Chapter 9: Basic Shapes

Contents

• • 9.1. Introduction and definitions
  • 9.2. The ‘rect’ element
  • 9.3. The ‘circle’ element
  • 9.4. The ‘ellipse’ element
  • 9.5. The ‘line’ element
  • 9.6. The ‘polyline’ element
  • 9.7. The ‘polygon’ element
  • 9.8. DOM interfaces
    • 9.8.1. Interface SVGRectElement
    • 9.8.2. Interface SVGCircleElement
    • 9.8.3. Interface SVGEllipseElement
    • 9.8.4. Interface SVGLineElement
    • 9.8.5. Interface SVGAnimatedPoints
    • 9.8.6. Interface SVGPointList
    • 9.8.7. Interface SVGPolylineElement
    • 9.8.8. Interface SVGPolygonElement

9.1. Introduction and definitions

basic shape

shape

shape elements

A graphics element that is defined by some combination of straight lines and curves. Specifically: ‘circle’, ‘ellipse’, ‘line’, ‘path’, ‘polygon’, ‘polyline’ and ‘rect’.

SVG contains the following set of basic shape elements:

• rectangles (including optional rounded corners), created with the ‘rect’ element,
• circles, created with the ‘circle’ element,
• ellipses, created with the ‘ellipse’ element,
• straight lines, created with the ‘line’ element,
• polylines, created with the ‘polyline’ element, and
• polygons, created with the ‘polygon’ element.

Mathematically, these shape elements are equivalent to a ‘path’ element that would construct the same shape. The basic shapes may be stroked, filled and used as clip paths. All of the properties available for ‘path’ elements also apply to the basic shapes.

The equivalent path and algorithm to compute the stroke for each shape are defined in the shape sections below.

9.2. The ‘rect’ element

The ‘rect’ element defines a rectangle which is axis-aligned with the current local coordinate system. Rounded rectangles can be achieved by setting appropriate values for attributes ‘rx’ and ‘ry’.

The ‘x’ and ‘y’ coordinates refer to the left and top edges of the rectangle, in the current user coordinate system.

The ‘width’ and ‘height’ of the rectangle. A negative value for either attribute is an error (see Error processing). A value of zero for either attribute disables rendering of the element.

For rounded rectangles, the x- and y-axis radii of the ellipse used to round off the corners of the rectangle. A negative value for either attribute is an error (see Error processing).

The values used for the x- and y-axis rounded corner radii are determined implicitly if the ‘rx’ or ‘ry’ attributes (or both) are not specified, or are specified but with invalid values. The values are also subject to clamping so that the lengths of the straight segments of the rectangle are never negative. The effective values for ‘rx’ and ‘ry’ are determined by following these steps in order:

1. Let rx and ry be length values.
2. If neither ‘rx’ nor ‘ry’ are properly specified, then set both rx and ry to 0. (This will result in square corners.)
3. Otherwise, if a properly specified value is provided for ‘rx’, but not for ‘ry’, then set both rx and ry to the value of ‘rx’.
4. Otherwise, if a properly specified value is provided for ‘ry’, but not for ‘rx’, then set both rx and ry to the value of ‘ry’.
5. Otherwise, both ‘rx’ and ‘ry’ were specified properly. Set rx to the value of ‘rx’ and ry to the value of ‘ry’.
6. If rx is greater than half of ‘width’, then set rx to half of ‘width’.
7. If ry is greater than half of ‘height’, then set ry to half of ‘height’.
8. The effective values of ‘rx’ and ‘ry’ are rx and ry, respectively.

Mathematically, a ‘rect’ element is mapped to an equivalent ‘path’ element as follows: (Note: all coordinate and length values are first converted into local coordinate system coordinates according to Units.)

