CSS Masking Level 1

Abstract

CSS Masking provides two means for partially or fully hiding portions of visual elements: masking and clipping.

Masking describes how to use another graphical element or image as a luminance or alpha mask. Typically, rendering an element via CSS or SVG can conceptually described as if the element, including its children, are drawn into a buffer and then that buffer is composited into the element's parent. Luminance and alpha masks influence the transparency of this buffer before the compositing stage.

Clipping describes the visible region of visual elements. The region can be described by using certain SVG graphics elements or basic shapes. Anything outside of this region is not rendered.

Status of this document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

The (archived) public mailing list public-fx@w3.org (see instructions) is preferred for discussion of this specification. When sending e-mail, please put the text “css-masking” in the subject, preferably like this: “[css-masking] …summary of comment…”

This document was produced by the CSS Working Group (part of the Style Activity) and the SVG Working Group (part of the Graphics Activity).

This document was produced by groups operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures (CSS) and a public list of any patent disclosures (SVG) made in connection with the deliverables of each group; these pages also include instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

Table of contents

• 1. Introduction
• 2. Module interactions
• 3. Values
• 4. Terminology
• 5. Layered Masks
  • 5.1. The ‘mask-image’ property
  • 5.2. The ‘mask-source-type’ property
  • 5.3. The ‘mask-repeat’ property
  • 5.4. The ‘mask-position’ property
  • 5.5. The ‘mask-clip’ property
  • 5.6. The ‘mask-origin’ property
  • 5.7. The ‘mask-size’ property
  • 5.8. The ‘mask’ property
  • 5.9. The ‘mask-type’ property
  • 5.10. The <mask> element
  • 5.11. The Mask Image Rendering Model
    • 5.11.1. Layering Multiple Mask Images
    • 5.11.2. Mask Source values
• 6. Box-Image Masks
  • 6.1. The ‘mask-box-image-source’ property
  • 6.2. The ‘mask-box-image-slice’ property
  • 6.3. The ‘mask-box-image-width’ property
  • 6.4. The ‘mask-box-image-outset’ property
  • 6.5. The ‘mask-box-image-repeat’ property
  • 6.6. The ‘mask-box-image’ property
  • 6.7. Masking with the mask-box-image
• 7. Clipping paths
  • 7.1. The ‘clip-path’ property
  • 7.2. The ‘clip-rule’ property
  • 7.3. The ‘clip’ property
  • 7.4. The <clipPath> element
  • 7.5. Clipping paths, geometry, and pointer events
• 8. Security
  • 8.1. Origin Restrictions
• 9. DOM interfaces
  • 9.1. Interface SVGClipPathElement
  • 9.2. Interface SVGMaskElement
• 10. Conformance
  • 10.1. Conformance to CSS Background and Borders
• Acknowledgments
• References
  • Normative references
  • Other references
• Changes since last publication
• Property index
• Index

1. Introduction

With CSS Masking it is possible to specify another graphics element, shape or image file to be used as clipping region or an luminance or alpha mask for compositing the current object into the background.

The effect of applying a mask is as if the mask images or mask sources are rendered into an off-screen image which has been initialized to transparent black. The graphical object to which the mask is applied will be painted onto the background through the mask, thus completely or partially masking out parts of the graphical object.

Masks are applied using the ‘mask-image’ or ‘mask-box-image-source’ properties.

The ‘mask-image’ property may reference a <mask> element as the mask source. Alternatively, for many simple uses, the ‘mask-image’ property may refer directly to images to be used as mask forgoing the need for an explicit <mask> element.
The ‘mask’ property serves as a shorthand property for ‘mask-image’ and other characterizing properties.

The ‘mask-box-image-source’ property splits a mask image into 9 pieces. The pieces may be sliced, scaled and stretched in various ways to fit the size of the mask image area.
The ‘mask-box-image’ property serves as a shorthand property for ‘mask-box-image-source’ and other characterizing properties.

Clipping path defines an outline (in the absence of anti-aliasing) of a visual element where everything on the "inside" of the outline is allowed to show through but everything on the outside is masked out. A clipping path is specified by vector paths or shapes.

The ‘clip-path’ property can reference SVG graphics element or use specified basic shapes as clipping path.

A luminance mask (middle) is applied on a shape filled with a gradient (left). This results in a masked shape (right).

2. Module interactions

This specification defines a set of CSS properties that affect the visual rendering of elements to which those properties are applied; these effects are applied after elements have been sized and positioned according to the Visual formatting model from [CSS21]. Some values of these properties result in the creation of a stacking context. Furthermore, this specification replaces the section Clipping: the ‘clip’ property from [CSS21].

