coordinate Systems, Transformations and Units

For all media, the canvas describes "the space where the SVG content is rendered." The canvas is infinite for each dimension of the space, but rendering occurs relative to a finite rectangular region of the canvas. This finite rectangular region is called the SVG viewport. For visual media ([CSS21], section 7.3.1) the SVG viewport is the viewing area where the user sees the SVG content.

The size of the SVG viewport (i.e., its width and height) is determined by a negotiation process (see Establishing the size of the initial viewport) between the SVG document fragment and its parent (real or implicit). Once that negotiation process is completed, the SVG user agent is provided the following information:

Using the above information, the SVG user agent determines the SVG viewport, an initial viewport coordinate system and an initial local coordinate system such that the two coordinates systems are identical. Both coordinates systems are established such that the origin matches the origin of the viewport (for the root viewport, the viewport origin is at the top/left corner), and one unit in the initial coordinate system equals one CSS pixel in the SVG viewport. (See Initial coordinate system.)

New SVG viewports can be established. By establishing a new viewport, you can redefine the meaning of percentages units and provide a new reference rectangle for "fitting" a graphic relative to a particular rectangular area.

7.2. The initial viewport

SVG content can be embedded (by reference or inline) within a containing document. This containing document might include attributes, properties and/or other parameters (explicit or implicit) which specify or provide hints about the dimensions of the viewport for the SVG content. SVG content itself optionally can provide information about the appropriate viewport region for the content via the ‘width’ and ‘height’ presentation attributes on the outermost svg element. If SVG is embedded within a containing element.

The ‘width’ presentation attribute on the outermost svg element establishes the viewport's width, unless the following conditions are met:

Under these conditions, the positioning properties establish the viewport's width.

Similarly, if there are positioning properties specified on the referencing element or on the outermost svg element that are sufficient to establish the height of the viewport, then these positioning properties establish the viewport's height; otherwise, the ‘height’ presentation attribute on the outermost svg element establishes the viewport's height.

If the ‘width’ or ‘height’ presentation attributes on the outermost svg element are in user units (i.e., no unit identifier has been provided), then the value is assumed to be equivalent to the same number of "px" units (see Units).

7.3. The initial coordinate system

For the outermost svg element, the SVG user agent determines an initial viewport coordinate system and an initial local coordinate system such that the two coordinates systems are identical. The origin of both coordinate systems is at the origin of the viewport, and one unit in the initial coordinate system equals one "pixel" (i.e., a px unit as defined in CSS 2.1 ([CSS21], section 4.3.2) in the SVG viewport. In most cases, such as stand-alone SVG documents or SVG document fragments embedded (by reference or inline) within XML parent documents where the parent's layout is determined by CSS [CSS21] or XSL [XSL], the initial viewport coordinate system (and therefore the initial user coordinate system) has its origin at the top/left of the viewport, with the positive x-axis pointing towards the right, the positive y-axis pointing down, and text rendered with an "upright" orientation, which means glyphs are oriented such that Roman characters and full-size ideographic characters for Asian scripts have the top edge of the corresponding glyphs oriented upwards and the right edge of the corresponding glyphs oriented to the right.

If the SVG implementation is part of a user agent which supports styling XML documents using CSS 2.1 compatible px units, then the SVG user agent should set its initial value for the size of a px unit in real world units to match the value used for other XML styling operations; otherwise, if the user agent can determine the size of a px unit from its environment, it should use that value; otherwise, it should choose an appropriate size for one px unit. In all cases, the size of a px must be in conformance with the rules described in CSS 2.1 ([CSS21], section 4.3.2).

Example InitialCoords below shows that the initial coordinate system has the origin at the top/left with the x-axis pointing to the right and the y-axis pointing down. The initial user coordinate system has one user unit equal to the parent (implicit or explicit) user agent's "pixel".

7.4. The ‘transform’ property

See the CSS3 Transforms spec for the description of the ‘transform’ property [CSS3TRANSFORMS].

7.5. The ‘viewBox’ attribute

Name Value Initial value Animatable viewBox [<min-x> <min-y> <width> <height>] As if not specified. yes

Name	Value	Initial value	Animatable
viewBox	[<min-x> <min-y> <width> <height>]	As if not specified.	yes

<min-x>, <min-x>, <width>, <height> = <number>

Transform on the ‘svg’ element is a bit special due to the ‘viewBox’ attribute. The transform should be applied as if the ‘svg’ had a parent element with that transform set.

RESOLUTION: transform property applies conceptually to the outside of the 'svg' element and there is no difference between presentation attribute and style property (in terms of the visual result).

It is often desirable to specify that a given set of graphics stretch to fit a particular container element. The ‘viewBox’ attribute provides this capability.

All elements that establish a new viewport (see elements that establish viewports), plus the ‘marker’, ‘pattern’ and ‘view’ elements have attribute ‘viewBox’. The value of the ‘viewBox’ attribute is a list of four numbers <min-x>, <min-y>, <width> and <height>, separated by whitespace and/or a comma, which specify a rectangle in user space which should be mapped to the bounds of the viewport established by the given element, taking into account attribute ‘preserveAspectRatio’. If specified, an additional transformation is applied to all descendants of the given element to achieve the specified effect.

A negative value for <width> or <height> is an error (see Error processing). A value of zero disables rendering of the element.

Example ViewBox illustrates the use of the ‘viewBox’ attribute on the outermost svg element to specify that the SVG content should stretch to fit bounds of the viewport.

<?xml version="1.0" standalone="no"?>
<svg width="300px" height="200px"
     viewBox="0 0 1500 1000" preserveAspectRatio="none"
     xmlns="http://www.w3.org/2000/svg">
  <desc>Example ViewBox - uses the viewBox
   attribute to automatically create an initial user coordinate
   system which causes the graphic to scale to fit into the
   viewport no matter what size the viewport is.</desc>
  <!-- This rectangle goes from (0,0) to (1500,1000) in local coordinate system.
       Because of the viewBox attribute above,
       the rectangle will end up filling the entire area
       reserved for the SVG content. -->
  <rect x="0" y="0" width="1500" height="1000"
        fill="yellow" stroke="blue" stroke-width="12"  />
  <!-- A large, red triangle -->
  <path fill="red"  d="M 750,100 L 250,900 L 1250,900 z"/>
  <!-- A text string that spans most of the viewport -->
  <text x="100" y="600" font-size="200" font-family="Verdana" >
    Stretch to fit
  </text>
</svg>

Example ViewBox
Rendered into viewport with width=300px, height=200px		Rendered into viewport with width=150px, height=200px

View this example as SVG (SVG-enabled browsers only)

The effect of the ‘viewBox’ attribute is that the user agent automatically supplies the appropriate transformation matrix to map the specified rectangle in local coordinate system to the bounds of a designated region (often, the viewport). To achieve the effect of the example on the left, with viewport dimensions of 300 by 200 pixels, the user agent needs to automatically insert a transformation which scales both X and Y by 0.2. The effect is equivalent to having a viewport of size 300px by 200px and the following supplemental transformation in the document, as follows:

<?xml version="1.0" standalone="no"?>
<svg width="300px" height="200px"
     xmlns="http://www.w3.org/2000/svg">
  <g transform="scale(0.2)">
    <!-- Rest of document goes here -->
  </g>
</svg>

To achieve the effect of the example on the right, with viewport dimensions of 150 by 200 pixels, the user agent needs to automatically insert a transformation which scales X by 0.1 and Y by 0.2. The effect is equivalent to having a viewport of size 150px by 200px and the following supplemental transformation in the document, as follows:

<?xml version="1.0" standalone="no"?>
<svg width="150px" height="200px"
     xmlns="http://www.w3.org/2000/svg">
  <g transform="scale(0.1 0.2)">
    <!-- Rest of document goes here -->
  </g>
</svg>

Note that in some cases the user agent will need to supply a translate transformation in addition to a scale transformation. For example, on an outermost svg element, a translate transformation will be needed if the ‘viewBox’ attributes specifies values other than zero for <min-x> or <min-y>.