• perform an absolute moveto operation to location (x+rx,y), where x is the value of the ‘rect’ element's ‘x’ attribute converted to user space, rx is the effective value of the ‘rx’ attribute converted to local coordinate system and y is the value of the ‘y’ attribute converted to local coordinate system
• perform an absolute horizontal lineto operation to location (x+width-rx,y), where width is the ‘rect’ element's ‘width’ attribute converted to user space
• perform an absolute elliptical arc operation to coordinate (x+width,y+ry), where the effective values for the ‘rx’ and ‘ry’ attributes on the ‘rect’ element converted to local coordinate system are used as the rx and ry attributes on the elliptical arc command, respectively, the x-axis-rotation is set to zero, the large-arc-flag is set to zero, and the sweep-flag is set to one
• perform a absolute vertical lineto to location (x+width,y+height-ry), where height is the ‘rect’ element's ‘height’ attribute converted to user space
• perform an absolute elliptical arc operation to coordinate (x+width-rx,y+height)
• perform an absolute horizontal lineto to location (x+rx,y+height)
• perform an absolute elliptical arc operation to coordinate (x,y+height-ry)
• perform an absolute absolute vertical lineto to location (x,y+ry)
• perform an absolute elliptical arc operation to coordinate (x+rx,y)

Path decomposition resolved during teleconference on June 3rd, 2013.

Example rect01 shows a rectangle with sharp corners. The ‘rect’ element is filled with yellow and stroked with navy.

<?xml version="1.0" standalone="no"?>
<svg width="12cm" height="4cm" viewBox="0 0 1200 400"
     xmlns="http://www.w3.org/2000/svg" version="1.1">
  <desc>Example rect01 - rectangle with sharp corners</desc>

  <!-- Show outline of canvas using 'rect' element -->
  <rect x="1" y="1" width="1198" height="398"
        fill="none" stroke="blue" stroke-width="2"/>

  <rect x="400" y="100" width="400" height="200"
        fill="yellow" stroke="navy" stroke-width="10"  />
</svg>

Example rect01

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

Example rect02 shows two rounded rectangles. The ‘rx’ specifies how to round the corners of the rectangles. Note that since no value has been specified for the ‘ry’ attribute, it will be assigned the same value as the ‘rx’ attribute.

<?xml version="1.0" standalone="no"?>
<svg width="12cm" height="4cm" viewBox="0 0 1200 400"
     xmlns="http://www.w3.org/2000/svg" version="1.1">
  <desc>Example rect02 - rounded rectangles</desc>

  <!-- Show outline of canvas using 'rect' element -->
  <rect x="1" y="1" width="1198" height="398"
        fill="none" stroke="blue" stroke-width="2"/>

  <rect x="100" y="100" width="400" height="200" rx="50"
        fill="green" />

  <g transform="translate(700 210) rotate(-30)">
    <rect x="0" y="0" width="400" height="200" rx="50"
          fill="none" stroke="purple" stroke-width="30" />
  </g>
</svg>

Example rect02

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

9.3. The ‘circle’ element

The ‘circle’ element defines a circle based on a center point and a radius.

The ‘cx’ and ‘cy’ attributes define the coordinates of the center of the circle.

The ‘r’ attribute defines the radius of the circle. A negative value is an error (see Error processing). A value of zero disables rendering of the element.

Mathematically, a ‘circle’ element is mapped to an equivalent ‘path’ element that consists of four elliptical arc segments, each covering a quarter of the circle. The path begins at the "3 o'clock" point on the radius and proceeds in a clock-wise direction (before any transformations).

Path decomposition resolved during teleconference on June 3rd, 2013.

Example circle01 consists of a ‘circle’ element that is filled with red and stroked with blue.

<?xml version="1.0" standalone="no"?>
<svg width="12cm" height="4cm" viewBox="0 0 1200 400"
     xmlns="http://www.w3.org/2000/svg" version="1.1">
  <desc>Example circle01 - circle filled with red and stroked with blue</desc>

  <!-- Show outline of canvas using 'rect' element -->
  <rect x="1" y="1" width="1198" height="398"
        fill="none" stroke="blue" stroke-width="2"/>

  <circle cx="600" cy="200" r="100"
        fill="red" stroke="blue" stroke-width="10"  />
</svg>

Example circle01

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

9.4. The ‘ellipse’ element

The ‘ellipse’ element defines an ellipse which is axis-aligned with the current local coordinate system based on a center point and two radii.