The compositing model follows the SVG compositing model [SVG11]: first any filter effect [FILTER-EFFECTS] is applied, then any clipping, masking and opacity [CSS3COLOR]. These effects all apply after any other CSS effects such as ‘border’ [CSS3BG].

3. Values

This specification follows the CSS property definition conventions from [CSS21]. Basic shapes are defined in CSS Exclusions and Shapes Module Level 3 [CSS3-EXCLUSIONS]. Value types not defined in these specifications are defined in CSS Values and Units Module Level 3 [CSS3VAL].

In addition to the property-specific values listed in their definitions, all properties defined in this specification also accept the inherit keyword as their property value. For readability it has not been repeated explicitly.

4. Terminology

When used in this specification, terms have the meanings assigned in this section.

bounding client rect

The union of all border boxes for the element that has an associated CSS layout box and is not in the http://www.w3.org/2000/svg namespace and its descendant elements. Or the object bounding box [SVG11], if the element does not have an associated CSS layout box and is in the http://www.w3.org/2000/svg namespace. (See getBoundingClientRect [CSSOM-VIEW].)

local coordinate system

user coordinate system

In general, a coordinate system defines locations and distances on the current canvas. The current local coordinate system (also user coordinate system) is the coordinate system that is currently active and which is used to define how coordinates and lengths are located and computed, respectively, on the current canvas [CSS3-TRANSFORMS].

For elements that have an associated CSS layout box, the current user coordinate system has its origin at the top-left corner of the bounding client rect and one unit equals one CSS pixel. The viewport for resolving percentage values is defined by the width and height of the bounding client rect.

If the element does not have an associated CSS layout box and is in the http://www.w3.org/2000/svg namespace, the current local coordinate system has its origin at the top-left corner of the element's nearest viewport.

object bounding box units

The bounding client rect defines the coordinate system in which to resolve values, as defined in object bounding box units [SVG11].

mask

An off-screen image which has been initialized to transparent black. The graphical object to which the mask is applied will be painted onto the background through the mask, thus completely or partially masking out parts of the graphical object.

mask source

A mask defined by a <mask> element.

clipping path

A combination of SVG graphics element or basic shapes which serve as the outline of a visual element where everything on the "inside" of the outline is allowed to show through but everything on the outside is masked out. (See ‘clip-path’.)

Definitions of CSS properties and values in this specification are analogous to definitions in CSS Backgrounds and Borders [CSS3BG]. To avoid redundancy, this specification relies on descriptions and definitions of CSS Backgrounds and Borders. The following terms in CSS Backgrounds and Borders have the following meaning in this specification:

5. Layered Masks

5.1. The ‘mask-image’ property

This property sets the mask image(s) or the mask source of an element. Images are drawn with the first specified one on top (closest to the user) and each subsequent image behind the previous one. Where:

<mask-reference> = <mask-image># | <mask-source>

<mask-image> = <image> | none 

<mask-source> = <url> | child | <child-selector>

<child-selector> = select(<compound-selector>#)

A <mask-source> can just be specified as a single value and can not contribute to a list of <mask-reference> values. Otherwise the specified value of the property is invalid. (See examples below.)

<url>

A URL reference to a <mask> element (for example ‘url(commonmasks.svg#mask)’).

‘child’

A keyword to indicate that the last direct child <mask> element of the element the ‘mask-image’ property is applied should be used as the mask source. It is equivalent to ‘select(mask:last-of-type)’.

<child-selector>

A functional notation [CSS3VAL] accepting a comma-separated list of compound selectors that represents first matching child <mask> element in tree order (as defined in [DOM]).

<compound-selector>

A compound selector as defined in [SELECTORS4].

<image>

A 2D image. (See [CSS3-IMAGES].)

‘none’

A value of ‘none’ counts as an image layer but does not mask the element.

If the URI in the ‘url()’ function has a fragment identifier, the <mask-reference> is treated as <mask-source>, otherwise as <mask-image>.

A computed value of other than ‘none’ results in the creation of a stacking context [CSS21] the same way that CSS ‘opacity’ [CSS3COLOR] does.

If the URI reference of a mask source is not valid (e.g it points to an object that doesn't exist or the object is not a <mask> element), no masking is applied.

A mask image that is empty (zero width or zero height), that fails to download, is non-existent, or that cannot be displayed (e.g. because it is not in a supported image format) is ignored. No masking is applied.