If both ‘transform’ (or ‘patternTransform’) and ‘viewBox’ are applied to an element two new coordinate systems are established. ‘transform’ establishes the first new coordinate system for the element. ‘viewBox’ establishes a second coordinate system for all descendants of the element. The first coordinate system is post-multiplied by the second coordinate system.

Unlike the ‘transform’ property, the automatic transformation that is created due to a ‘viewBox’ does not affect the ‘x’, ‘y’, ‘width’ and ‘height’ attributes (or in the case of the ‘marker’ element, the ‘markerWidth’ and ‘markerHeight’ attributes) on the element with the ‘viewBox’ attribute. Thus, in the example above which shows an ‘svg’ element which has ‘width’ and ‘height’ presentation attributes and a ‘viewBox’ attribute, the ‘width’ and ‘height’ represent values in the coordinate system that exists before the ‘viewBox’ transformation is applied. On the other hand, like the ‘transform’ property, it does establish a new coordinate system for all other attributes and for descendant elements.

7.6. The ‘preserveAspectRatio’ attribute

Name Value Initial value Animatable preserveAspectRatio <align> <meetOrSlice>? xMidYMid meet yes

Name	Value	Initial value	Animatable
preserveAspectRatio	<align> <meetOrSlice>?	xMidYMid meet	yes

<align> = none | xMinYMin | xMidYMin | xMaxYMin | xMinYMid | xMidYMid | xMaxYMid | xMinYMax | xMidYMax | xMaxYMax

<meetOrSlice> = meet | slice

Indicates whether or not to force uniform scaling. Applies to all elements that establish a new viewport (see elements that establish viewports), plus the ‘image’, ‘marker’, ‘pattern’ and ‘view’ elements

In some cases, typically when using the ‘viewBox’ attribute, it is desirable that the graphics stretch to fit non-uniformly to take up the entire viewport. In other cases, it is desirable that uniform scaling be used for the purposes of preserving the aspect ratio of the graphics.

For elements that establish a new viewport (see elements that establish viewports), plus the ‘marker’, ‘pattern’ and ‘view’ elements, ‘preserveAspectRatio’ only applies when a value has been provided for ‘viewBox’ on the same element. For these elements, if attribute ‘viewBox’ is not provided, then ‘preserveAspectRatio’ is ignored.

For ‘image’ elements, ‘preserveAspectRatio’ indicates how referenced images should be fitted with respect to the reference rectangle and whether the aspect ratio of the referenced image should be preserved with respect to the current user coordinate system.

The <align> parameter indicates whether to force uniform scaling and, if so, the alignment method to use in case the aspect ratio of the ‘viewBox’ doesn't match the aspect ratio of the SVG viewport. The <align> parameter must be one of the following strings:

none - Do not force uniform scaling. Scale the graphic content of the given element non-uniformly if necessary such that the element's bounding box exactly matches the viewport rectangle.
(Note: if <align> is none, then the optional <meetOrSlice> value is ignored.)
xMinYMin - Force uniform scaling.
Align the <min-x> of the element's ‘viewBox’ with the smallest X value of the viewport.
Align the <min-y> of the element's ‘viewBox’ with the smallest Y value of the viewport.
xMidYMin - Force uniform scaling.
Align the midpoint X value of the element's ‘viewBox’ with the midpoint X value of the viewport.
Align the <min-y> of the element's ‘viewBox’ with the smallest Y value of the viewport.
xMaxYMin - Force uniform scaling.
Align the <min-x>+<width> of the element's ‘viewBox’ with the maximum X value of the viewport.
Align the <min-y> of the element's ‘viewBox’ with the smallest Y value of the viewport.
xMinYMid - Force uniform scaling.
Align the <min-x> of the element's ‘viewBox’ with the smallest X value of the viewport.
Align the midpoint Y value of the element's ‘viewBox’ with the midpoint Y value of the viewport.
xMidYMid (the initial value) - Force uniform scaling.
Align the midpoint X value of the element's ‘viewBox’ with the midpoint X value of the viewport.
Align the midpoint Y value of the element's ‘viewBox’ with the midpoint Y value of the viewport.
xMaxYMid - Force uniform scaling.
Align the <min-x>+<width> of the element's ‘viewBox’ with the maximum X value of the viewport.
Align the midpoint Y value of the element's ‘viewBox’ with the midpoint Y value of the viewport.
xMinYMax - Force uniform scaling.
Align the <min-x> of the element's ‘viewBox’ with the smallest X value of the viewport.
Align the <min-y>+<height> of the element's ‘viewBox’ with the maximum Y value of the viewport.
xMidYMax - Force uniform scaling.
Align the midpoint X value of the element's ‘viewBox’ with the midpoint X value of the viewport.
Align the <min-y>+<height> of the element's ‘viewBox’ with the maximum Y value of the viewport.
xMaxYMax - Force uniform scaling.
Align the <min-x>+<width> of the element's ‘viewBox’ with the maximum X value of the viewport.
Align the <min-y>+<height> of the element's ‘viewBox’ with the maximum Y value of the viewport.

The <meetOrSlice> parameter is optional and, if provided, is separated from the <align> value by one or more spaces and then must be one of the following strings:

meet (the default) - Scale the graphic such that:
- aspect ratio is preserved
- the entire ‘viewBox’ is visible within the viewport
- the ‘viewBox’ is scaled up as much as possible, while still meeting the other criteria
In this case, if the aspect ratio of the graphic does not match the viewport, some of the viewport will extend beyond the bounds of the ‘viewBox’ (i.e., the area into which the ‘viewBox’ will draw will be smaller than the viewport).
slice - Scale the graphic such that:
- aspect ratio is preserved
- the entire viewport is covered by the ‘viewBox’
- the ‘viewBox’ is scaled down as much as possible, while still meeting the other criteria
In this case, if the aspect ratio of the ‘viewBox’ does not match the viewport, some of the ‘viewBox’ will extend beyond the bounds of the viewport (i.e., the area into which the ‘viewBox’ will draw is larger than the viewport).

Example PreserveAspectRatio illustrates the various options on ‘preserveAspectRatio’. The example creates several new viewports by including ‘svg’ sub-elements embedded inside the outermost svg element (see Establishing a new viewport).