The ‘cx’ and ‘cy’ coordinates define the center of the ellipse.

The ‘cx’ and ‘cy’ attributes define the x- and y-axis radii of the ellipse. A negative value for either attribute is an error (see Error processing). A value of zero disables rendering of the element.

Mathematically, an ‘ellipse’ element is mapped to an equivalent ‘path’ element that consists of four elliptical arc segments, each covering a quarter of the ellipse. The path begins at the "3 o'clock" point on the radius and proceeds in a clock-wise direction (before any transformation).

Path decomposition resolved during teleconference on June 3rd, 2013.

Example ellipse01 below specifies the coordinates of the two ellipses in the user coordinate system established by the ‘viewBox’ attribute on the ‘svg’ element and the ‘transform’ property on the ‘g’ and ‘ellipse’ elements. Both ellipses use the default values of zero for the ‘cx’ and ‘cy’ attributes (the center of the ellipse). The second ellipse is rotated.

<?xml version="1.0" standalone="no"?>
<svg width="12cm" height="4cm" viewBox="0 0 1200 400"
     xmlns="http://www.w3.org/2000/svg" version="1.1">
  <desc>Example ellipse01 - examples of ellipses</desc>

  <!-- Show outline of canvas using 'rect' element -->
  <rect x="1" y="1" width="1198" height="398"
        fill="none" stroke="blue" stroke-width="2" />

  <g transform="translate(300 200)">
    <ellipse rx="250" ry="100"
          fill="red"  />
  </g>

  <ellipse transform="translate(900 200) rotate(-30)" 
        rx="250" ry="100"
        fill="none" stroke="blue" stroke-width="20"  />

</svg>

Example ellipse01

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

9.5. The ‘line’ element

The ‘line’ element defines a line segment that starts at one point and ends at another.

Attribute definitions:

Name	Value	Initial value	Animatable
x1, y1	<length> | <percentage> | <number>	0	yes

The x- and y-axis coordinates of the start of the line.

Name	Value	Initial value	Animatable
x2, y2	<length> | <percentage> | <number>	0	yes

The x- and y-axis coordinates of the end of the line.

Mathematically, a ‘line’ element can be mapped to an equivalent ‘path’ element as follows: (Note: all coordinate and length values are first converted into local coordinate system coordinates according to Units.)

• perform an absolute moveto operation to absolute location (x1,y1), where x1 and y1 are the values of the ‘line’ element's ‘x1’ and ‘y1’ attributes converted to local coordinate system, respectively
• perform an absolute lineto operation to absolute location (x2,y2), where x2 and y2 are the values of the ‘line’ element's ‘x2’ and ‘y2’ attributes converted to local coordinate system, respectively

Because ‘line’ elements are single lines and thus are geometrically one-dimensional, they have no interior; thus, ‘line’ elements are never filled (see the ‘fill’ property).

Example line01 below specifies the coordinates of the five lines in the user coordinate system established by the ‘viewBox’ attribute on the ‘svg’ element. The lines have different thicknesses.

<?xml version="1.0" standalone="no"?>
<svg width="12cm" height="4cm" viewBox="0 0 1200 400"
     xmlns="http://www.w3.org/2000/svg" version="1.1">
  <desc>Example line01 - lines expressed in user coordinates</desc>

  <!-- Show outline of canvas using 'rect' element -->
  <rect x="1" y="1" width="1198" height="398"
        fill="none" stroke="blue" stroke-width="2" />

  <g stroke="green" >
    <line x1="100" y1="300" x2="300" y2="100"
            stroke-width="5"  />
    <line x1="300" y1="300" x2="500" y2="100"
            stroke-width="10"  />
    <line x1="500" y1="300" x2="700" y2="100"
            stroke-width="15"  />
    <line x1="700" y1="300" x2="900" y2="100"
            stroke-width="20"  />
    <line x1="900" y1="300" x2="1100" y2="100"
            stroke-width="25"  />
  </g>
</svg>

Example line01

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

9.6. The ‘polyline’ element

The ‘polyline’ element defines a set of connected straight line segments. Typically, ‘polyline’ elements define open shapes.