See the section “Layering multiple mask images” for how ‘mask-image’ interacts with other comma-separated mask properties to form each mask image layer.

body { mask-image: linear-gradient(black 0%, transparent 100%) }
p { mask-image: none }
div { mask-image: url(tl.png), url(tr.png) }

img { mask-image: url(#mask) }
span { mask-image: url(resources.svg#mask2) }

p { mask-image: url(#mask), url(tl.png) }
img { mask-image: url(#mask), url(resources.svg#mask2) }

5.2. The ‘mask-source-type’ property

The ‘mask-source-type’ property indicates whether the <mask-reference> is treated as luminance mask or alpha mask. (See Mask Source values.)

<source-type> = alpha | luminance | auto

Values have the following meanings:

‘alpha’

A value of ‘alpha’ indicates that the alpha values of the mask image should be used as the mask values. See Calculating mask values.

‘luminance’

A value of ‘luminance’ indicates that the luminance values of the mask image should be used as the mask values. See Calculating mask values.

‘auto’

If the <mask-reference> of the ‘mask-image’ property is of type <mask-source> the luminance values of the mask image should be used as the mask values.

If the <mask-reference> of the ‘mask-image’ property is of type <mask-image> the alpha values of the mask image should be used as the mask values.

<mask id="SVGMask" mask-type="alpha" maskContentUnits="objectBoundingBox">
  <radialGradient id="radialFill">
    <stop stop-color="white" offset="0"/>
    <stop stop-color="black" offset="1"/>
  </radialGradient>
  <circle fill="url(#radialFill)" cx="0.5" cy="0.5" r="0.5"/>
</mask>

<style>
  rect {
    mask-image: url(#SVGMask);
    mask-source-type: luminance;
  }
</style>

<rect width="200" height="200" fill="green"/>

5.3. The ‘mask-repeat’ property

Specifies how mask images are tiled after they have been sized and positioned.

See Tiling Images: the ‘background-repeat’ property [CSS3BG] for the definitions of the property values.

body {
  background-color: blue;
  mask-image: url(dot-mask.png) luminance;
  mask-repeat: space;
}

See the section “Layering multiple mask images” for how ‘mask-repeat’ interacts with other comma-separated mask properties to form each mask image layer.

5.4. The ‘mask-position’ property

See Positioning Images: the ‘background-position’ property [CSS3BG] for the definitions of the property values.

body {
  mask-image: url("logo.png");
  mask-position: 100% 100%;
  mask-repeat: no-repeat;
}

mask-position: right 3em bottom 10px

See the section “Layering multiple mask images” for how ‘mask-position’ interacts with other comma-separated mask properties to form each mask image layer.

5.5. The ‘mask-clip’ property

Determines the mask painting area, which determines the area that is affected by the mask. The painted content of an element may be restricted to this area.

Values have the following meanings:

‘border-box’

The painted content is rectricted to (clipped to) the border box respectively painting box for objects without associated layout box.

‘padding-box’

The painted content is rectricted to (clipped to) the padding box.

‘content-box’

The painted content is rectricted to (clipped to) the content box respectively object bounding box [SVG11] for objects without associated layout box.

‘no-clip’

The painted content is not restricted (not clipped). The mask painting area is set to the bounding client rect.

Note that there is no corresponding box for ‘padding-box’ on objects without associated layout box and ‘content-box’ is choosen instead.

See the section “Layering multiple mask images” for how ‘mask-clip’ interacts with other comma-separated mask properties to form each mask image layer.

5.6. The ‘mask-origin’ property

For elements rendered as a single box, specifies the mask positioning area. For elements rendered as multiple boxes (e.g., inline boxes on several lines, boxes on several pages) specifies which boxes ‘box-decoration-break’ operates on to determine the mask positioning area(s).

‘padding-box’

The position is relative to the padding box. (For single boxes ‘0 0’ is the upper left corner of the padding edge, ‘100% 100%’ is the lower right corner.)

‘border-box’

The position is relative to the border box respectively painting box for objects without associated layout box.

‘content-box’

The position is relative to the content box respectively object bounding box [SVG11] for objects without associated layout box.

Note that there is no corresponding box for ‘padding-box’ on objects without associated layout box and ‘content-box’ is choosen instead.

Note that if ‘mask-clip’ is ‘padding-box’, ‘mask-origin’ is ‘border-box’, ‘mask-position’ is ‘top left’ (the initial value), and the element has a non-zero border, then the top and left of the mask image will be clipped.

See the section “Layering multiple mask images” for how ‘mask-origin’ interacts with other comma-separated mask properties to form each mask image layer.

5.7. The ‘mask-size’ property

Specifies the size of the mask images.

See Sizing Images: the ‘background-size’ property [CSS3BG] for the definitions of the property values.

See the section “Layering multiple mask images” for how ‘mask-size’ interacts with other comma-separated mask properties to form each mask image layer.