<svg width="450px" height="300px"
     xmlns="http://www.w3.org/2000/svg">
  <desc>Example PreserveAspectRatio - illustrates preserveAspectRatio attribute</desc>
  <defs>
	<g id="smile">
<rect x='.5' y='.5' width='29' height='39' fill='black' stroke='red'/>
<g transform='translate(0, 5)'>
<circle cx='15' cy='15' r='10' fill='yellow'/>
<circle cx='12' cy='12' r='1.5' fill='black'/>
<circle cx='17' cy='12' r='1.5' fill='black'/>
<path d='M 10 19 A 8 8 0 0 0 20 19' stroke='black' stroke-width='2'/>
</g>
</g>
</defs>
  <rect x="1" y="1" width="448" height="298"
        fill="none" stroke="blue"/>
  <g font-size="9">
    <text x="10" y="30">SVG to fit</text>
    <g transform="translate(20,40)"><use href="#smile" /></g>
    <text x="10" y="110">Viewport 1</text>
    <g transform="translate(10,120)"><rect x='.5' y='.5' width='49' height='29' fill='none' stroke='blue'/>;</g>
    <text x="10" y="180">Viewport 2</text>
    <g transform="translate(20,190)"><rect x='.5' y='.5' width='29' height='59' fill='none' stroke='blue'/>;</g>

    <g id="meet-group-1" transform="translate(100, 60)">
      <text x="0" y="-30">--------------- meet ---------------</text>
      <g><text y="-10">xMin*</text><rect x='.5' y='.5' width='49' height='29' fill='none' stroke='blue'/>;
        <svg preserveAspectRatio="xMinYMin meet" viewBox="0 0 30 40"
             width="50" height="30"><use href="#smile" /></svg></g>
      <g transform="translate(70,0)"><text y="-10">xMid*</text><rect x='.5' y='.5' width='49' height='29' fill='none' stroke='blue'/>;
        <svg preserveAspectRatio="xMidYMid meet" viewBox="0 0 30 40"
             width="50" height="30"><use href="#smile" /></svg></g>
      <g transform="translate(0,70)"><text y="-10">xMax*</text><rect x='.5' y='.5' width='49' height='29' fill='none' stroke='blue'/>;
        <svg preserveAspectRatio="xMaxYMax meet" viewBox="0 0 30 40"
             width="50" height="30"><use href="#smile" /></svg></g>
    </g>

    <g id="meet-group-2" transform="translate(250, 60)">
      <text x="0" y="-30">---------- meet ----------</text>
      <g><text y="-10">*YMin</text><rect x='.5' y='.5' width='29' height='59' fill='none' stroke='blue'/>;
        <svg preserveAspectRatio="xMinYMin meet" viewBox="0 0 30 40"
             width="30" height="60"><use href="#smile" /></svg></g>
      <g transform="translate(50, 0)"><text y="-10">*YMid</text><rect x='.5' y='.5' width='29' height='59' fill='none' stroke='blue'/>;
        <svg preserveAspectRatio="xMidYMid meet" viewBox="0 0 30 40"
             width="30" height="60"><use href="#smile" /></svg></g>
      <g transform="translate(100, 0)"><text y="-10">*YMax</text><rect x='.5' y='.5' width='29' height='59' fill='none' stroke='blue'/>;
        <svg preserveAspectRatio="xMaxYMax meet" viewBox="0 0 30 40"
             width="30" height="60"><use href="#smile" /></svg></g>
    </g>

    <g id="slice-group-1" transform="translate(100, 220)">
      <text x="0" y="-30">---------- slice ----------</text>
      <g><text y="-10">xMin*</text><rect x='.5' y='.5' width='29' height='59' fill='none' stroke='blue'/>;
        <svg preserveAspectRatio="xMinYMin slice" viewBox="0 0 30 40"
             width="30" height="60"><use href="#smile" /></svg></g>
      <g transform="translate(50,0)"><text y="-10">xMid*</text><rect x='.5' y='.5' width='29' height='59' fill='none' stroke='blue'/>;
        <svg preserveAspectRatio="xMidYMid slice" viewBox="0 0 30 40"
             width="30" height="60"><use href="#smile" /></svg></g>
      <g transform="translate(100,0)"><text y="-10">xMax*</text><rect x='.5' y='.5' width='29' height='59' fill='none' stroke='blue'/>;
        <svg preserveAspectRatio="xMaxYMax slice" viewBox="0 0 30 40"
             width="30" height="60"><use href="#smile" /></svg></g>
    </g>

    <g id="slice-group-2" transform="translate(250, 220)">
      <text x="0" y="-30">--------------- slice ---------------</text>
      <g><text y="-10">*YMin</text><rect x='.5' y='.5' width='49' height='29' fill='none' stroke='blue'/>;
        <svg preserveAspectRatio="xMinYMin slice" viewBox="0 0 30 40"
             width="50" height="30"><use href="#smile" /></svg></g>
      <g transform="translate(70,0)"><text y="-10">*YMid</text><rect x='.5' y='.5' width='49' height='29' fill='none' stroke='blue'/>;
        <svg preserveAspectRatio="xMidYMid slice" viewBox="0 0 30 40"
             width="50" height="30"><use href="#smile" /></svg></g>
      <g transform="translate(140,0)"><text y="-10">*YMax</text><rect x='.5' y='.5' width='49' height='29' fill='none' stroke='blue'/>;
        <svg preserveAspectRatio="xMaxYMax slice" viewBox="0 0 30 40"
             width="50" height="30"><use href="#smile" /></svg></g>
    </g>   
  </g>
</svg>

Example PreserveAspectRatio

7.7. Establishing a new SVG viewport

Note that this section handles the SVG viewport which is different from the term "viewport" in CSS.

At any point in an SVG drawing, you can establish a new SVG viewport into which all contained graphics is drawn by including an ‘svg’ element inside SVG content. By establishing a new SVG viewport, you also implicitly establish a new viewport coordinate system, a new user coordinate system. Additionally, there is a new meaning for percentage units defined to be relative to the current SVG viewport since a new SVG viewport has been established (see Units).

The bounds of the new SVG viewport are defined by the ‘x’, ‘y’, ‘width’ and ‘height’ attributes on the element establishing the new SVG viewport, such as an ‘svg’ element. Both the new viewport coordinate system and the new user coordinate system have their origins at (‘x’, ‘y’), where ‘x’ and ‘y’ represent the value of the corresponding attributes on the element establishing the SVG viewport. The orientation of the new viewport coordinate system and the new user coordinate system correspond to the orientation of the current user coordinate system for the element establishing the SVG viewport. A single unit in the new viewport coordinate system and the new user coordinate system are the same size as a single unit in the current user coordinate system for the element establishing the SVG viewport.

Here is an example:

<?xml version="1.0" standalone="no"?>
<svg width="4in" height="3in"
     xmlns="http://www.w3.org/2000/svg">
  <desc>This SVG drawing embeds another one,
    thus establishing a new SVG viewport
  </desc>
  <!-- The following statement establishing a new SVG viewport
       and renders SVG drawing B into that viewport -->
  <svg x="25%" y="25%" width="50%" height="50%">
     <!-- drawing B goes here -->
  </svg>
</svg>

For an extensive example of creating new SVG viewports, see Example PreserveAspectRatio.

The following elements establish new SVG viewports:

The ‘svg’ element
A ‘symbol’ element define new viewports whenever they are instanced by a ‘use’ element.
An ‘image’ element that references an SVG file will result in the establishment of a temporary new SVG viewport since the referenced resource by definition will have an ‘svg’ element.
An ‘iframe’ element that references an SVG file establishes new SVG viewport like the situation of ‘image’ element.
A ‘foreignObject’ element creates a new SVG viewport for rendering the content that is within the element. (It also creates a new initial containing block.)