Attribute definitions:

Name	Value	Initial value	Animatable
points	<points>	(none)	yes

where:

<points> =

[<number> ,? <number> ,?]+

The points that make up the polyline. All coordinate values are in the user coordinate system.

If an odd number of coordinates is provided, then the element is in error, with the same user agent behavior as occurs with an incorrectly specified ‘path’ element. In such error cases the user agent will drop the last, odd coordinate and otherwise render the shape.

The initial value, (none), indicates that the polyline element is valid but does not render.

Mathematically, a ‘polyline’ element can be mapped to an equivalent ‘path’ element as follows:

• perform an absolute moveto operation to the first coordinate pair in the list of points
• for each subsequent coordinate pair, perform an absolute lineto operation to that coordinate pair.

Example polyline01 below specifies a polyline in the user coordinate system established by the ‘viewBox’ attribute on the ‘svg’ element.

<?xml version="1.0" standalone="no"?>
<svg width="12cm" height="4cm" viewBox="0 0 1200 400"
     xmlns="http://www.w3.org/2000/svg" version="1.1">
  <desc>Example polyline01 - increasingly larger bars</desc>

  <!-- Show outline of canvas using 'rect' element -->
  <rect x="1" y="1" width="1198" height="398"
        fill="none" stroke="blue" stroke-width="2" />

  <polyline fill="none" stroke="blue" stroke-width="10" 
            points="50,375
                    150,375 150,325 250,325 250,375
                    350,375 350,250 450,250 450,375
                    550,375 550,175 650,175 650,375
                    750,375 750,100 850,100 850,375
                    950,375 950,25 1050,25 1050,375
                    1150,375" />
</svg>

Example polyline01

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

9.7. The ‘polygon’ element

The ‘polygon’ element defines a closed shape consisting of a set of connected straight line segments.

Attribute definitions:

Name	Value	Initial value	Animatable
points	<points>	(none)	yes

The points that make up the polygon. All coordinate values are in the user coordinate system.

If an odd number of coordinates is provided, then the element is in error, with the same user agent behavior as occurs with an incorrectly specified ‘path’ element.

The initial value, (none), indicates that the polygon element is valid, but does not render.

Mathematically, a ‘polygon’ element can be mapped to an equivalent ‘path’ element as follows:

• perform an absolute moveto operation to the first coordinate pair in the list of points
• for each subsequent coordinate pair, perform an absolute lineto operation to that coordinate pair
• perform a closepath command

Example polygon01 below specifies two polygons (a star and a hexagon) in the user coordinate system established by the ‘viewBox’ attribute on the ‘svg’ element.

<?xml version="1.0" standalone="no"?>
<svg width="12cm" height="4cm" viewBox="0 0 1200 400"
     xmlns="http://www.w3.org/2000/svg" version="1.1">
  <desc>Example polygon01 - star and hexagon</desc>

  <!-- Show outline of canvas using 'rect' element -->
  <rect x="1" y="1" width="1198" height="398"
        fill="none" stroke="blue" stroke-width="2" />

  <polygon fill="red" stroke="blue" stroke-width="10" 
            points="350,75  379,161 469,161 397,215
                    423,301 350,250 277,301 303,215
                    231,161 321,161" />
  <polygon fill="lime" stroke="blue" stroke-width="10" 
            points="850,75  958,137.5 958,262.5
                    850,325 742,262.6 742,137.5" />
</svg>

Example polygon01

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

9.8. DOM interfaces

9.8.1. Interface SVGRectElement

interface SVGRectElement : SVGGeometryElement {
  readonly attribute SVGAnimatedLength x;
  readonly attribute SVGAnimatedLength y;
  readonly attribute SVGAnimatedLength width;
  readonly attribute SVGAnimatedLength height;
  readonly attribute SVGAnimatedLength rx;
  readonly attribute SVGAnimatedLength ry;
};

Attributes:

x (readonly SVGAnimatedLength)

Corresponds to attribute ‘x’ on the given ‘rect’ element.

y (readonly SVGAnimatedLength)

Corresponds to attribute ‘y’ on the given ‘rect’ element.

width (readonly SVGAnimatedLength)

Corresponds to attribute ‘width’ on the given ‘rect’ element.

height (readonly SVGAnimatedLength)

Corresponds to attribute ‘height’ on the given ‘rect’ element.

rx (readonly SVGAnimatedLength)

Corresponds to attribute ‘rx’ on the given ‘rect’ element.

ry (readonly SVGAnimatedLength)

Corresponds to attribute ‘ry’ on the given ‘rect’ element.

9.8.2. Interface SVGCircleElement

interface SVGCircleElement : SVGGeometryElement {
  readonly attribute SVGAnimatedLength cx;
  readonly attribute SVGAnimatedLength cy;
  readonly attribute SVGAnimatedLength r;
};

Attributes:

cx (readonly SVGAnimatedLength)

Corresponds to attribute ‘cx’ on the given ‘circle’ element.

cy (readonly SVGAnimatedLength)

Corresponds to attribute ‘cy’ on the given ‘circle’ element.

r (readonly SVGAnimatedLength)

Corresponds to attribute ‘r’ on the given ‘circle’ element.

9.8.3. Interface SVGEllipseElement

interface SVGEllipseElement : SVGGeometryElement {
  readonly attribute SVGAnimatedLength cx;
  readonly attribute SVGAnimatedLength cy;
  readonly attribute SVGAnimatedLength rx;
  readonly attribute SVGAnimatedLength ry;
};

Attributes:

cx (readonly SVGAnimatedLength)

Corresponds to attribute ‘cx’ on the given ‘ellipse’ element.

cy (readonly SVGAnimatedLength)

Corresponds to attribute ‘cy’ on the given ‘ellipse’ element.

rx (readonly SVGAnimatedLength)

Corresponds to attribute ‘rx’ on the given ‘ellipse’ element.

ry (readonly SVGAnimatedLength)

Corresponds to attribute ‘ry’ on the given ‘ellipse’ element.

9.8.4. Interface SVGLineElement

interface SVGLineElement : SVGGeometryElement {
  readonly attribute SVGAnimatedLength x1;
  readonly attribute SVGAnimatedLength y1;
  readonly attribute SVGAnimatedLength x2;
  readonly attribute SVGAnimatedLength y2;
};

Attributes:

x1 (readonly SVGAnimatedLength)

Corresponds to attribute ‘x1’ on the given ‘line’ element.

y1 (readonly SVGAnimatedLength)

Corresponds to attribute ‘y1’ on the given ‘line’ element.

x2 (readonly SVGAnimatedLength)

Corresponds to attribute ‘x2’ on the given ‘line’ element.

y2 (readonly SVGAnimatedLength)

Corresponds to attribute ‘y2’ on the given ‘line’ element.

9.8.5. Interface SVGAnimatedPoints

The SVGAnimatedPoints interface supports elements which have a ‘points’ attribute which holds a list of coordinate values and which support the ability to animate that attribute.

Additionally, the ‘points’ attribute on the original element accessed via the XML DOM (e.g., using the getAttribute() method call) will reflect any changes made to points.

[NoInterfaceObject]
interface SVGAnimatedPoints {
  readonly attribute SVGPointList points;
  readonly attribute SVGPointList animatedPoints;
};

Attributes:

points (readonly SVGPointList)

Provides access to the base (i.e., static) contents of the ‘points’ attribute.

animatedPoints (readonly SVGPointList)

Provides access to the current animated contents of the ‘points’ attribute. If the given attribute or property is being animated, contains the current animated value of the attribute or property. If the given attribute or property is not currently being animated, contains the same value as points.

9.8.6. Interface SVGPointList

This interface defines a list of DOMPoint objects.

SVGPointList has the same attributes and methods as other SVGxxxList interfaces. Implementers may consider using a single base class to implement the various SVGxxxList interfaces.

The supported property indices of an SVGPointList object is all non-negative integers less than the length of the list.

interface SVGPointList {

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

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

Attributes:

length (readonly unsigned long)

The number of items in the list.

numberOfItems (readonly unsigned long)

The number of items in the list.