Whether a new SVG viewport also establishes a new additional clipping path is determined by the value of the ‘overflow’ property on the element that establishes the new SVG viewport.

7.8. Units

SVG follows the description and definition of common values and units from the CSS Values and Units Module [CSS3VALUES] for attributes, presentation attributes and CSS properties. Each attribute and property must specify the used component value type. Subsequent or extending specifications published by the CSS WG or SVG WG may extend basic data types or add new data types.

For <percentage> values that are defined to be relative to the size of viewport:

For any x-coordinate value or width value expressed as a percentage of the viewport, the value to use is the specified percentage of the actual-width in user units for the nearest containing viewport, where actual-width is the width dimension of the viewport element within the user coordinate system for the viewport element.
For any y-coordinate value or height value expressed as a percentage of the viewport, the value to use is the specified percentage of the actual-height in user units for the nearest containing viewport, where actual-height is the height dimension of the viewport element within the user coordinate system for the viewport element.
For any other length value expressed as a percentage of the viewport, the percentage is calculated as the specified percentage of sqrt((actual-width)**2 + (actual-height)**2)/sqrt(2).

Example Units below illustrates some of the processing rules for different types of units.

<?xml version="1.0" standalone="no"?>
<svg width="400px" height="200px" viewBox="0 0 4000 2000"
     xmlns="http://www.w3.org/2000/svg">
  <title>Example Units</title>
  <desc>Illustrates various units options</desc>

  <!-- Frame the picture -->
  <rect x="5" y="5" width="3990" height="1990" 
        fill="none" stroke="blue" stroke-width="10"/>

  <g fill="blue" stroke="red" font-family="Verdana" font-size="150">
    <!-- Absolute unit specifiers -->
    <g transform="translate(400,0)">
      <text x="-50" y="300" fill="black" stroke="none">Abs. units:</text>
      <rect x="0" y="400" width="4in" height="2in" stroke-width=".4in"/>
      <rect x="0" y="750" width="384" height="192" stroke-width="38.4"/>
      <g transform="scale(2)">
        <rect x="0" y="600" width="4in" height="2in" stroke-width=".4in"/>
      </g>
    </g>

    <!-- Relative unit specifiers -->
    <g transform="translate(1600,0)">
      <text x="-50" y="300" fill="black" stroke="none">Rel. units:</text>
      <rect x="0" y="400" width="2.5em" height="1.25em" stroke-width=".25em"/>
      <rect x="0" y="750" width="375" height="187.5" stroke-width="37.5"/>
      <g transform="scale(2)">
        <rect x="0" y="600" width="2.5em" height="1.25em" stroke-width=".25em"/>
      </g>
    </g>

    <!-- Percentages -->
    <g transform="translate(2800,0)">
      <text x="-50" y="300" fill="black" stroke="none">Percentages:</text>
      <rect x="0" y="400" width="10%" height="10%" stroke-width="1%"/>
      <rect x="0" y="750" width="400" height="200" stroke-width="31.62"/>
      <g transform="scale(2)">
        <rect x="0" y="600" width="10%" height="10%" stroke-width="1%"/>
      </g>
    </g>
  </g>
</svg>

Example Units

The three rectangles on the left demonstrate the use of one of the absolute unit identifiers, the "in" unit (inch). CSS defines 1 inch to be equal to 96 pixels. Therefore, the topmost rectangle, which is specified in inches, is exactly the same size as the middle rectangle, which is specified in user units such that there are 96 user units for each corresponding inch in the topmost rectangle. The bottom rectangle of the group illustrates what happens when values specified in inches are scaled.

The three rectangles in the middle demonstrate the use of one of the relative unit identifiers, the "em" unit. Because the ‘font-size’ property has been set to 150 on the outermost ‘g’ element, each "em" unit is equal to 150 user units. The topmost rectangle, which is specified in "em" units, is exactly the same size as the middle rectangle, which is specified in user units such that there are 150 user units for each corresponding "em" unit in the topmost rectangle. The bottom rectangle of the group illustrates what happens when values specified in "em" units are scaled.

The three rectangles on the right demonstrate the use of percentages. Note that the width and height of the viewport in the user coordinate system for the viewport element (in this case, the outermost svg element) are 4000 and 2000, respectively, because processing the ‘viewBox’ attribute results in a transformed user coordinate system. The topmost rectangle, which is specified in percentage units, is exactly the same size as the middle rectangle, which is specified in equivalent user units. In particular, note that the ‘stroke-width’ property in the middle rectangle is set to 1% of the sqrt((actual-width)**2 + (actual-height)**2) / sqrt(2), which in this case is .01*sqrt(4000*4000+2000*2000)/sqrt(2), or 31.62. The bottom rectangle of the group illustrates what happens when values specified in percentage units are scaled.

7.9. Bounding boxes

bounding box: The bounding box (or "bbox") of an element is the tightest fitting rectangle aligned with the axes of that element's user coordinate system that entirely encloses it and its descendants.

Three kinds of bounding boxes can be computed for an element:

The object bounding box is the bounding box that contains only an element's geometric shape. For basic shapes, this is the area that is filled. Unless otherwise specified, this is what is meant by the unqualified term "bounding box".
The stroke bounding box is the bounding box that contains an element's geometric shape and its stroke shape.
The decorated bounding box is the bounding box that contains an element's geometric shape, its stroke shape and its markers.

Note that the values of the ‘opacity’, ‘visibility’, ‘fill’, ‘fill-opacity’, ‘fill-rule’, ‘stroke-dasharray’ and ‘stroke-dashoffset’ properties on an element have no effect on the bounding box of an element.

For curved shapes, the bounding box must enclose all portions of the shape along the edge, not just end points. Note that control points for a curve which are not defined as lying along the line of the resulting curve (e.g., the second coordinate pair of a Cubic Bézier command) must not contribute to the dimensions of the bounding box (though those points may fall within the area of the bounding box, if they lie within the shape itself, or along or close to the curve). For example, control points of a curve that are at a further distance than the curve edge, from the non-enclosing side of the curve edge, must be excluded from the bounding box.

Image showing the object bounding box of a quadratic Bézier curve.

The path 'M20,50 L35,100 H120 V50 Q70,10 20,50' is shown in light blue. On the left, a correct object bounding box of the path is shown. Note that it does not include the top-most control point of the curve, but it does include all of the blue shape, even the parts that lie outside of the convex hull of the control points.

Even if an element is not in the rendering tree – due to it being 'display: none', within a ‘defs’ element, not usually rendered like a ‘symbol’ element or not currently present in the document tree – it still has a bounding box. A call to getBBox on the element will return the same rectangle as if the element were rendered. However, an element that is not in the rendering tree does not contribute to the bounding box of any ancestor element.

The following example defines a number of elements. The expected object bounding box for each element with an ID is shown below.

<svg xmlns="http://www.w3.org/2000/svg">

  <title>Bounding Box Calculation</title>
  <desc>Examples of elements with different bounding box results based on context.</desc>

  <defs id="defs-1">
     <rect id="rect-1" x="20" y="20" width="40" height="40" fill="blue" />
  </defs>

  <g id="group-1">
    <use id="use-1" href="#rect-1" x="10" y="10" />

    <g id="group-2" display="none">
      <rect id="rect-2" x="10" y="10" width="100" height="100" fill="red" />
    </g>
  </g>
</svg>

Element ID	Bounding Box Result
"`defs-1`"	{0, 0, 0, 0}
"`rect-1`"	{20, 20, 40, 40}
"`group-1`"	{30, 30, 40, 40}
"`use-1`"	{30, 30, 40, 40}
"`group-2`"	{10, 10, 100, 100}
"`rect-2`"	{10, 10, 100, 100}

For text content elements, for the purposes of the bounding box calculation, each glyph must be treated as a separate graphics element. he calculations must assume that all glyphs occupy the full glyph cell. For example, for horizontal text, the calculations must assume that each glyph extends vertically to the full ascent and descent values for the font. An exception to this is when the ‘inline-size’ presentation attribute has been specified on the ‘text’ element, in which case the element's content area is its bounding box.

Because declarative or scripted animation can change the shape, size, and position of an element, the bounding box is mutable. Thus, the bounding box for an element shall reflect the current values for the element at the snapshot in time at which the bounding box is requested, whether through a script call or as part of a declarative or linking syntax.

An element which has zero width, zero height, or both (such as a vertical or horizontal line, or a ‘rect’ element with a zero ‘width’ or ‘height’) still has a bounding box, with a positive value for the positive dimension, or with '0' for both the width and height if no positive dimension is specified. Similarly, subpaths segments of a ‘path’ element with zero width and height must be included in that element's geometry for the sake of the bounding box.

An element with no position specified (such as a ‘path’ element with a value of (none) for the ‘d’ attribute) is positioned at the point (0,0) for the purposes of calculating a bounding box.

Note that elements whose DOM object does not derive from SVGGraphicsElement (such as gradient elements) do not have a bounding box, and thus have no interface to request a bounding box.

Elements in the rendering tree which reference unresolved resources shall still have a bounding box, defined by the position and dimensions specified in their attributes, or by the initial value for those attributes if no values are supplied. For example, the element <use href="#bad" x="10" y="10"/> would have a bounding box with an x and y of 10 and a width and height of 0.

The following algorithm defines how to compute a bounding box for a given element. The inputs to the algorithm are:

element, the element we are computing a bounding box for;
space, a coordinate space in which the bounding box will be computed;
fill, a boolean indicating whether the bounding box includes the geometry of the element and its descendants;
stroke, a boolean indicating whether the bounding box includes the stroke of the element and its descendants;
markers, a boolean indicating whether the bounding box includes the markers of the element and its descendants; and
clipped, a boolean indicating whether the bounding box is affected by any clipping paths applied to the element and its descendants.

Need to define what the union of rectangles with no area means.

The algorithm to compute the bounding box is as follows, depending on the type of element:

a shape

a text content element

an ‘a’ element within a text content element

Let box be a rectangle initialized to (0, 0, 0, 0).
Let fill-shape be the equivalent path of element if it is a shape, or a shape that includes each of the glyph cells corresponding to the text within the elements otherwise.
Need to update this take into account ‘inline-size’ on ‘text’.
If fill is true, then set box to the tightest rectangle in the coordinate system space that contains fill-shape.
The values of the ‘fill’, ‘fill-opacity’ and ‘fill-rule’ properties do not affect fill-shape.
If stroke is true and the element's ‘stroke’ is anything other than none, then set box to be the union of box and the tightest rectangle in coordinate system space that contains the stroke shape of the element, with the assumption that the element has no dash pattern.
The values of the ‘stroke-opacity’, ‘stroke-dasharray’ and ‘stroke-dashoffset’ do not affect the calculation of the stroke shape.
If markers is true, then for each marker marker rendered on the element:
1. For each descendant graphics element child of the ‘marker’ element that defines marker's content:
  1. If child has an ancestor element within the ‘marker’ that is 'display: none', has a failing conditional processing attribute, or is not an ‘a’, ‘g’, ‘svg’ or ‘switch’ element, then continue to the next descendant graphics element.
  2. Otherwise, set box to be the union of box and the result of invoking the algorithm to compute a bounding box with child as the element, space as the target coordinate space, true for fill, stroke and markers, and clipped for clipped.
If clipped is true and the value of ‘clip-path’ on element is not none, then set box to be the tighest rectangle in coordinate system space that contains the intersection of box and the clipping path.
Return box.

a container element

‘use’

Let box be a rectangle initialized to (0, 0, 0, 0).
Let parent be the container element if it is one, or the root of the ‘use’ element's shadow tree otherwise.
For each descendant graphics element child of parent:
1. If child has an ancestor element within parent that is 'display: none', has a failing conditional processing attribute, or is not an ‘a’, ‘g’, ‘svg’ or ‘switch’ element, then continue to the next descendant graphics element.
2. Otherwise, set box to be the union of box and the result of invoking the algorithm to compute a bounding box with child as the element and the same values for space, fill, stroke, markers and clipped as the corresponding algorithm input values.
Return box.

‘canvas’

‘foreignObject’

‘iframe’

‘image’

‘video’

Return the tightest rectangle in coordinate space space that contains the rectangle defined by the ‘x’, ‘y’, ‘width’ and ‘height’ attributes of the element.
The fill, stroke and markers input arguments to this algorithm do not affect the bounding box returned for these elements.

The object bounding box, stroke bounding box or decorated bounding box of an element is the result of invoking the bounding box computation algorithm above with the following arguments: element is the element itself; space is the element's user coordinate system; fill is true; stroke is true if we are computing the stroke bounding box or decorated bounding box, and false othwerise; markers is true if we are computing the decorated bounding box, and false otherwise; and clipped is false.

7.10. Object bounding box units

The following elements offer the option of expressing coordinate values and lengths as fractions (and, in some cases, percentages) of the object bounding box, by setting a specified attribute to 'objectBoundingBox' on the given element:

Element	Attribute	Effect
‘linearGradient’	‘gradientUnits’	Indicates that the attributes which specify the gradient vector (‘x1’, ‘y1’, ‘x2’, ‘y2’) represent fractions or percentages of the bounding box of the element to which the gradient is applied.
‘radialGradient’	‘gradientUnits’	Indicates that the attributes which specify the center (‘cx’, ‘cy’), the radius (‘r’) and focus (‘fx’, ‘fy’) represent fractions or percentages of the bounding box of the element to which the gradient is applied.
‘mesh’	‘gradientUnits’	Indicates that the attributes which specify the paint server mesh starting point (‘x’, ‘y’) represent fractions or percentages and that mesh path data represents fractions of the bounding box of the element to which the mesh is applied. If the mesh is rendered as a graphics object (i.e., not as a paint server) the current viewport is used in place of a bounding box.
‘pattern’	‘patternUnits’	Indicates that the attributes which define how to tile the pattern (‘x’, ‘y’, ‘width’, ‘height’) are established using the bounding box of the element to which the pattern is applied.
‘pattern’	‘patternContentUnits’	Indicates that the user coordinate system for the contents of the pattern is established using the bounding box of the element to which the pattern is applied.
‘clipPath’	‘clipPathUnits’	Indicates that the user coordinate system for the contents of the ‘clipPath’ element is established using the bounding box of the element to which the clipping path is applied.
‘mask’	‘maskUnits’	Indicates that the attributes which define the masking region (‘x’, ‘y’, ‘width’, ‘height’) is established using the bounding box of the element to which the mask is applied.
‘mask’	‘maskContentUnits’	Indicates that the user coordinate system for the contents of the ‘mask’ element are established using the bounding box of the element to which the mask is applied.
‘filter’	‘filterUnits’	Indicates that the attributes which define the filter effects region (‘x’, ‘y’, ‘width’, ‘height’) represent fractions or percentages of the bounding box of the element to which the filter is applied.
‘filter’	‘primitiveUnits’	Indicates that the various length values within the filter primitives represent fractions or percentages of the bounding box of the element to which the filter is applied.