Operations:

void clear()

Clears all existing current items from the list, with the result being an empty list.

Exceptions

DOMException, code NO_MODIFICATION_ALLOWED_ERR

Raised when the list cannot be modified.

DOMPoint initialize(DOMPoint newItem)

Clears all existing current items from the list and re-initializes the list to hold a single item specified by the parameter. If newItem is in a list, then a new SVGPoint object is created with the same values as newItem and this item is inserted into the list. Otherwise, newItem itself is inserted into the list.

Parameters

1. DOMPoint newItem
   The item which should become the only member of the list.

Returns

The item being inserted into the list.

Exceptions

DOMException, code NO_MODIFICATION_ALLOWED_ERR

Raised when the list cannot be modified.

DOMPoint getItem(unsigned long index)

Returns the specified item from the list. The returned item is the item itself and not a copy. Any changes made to the item are immediately reflected in the list.

Parameters

1. unsigned long index
   The index of the item from the list which is to be returned. The first item is number 0.

Returns

The selected item.

Exceptions

DOMException, code INDEX_SIZE_ERR

Raised if the index number is greater than or equal to numberOfItems.

DOMPoint insertItemBefore(DOMPoint newItem, unsigned long index)

Inserts a new item into the list at the specified position. The first item is number 0. If newItem is already in a list, then a new SVGPoint object is created with the same values as newItem and this item is inserted into the list. Otherwise, newItem itself is inserted into the list.

Parameters

1. DOMPoint newItem
   The item which is to be inserted into the list.
2. unsigned long index
   The index of the item before which the new item is to be inserted. The first item is number 0. If the index is equal to 0, then the new item is inserted at the front of the list. If the index is greater than or equal to numberOfItems, then the new item is appended to the end of the list.

Returns

The inserted item.

Exceptions

DOMException, code NO_MODIFICATION_ALLOWED_ERR

Raised when the list cannot be modified.

DOMPoint replaceItem(DOMPoint newItem, unsigned long index)

Replaces an existing item in the list with a new item. If newItem is already in a list, it is removed from its previous list before it is inserted into this list. The inserted item is the item itself and not a copy. If the item is already in this list, note that the index of the item to replace is before the removal of the item.

Parameters

1. DOMPoint newItem
   The item which is to be inserted into the list.
2. unsigned long index
   The index of the item which is to be replaced. The first item is number 0.

Returns

The inserted item.

Exceptions

DOMException, code NO_MODIFICATION_ALLOWED_ERR

Raised when the list cannot be modified.

DOMException, code INDEX_SIZE_ERR

Raised if the index number is greater than or equal to numberOfItems.

DOMPoint removeItem(unsigned long index)

Removes an existing item from the list.

Parameters

1. unsigned long index
   The index of the item which is to be removed. The first item is number 0.

Returns

The removed item.

Exceptions

DOMException, code NO_MODIFICATION_ALLOWED_ERR

Raised when the list cannot be modified.

DOMException, code INDEX_SIZE_ERR

Raised if the index number is greater than or equal to numberOfItems.

DOMPoint appendItem(DOMPoint newItem)

Inserts a new item at the end of the list. If newItem is already in a list, it is removed from its previous list before it is inserted into this list. The inserted item is the item itself and not a copy.

Parameters

1. DOMPoint newItem
   The item which is to be inserted. The first item is number 0.

Returns

The inserted item.

Exceptions

DOMException, code NO_MODIFICATION_ALLOWED_ERR

Raised when the list cannot be modified.

setter void (unsigned long index, DOMPoint newItem)

Replaces the item at index index with newItem.

9.8.7. Interface SVGPolylineElement

The SVGPolylineElement interface corresponds to the ‘polyline’ element.

interface SVGPolylineElement : SVGGeometryElement {
};

SVGPolylineElement implements SVGAnimatedPoints;

9.8.8. Interface SVGPolygonElement

The SVGPolygonElement interface corresponds to the ‘polygon’ element.

interface SVGPolygonElement : SVGGeometryElement {
};

SVGPolygonElement implements SVGAnimatedPoints;