In the discussion that follows, the term applicable element is the element to which the given effect applies. For gradients and patterns, the applicable element is the graphics element which has its ‘fill’ or ‘stroke’ property referencing the given gradient or pattern. (See Inheritance of Painting Properties. For special rules concerning text elements, see the discussion of object bounding box units and text elements.) For clipping paths, masks and filters, the applicable element can be either a container element or a graphics element.

When keyword objectBoundingBox is used, then the effect is as if a supplemental transformation matrix were inserted into the list of nested transformation matrices to create a new user coordinate system.

First, the (minx,miny) and (maxx,maxy) coordinates are determined by the extends of the object bounding box of the applicable element.

Then, coordinate (0,0) in the new user coordinate system is mapped to the (minx,miny) corner of the tight bounding box within the user coordinate system of the applicable element and coordinate (1,1) in the new user coordinate system is mapped to the (maxx,maxy) corner of the tight bounding box of the applicable element. In most situations, the following transformation matrix produces the correct effect:

[ (maxx-minx) 0 0 (maxy-miny) minx miny ]

When percentages are used with attributes that define the gradient vector, the pattern tile, the filter region or the masking region, a percentage represents the same value as the corresponding decimal value (e.g., 50% means the same as 0.5). If percentages are used within the content of a ‘pattern’, ‘clipPath’, ‘mask’ or ‘filter’ element, these values are treated according to the processing rules for percentages as defined in Units.

Any numeric value can be specified for values expressed as a fraction or percentage of object bounding box units. In particular, fractions less are zero or greater than one and percentages less than 0% or greater than 100% can be specified.

Keyword objectBoundingBox should not be used when the geometry of the applicable element has no width or no height, such as the case of a horizontal or vertical line, even when the line has actual thickness when viewed due to having a non-zero stroke width since stroke width is ignored for bounding box calculations. When the geometry of the applicable element has no width or height and objectBoundingBox is specified, then the given effect (e.g., a gradient or a filter) will be ignored.

7.11. Intrinsic sizing properties of the viewport of SVG content

SVG needs to specify how to calculate some intrinsic sizing properties to enable inclusion within other host documents. The intrinsic width and height of the SVG viewport of SVG content must be determined from the ‘width’ and ‘height’ properties. If either of these are not specified, the used value is the initial value 'auto'. Specifically, percentage values do not provide an intrinsic width or height.

The intrinsic aspect ratio of the SVG viewport of SVG content is necessary for example, when including SVG from an ‘object’ element in HTML styled with CSS. It is possible (indeed, common) for an SVG graphic to have an intrinsic aspect ratio but not to have an intrinsic width or height. The intrinsic aspect ratio must be calculated based upon the following rules:

In this example the intrinsic aspect ratio of the SVG viewport is 2:1. The intrinsic width is 10cm and the intrinsic height is 5cm.

In this example the intrinsic aspect ratio of the outermost SVG viewport is 1:1. An aspect ratio calculation in this case allows embedding in an object within a containing block that is only constrained in one direction.

Add more examples for the new auto value? E.g some of the examples provided by David Vest.

7.12. DOM interfaces

7.12.1. Interface SVGTransform

The SVGTransform interface is used to represent <transform-function> values that appear in the ‘transform’ property and its presentation attributes ‘transform’, ‘gradientTransform’ and ‘patternTransform’. An SVGTransform represents a single component in a transform list, such as a single scale(…) or matrix(…) value.

An SVGTransform object can be designated as read only, which means that attempts to modify the object will result in an exception being thrown, as described below.

An SVGTransform object can be associated with a particular element. The associated element is used to determine which element's ‘transform’ presentation attribute to update if the object reflects that attribute. Unless otherwise described, an SVGTransform object is not associated with any element.

Every SVGTransform object operates in one of three modes. It can:

reflect an element of a presentation attribute value (being exposed through the methods on the baseVal member of an SVGAnimatedTransformList),
reflect an element of the computed value of the ‘transform’ property (being exposed through the methods on the animVal member of an SVGAnimatedTransformList),
be detached, which is the case for SVGTransform objects created with createSVGTransform and createSVGTransformFromMatrix.

An SVGTransform object maintains an internal <transform-function> value, which is called its value. It also maintains a DOMMatrix object, which is called its matrix object, which is the object returned from the matrix IDL attribute. An SVGTransform object's matrix object is always kept synchronized with it its value.

interface SVGTransform {

  // Transform Types
  const unsigned short SVG_TRANSFORM_UNKNOWN = 0;
  const unsigned short SVG_TRANSFORM_MATRIX = 1;
  const unsigned short SVG_TRANSFORM_TRANSLATE = 2;
  const unsigned short SVG_TRANSFORM_SCALE = 3;
  const unsigned short SVG_TRANSFORM_ROTATE = 4;
  const unsigned short SVG_TRANSFORM_SKEWX = 5;
  const unsigned short SVG_TRANSFORM_SKEWY = 6;

  readonly attribute unsigned short type;
  [SameObject] readonly attribute DOMMatrix matrix;
  readonly attribute float angle;

  void setMatrix(DOMMatrixReadOnly matrix);
  void setTranslate(float tx, float ty);
  void setScale(float sx, float sy);
  void setRotate(float angle, float cx, float cy);
  void setSkewX(float angle);
  void setSkewY(float angle);
};

The numeric transform type constants defined on SVGTransform are used to represent the type of an SVGTransform's value. Their meanings are as follows:

Constant	Meaning
SVG_TRANSFORM_MATRIX	A matrix(…) value.
SVG_TRANSFORM_TRANSLATE	A translate(…) value.
SVG_TRANSFORM_SCALE	A scale(…) value.
SVG_TRANSFORM_ROTATE	A rotate(…) value.
SVG_TRANSFORM_SKEWX	A skewX(…) value.
SVG_TRANSFORM_SKEWY	A skewY(…) value.
SVG_TRANSFORM_UNKNOWN	Some other type of value.

The use of numeric transform type constants is an anti-pattern and new constant values will not be introduced for any transform types supported by SVGTransform. If other types of transforms are supported and used, the SVGTransform uses the SVG_TRANSFORM_UNKNOWN type. See below for details on how the other properties of an SVGTransform operate with these types of transforms.

The type IDL attribute represents the type of transform item that the SVGTransform's value is. On getting type, the following steps are run:

If the SVGTransform's value is a matrix(…), translate(…), scale(…), rotate(…), skewX(…) or skewY(…) function, then return the corresponding constant value from the transform type table above.
Otherwise, return SVG_TRANSFORM_UNKNOWN.
For example, for a scaleX(…) or translate3d(…) transform, SVG_TRANSFORM_UNKNOWN would be returned.

The matrix IDL attribute represents the transform as a 4x4 homogeneous matrix, and on getting returns the SVGTransform's matrix object. When the matrix object is first created, its values are set to match the SVGTransform's transform function value, and is set to reflects the SVGTransform.

See the CSS Transforms specification for a description of how the different transform function types correspond to particular matrix values.

The angle IDL attribute represents the angle parameter of a rotate(…), skewX(…) or skewY(…) transform function. On getting, the following steps are run:

If the SVGTransform object's value is a rotate(…), skewX(…) or skewY(…) function, return its angle argument in degrees.
Otherwise, return 0.

The setMatrix method is used to set the SVGTransform to a given matrix value. When setMatrix(matrix) is called, the following steps are run:

If the SVGTransform object is read only, then throw a DOMException with code NO_MODIFICATION_ALLOWED_ERR.
If matrix would return true from its is2d method, then set the SVGTransform object's value to a matrix(…) value that represents the same matrix as matrix.
Otherwise, set the SVGTransform object's value to a matrix3d(…) value that represents the same matrix as matrix.
Copy the matrix component values from matrix into the SVGTransform object's matrix object.
If the SVGTransform object reflects an element of a presentation attribute value, then reserialize the reflected attribute.

The setTranslate, setScale, setRotate, setSkewX and setSkewY methods are used to set the SVGTransform to a new transform function value. When one of these methods is called, the following steps are run:

If the SVGTransform object is read only, then throw a DOMException with code NO_MODIFICATION_ALLOWED_ERR.
Set the SVGTransform object's value to a new transform function value, depending on which method was called:

setTranslate(tx, ty)

the new transform function value is translate(tx, ty)

setScale(sx, sy)

the new transform function value is scale(sx, sy)

setRotate(angle, cx, cy)

the new transform function value is rotate(angle, cx, cy)

setSkewX(angle)

the new transform function value is skewX(angle)

setSkewY(angle)

the new transform function value is skewY(angle)
Set the components of the SVGTransform object's matrix object to match the new transform function value.
If the SVGTransform object reflects an element of a presentation attribute value, then reserialize the reflected attribute.

This specification imposes additional requirements on the behavior of DOMMatrix objects beyond those described in the the Geometry Interfaces specification, so that they can be used to reflect presentation attributes that take transform values.

Every DOMMatrix object operates in one of two modes. It can:

reflect an SVGTransform (being exposed through the matrix IDL attribute on an SVGTransform), or
be detached, which is the case for DOMMatrix objects created using their constructor or with createSVGMatrix.

A DOMMatrix can be designated as read only, which means that attempts to modify the object will result in an exception being thrown. When assigning to any of a read only DOMMatrix's IDL attributes, or when invoking any of its mutable transform methods, a DOMException with code NO_MODIFICATION_ALLOWED_ERR must be thrown instead of updating the internal value.

Note that this applies only to the read-write DOMMatrix interface; the DOMMatrixReadOnly interface, which is not used for reflecting ‘transform’, will already throw an exception if an attempt is made to modify it.

When assigning to any of a writable DOMMatrix's IDL attributes, or when invoking any of its mutable transform methods, the following steps are run after updating the internal matrix value:

If the DOMMatrix reflects an SVGTransform, then:
1. If the DOMMatrix would return true from its is2d method, then set the SVGTransform object's value to a matrix(…) value that represents the same matrix as the DOMMatrix.
2. Otherwise, set the SVGTransform object's value to a matrix3d(…) value that represents the same matrix as the DOMMatrix.
3. If the SVGTransform object reflects an element of a presentation attribute value, then reserialize the reflected attribute.

7.12.2. Interface SVGTransformList

The SVGTransformList interface is a list interface whose elements are SVGTransform objects. An SVGTransformList represents a value that the ‘transform’ property can take, namely either a <transform-list> or the keyword none.

interface SVGTransformList {

  readonly attribute unsigned long length;
  readonly attribute unsigned long numberOfItems;

  void clear();
  SVGTransform initialize(SVGTransform newItem);
  getter SVGTransform getItem(unsigned long index);
  SVGTransform insertItemBefore(SVGTransform newItem, unsigned long index);
  SVGTransform replaceItem(SVGTransform newItem, unsigned long index);
  SVGTransform removeItem(unsigned long index);
  SVGTransform appendItem(SVGTransform newItem);
  setter void (unsigned long index, SVGTransform newItem);

  // Additional methods not common to other list interfaces.
  SVGTransform createSVGTransformFromMatrix(DOMMatrixReadOnly matrix);
  SVGTransform? consolidate();
};

The createSVGTransformFromMatrix method is used to create a new SVGTransform object from a matrix object. When the createSVGTransformFromMatrix(matrix) method is called, the following steps are run:

Let transform be a newly created SVGTransform object that is detached.
Follow the steps that would be run if the setMatrix method on transform were called, passing matrix as its argument.
Return transform.

The consolidate method is used to convert the transform list into an equivalent transformation using a single transform function. When the consolidate() method is called, the following steps are run:

If the SVGTransformList object is read only, then throw a DOMException with code NO_MODIFICATION_ALLOWED_ERR.
If the list is empty, return null.
Detach and then remove all elements in the list.
Let transform be a newly created SVGTransform object.
Let matrix be the matrix value obtained by beginning with an identity matrix, and then post-multiplying the value of the matrix object for each SVGTransform in the list, in order.
Set the components of transform's matrix object to the component values in matrix.
If transform's matrix object would return true from its is2d method, then set transform's value to a matrix(…) value that represents the same matrix as the matrix object.
Otherwise, set transform's value to a matrix3d(…) value that represents the same matrix as the matrix object.
Attach transform to this SVGTransformList.
Append transform to this list.
If the list reflects a presentation attribute, then reserialize the reflected attribute.
Return transform.

The behavior of all other interface members of SVGLengthList are defined in List interfaces.

7.12.3. Interface SVGAnimatedTransformList

An SVGAnimatedTransformList object is used to reflect the ‘transform’ property and its corresponding presentation attribute (which, depending on the element, is ‘transform’, ‘gradientTransform’ or ‘patternTransform’).

interface SVGAnimatedTransformList {
  [SameObject] readonly attribute SVGTransformList baseVal;
  [SameObject] readonly attribute SVGTransformList animVal;
};

The baseVal IDL attribute represents the value of the reflected presentation attribute. On getting baseVal, an SVGTransformList object is returned that reflects the given presentation attribute.

The animVal IDL attribute represents the computed value of the ‘transform’ property on an element. On getting animVal, an SVGTransformList object is returned that reflects the computed value of the ‘transform’ property on the given element.

The objects returned from baseVal and animVal must be distinct, even if there is no animation currently affecting the attribute.

7.12.4. Interface SVGPreserveAspectRatio

The SVGPreserveAspectRatio interface is used to represent values for the ‘preserveAspectRatio’ attribute.

An SVGPreserveAspectRatio object can be designated as read only, which means that attempts to modify the object will result in an exception being thrown, as described below.

Every SVGPreserveAspectRatio object operates in one of two modes. It can:

reflect the base value of a reflected ‘preserveAspectRatio’ attribute (being exposed through the methods on the baseVal member of an SVGAnimatedPreserveAspectRatio), or
reflect the animated value of a reflected ‘preserveAspectRatio’ attribute (being exposed through the methods on the animVal member of an SVGAnimatedPreserveAspectRatio).

interface SVGPreserveAspectRatio {

  // Alignment Types
  const unsigned short SVG_PRESERVEASPECTRATIO_UNKNOWN = 0;
  const unsigned short SVG_PRESERVEASPECTRATIO_NONE = 1;
  const unsigned short SVG_PRESERVEASPECTRATIO_XMINYMIN = 2;
  const unsigned short SVG_PRESERVEASPECTRATIO_XMIDYMIN = 3;
  const unsigned short SVG_PRESERVEASPECTRATIO_XMAXYMIN = 4;
  const unsigned short SVG_PRESERVEASPECTRATIO_XMINYMID = 5;
  const unsigned short SVG_PRESERVEASPECTRATIO_XMIDYMID = 6;
  const unsigned short SVG_PRESERVEASPECTRATIO_XMAXYMID = 7;
  const unsigned short SVG_PRESERVEASPECTRATIO_XMINYMAX = 8;
  const unsigned short SVG_PRESERVEASPECTRATIO_XMIDYMAX = 9;
  const unsigned short SVG_PRESERVEASPECTRATIO_XMAXYMAX = 10;

  // Meet-or-slice Types
  const unsigned short SVG_MEETORSLICE_UNKNOWN = 0;
  const unsigned short SVG_MEETORSLICE_MEET = 1;
  const unsigned short SVG_MEETORSLICE_SLICE = 2;

  attribute unsigned short align;
  attribute unsigned short meetOrSlice;
};

The numeric alignment type constants defined on SVGPreserveAspectRatio are used to represent the alignment keyword values that ‘preserveAspectRatio’ can take. Their meanings are as follows:

Constant	Meaning
SVG_PRESERVEASPECTRATIO_NONE	The none keyword.
SVG_PRESERVEASPECTRATIO_XMINYMIN	The xMinYMin keyword.
SVG_PRESERVEASPECTRATIO_XMIDYMIN	The xMidYMin keyword.
SVG_PRESERVEASPECTRATIO_XMAXYMIN	The xMaxYMin keyword.
SVG_PRESERVEASPECTRATIO_XMINYMID	The xMinYMid keyword.
SVG_PRESERVEASPECTRATIO_XMIDYMID	The xMidYMid keyword.
SVG_PRESERVEASPECTRATIO_XMAXYMID	The xMaxYMid keyword.
SVG_PRESERVEASPECTRATIO_XMINYMAX	The xMinYMax keyword.
SVG_PRESERVEASPECTRATIO_XMIDYMAX	The xMidYMax keyword.
SVG_PRESERVEASPECTRATIO_XMAXYMAX	The xMaxYMax keyword.
SVG_PRESERVEASPECTRATIO_UNKNOWN	Some other type of value.

Similarly, the numeric meet-or-slice type constants defined on SVGPreserveAspectRatio are used to represent the meet-or-slice keyword values that ‘preserveAspectRatio’ can take. Their meanings are as follows:

Constant	Meaning
SVG_MEETORSLICE_MEET	The meet keyword.
SVG_MEETORSLICE_SLICE	The slice keyword.
SVG_MEETORSLICE_UNKNOWN	Some other type of value.

The align IDL attribute represents the alignment keyword part of the ‘preserveAspectRatio’ value. On getting, the following steps are run:

Let value be a ‘preserveAspectRatio’ value, depending on the SVGPreserveAspectRatio object's mode:

reflecting the base value of a ‘preserveAspectRatio’ attribute

value is the current non-animated value of the attribute (using the attribute's initial value if it is not present or invalid).

reflecting the animated value of a ‘preserveAspectRatio’ attribute

If the attribute is currently being animated, then value is its current animated value. Otherwise, it is the current non-animated value of the attribute.
Return the constant value as specified in the alignment constant table above for the alignment keyword in value.

On setting align, the following steps are run:

If the SVGPreserveAspectRatio is read only, then throw a DOMException with code NO_MODIFICATION_ALLOWED_ERR.
If value is SVG_PRESERVEASPECTRATIO_UNKNOWN or does not have a corresponding entry in the alignment keyword table above, then return.
Let string be the corresponding keyword in the alignment keyword table above for value.
Append a single U+0020 SPACE character to string.
Let meet or slice be the value that would be returned from the meetOrSlice member on this SVGPreserveAspectRatio.
Append to string the corresponding keyword in the meet-or-slice keyword table above for meet or slice.
Set the reflected ‘preserveAspectRatio’ attribute to string.

The meetOrSlice IDL attribute represents the alignment keyword part of the ‘preserveAspectRatio’ value. On getting, the following steps are run:

Let value be a ‘preserveAspectRatio’ value, depending on the SVGPreserveAspectRatio object's mode:

reflecting the base value of a ‘preserveAspectRatio’ attribute

value is the current non-animated value of the attribute.

reflecting the animated value of a ‘preserveAspectRatio’ attribute

If the attribute is currently being animated, then value is its current animated value. Otherwise, it is the current non-animated value of the attribute.
If the meet-or-slice value is not present in value, then return SVG_MEETORSLICE_MEET.
Otherwise, the meet-or-slice value is present. Return the constant value as specified in the meet-or-slice constant table above for the meet-or-slice keyword in value.

On setting meetOrSlice, the following steps are run:

If the SVGPreserveAspectRatio is read only, then throw a DOMException with code NO_MODIFICATION_ALLOWED_ERR.
If value is SVG_MEETORSLICE_UNKNOWN or does not have a corresponding entry in the meet-or-slice keyword table above, then return.
Let align be the value that would be returned from the align member on this SVGPreserveAspectRatio.
Let string be the corresponding keyword in the alignment keyword table above for align.
Append a single U+0020 SPACE character to string.
Append to string the corresponding keyword in the meet-or-slice keyword table above for value.
Set the reflected ‘preserveAspectRatio’ attribute to string.

7.12.5. Interface SVGAnimatedPreserveAspectRatio

An SVGAnimatedPreserveAspectRatio object is used to reflect the ‘preserveAspectRatio’ attribute.

interface SVGAnimatedPreserveAspectRatio {
  [SameObject] readonly attribute SVGPreserveAspectRatio baseVal;
  [SameObject] readonly attribute SVGPreserveAspectRatio animVal;
};

The baseVal IDL attribute represents the current non-animated value of the reflected ‘preserveAspectRatio’ attribute. On getting baseVal, an SVGPreserveAspectRatio object is returned that reflects the base value of the ‘preserveAspectRatio’ attribute on the SVG element that the object with the reflcting IDL attribute of type SVGAnimatedPreserveAspectRatio was obtained from.

The animVal IDL attribute represents the current non-animated value of the reflected ‘preserveAspectRatio’ attribute. On getting animVal, an SVGPreserveAspectRatio object is returned that reflects the animated value of the ‘preserveAspectRatio’ attribute on the SVG element that the object with the reflcting IDL attribute of type SVGAnimatedPreserveAspectRatio was obtained from.