5.8. The ‘mask’ property

Where

<mask-layer> = <mask-reference> <source-type>? || <position> [ / <bg-size> ]? || <repeat-style> || <box> || [ <box> | no-clip ]

If one <box> value is present then it sets both ‘mask-origin’ and ‘mask-clip’ to that value. If two <box> values are present, then the first sets ‘mask-origin’ and the second ‘mask-clip’.

Note that the properties ‘mask-image’, ‘mask-repeat’, ‘mask-position’, ‘mask-clip’, ‘mask-origin’ and ‘mask-size’ have no affect on <mask> elements.

The ‘mask’ property does not influence the ‘mask-box-image’ properties. ‘mask: none’ does not remove any masking defined by ‘mask-box-image’.

5.9. The ‘mask-type’ property

The ‘mask-type’ property defines whether the content of the <mask> element is treated as as luminance mask or alpha mask, as described in Calculating mask values.

Values have the following meanings:

‘luminance’

Indicates that the luminance values of the mask should be used.

‘alpha’

Indicates that the alpha values of the mask should be used.

The ‘mask-type’ property allows the author of the <mask> element to specify the preferred masking operation. However, the author can override this preference by setting the ‘mask-source-type’ value to something different than ‘auto’ on the masked content.

<svg>
  <mask style="mask-type: luminance;" id="mask">
    ...
  </mask>
</svg>

<p style="mask-image: url(#mask); mask-source-type: auto;">
  This is the masked content.
</p>

<svg>
  <mask style="mask-type: luminance;" id="mask2">
    ...
  </mask>
</svg>

<p style="mask-image: url(#mask2); mask-source-type: alpha;">
  This is the masked content.
</p>

The ‘mask-type’ property is a presentation attribute for SVG elements.

5.10. The <mask> element

If at least one of the attributes ‘x’, ‘y’, ‘width’ or ‘height’ are specified, the given object and the rectangle defined by ‘x’, ‘y’, ‘width’ and ‘height’ establish a current clipping path. The rendered content of the mask must be clipped by this current clipping path.

CSS properties inherit into the <mask> element from its ancestors; properties do not inherit from the element referencing the <mask> element.

<mask> elements are never rendered directly; their only usage is as something that can be referenced using the ‘mask’ property. The ‘opacity’, ‘filter’ and ‘display’ properties do not apply to the <mask> element; thus, <mask> elements are not directly rendered even if the ‘display’ property is set to a value other than ‘none’, and <mask> elements are available for referencing even when the ‘display’ property on the <mask> element or any of its ancestors is set to none.

5.11. The Mask Image Rendering Model

The application of the ‘mask-image’ property to an element formatted with the CSS box model establishes a stacking context in the same way that CSS ‘opacity’ [CSS3COLOR] does, and all the element's descendants are rendered together as a group with the masking applied to the group as a whole.

The ‘mask-image’ property has no effect on the geometry or hit-testing of any element's CSS boxes.

5.11.1. Layering Multiple Mask Images

The mask of a box can have multiple layers. The number of layers is determined by the number of comma-separated values in the ‘mask-image’ property. Note that a value of ‘none’ still creates a layer. See Layering Multiple Background Images [CSS3BG].

All mask images are transformed to alpha masks (if necessary) and combined by compositing on the mask.

5.11.2. Mask Source values

A mask may be interpreted using one of two different methods with regards to calculating the mask values that will be multiplied with the target alpha values.

The first and simplest method of calculating the mask values is to use the alpha channel of the mask source. In this case the mask value at a given point is simply the value of the alpha channel at that point. The color channels do not contribute to the mask value.

The second method of calculating the mask values is to use the luminance of the mask source. In this case the mask value at a given point is computed from the color channel values and alpha channel value using the following procedure.

1. Compute a luminance value from the color channel values.
   • If the computed value of ‘color-interpolation’ on the <mask> element is ‘linearRGB’, convert the original image color values (potentially in the sRGB color space) to the linear RGB color space.
   • Then, using non-premultiplied RGB color values, apply the luminance-to-alpha coefficients (as defined in the <feColorMatrix> filter primitive [SVG11]) to convert the RGB color values to luminance values.
2. Multiply the computed luminance value by the corresponding alpha value to produce the mask value.

Regardless of the method used, the procedure for calculating mask values assumes the content of the mask is a four-channel RGBA graphics object. For other types of graphics objects, special handling is required as follows.

For a three-channel RGB graphics object that is used in a mask (e.g., when referencing a three-channel image file), the effect is as if the object were converted into a four-channel RGBA image with the alpha channel uniformly set to 1.

For a single-channel image that is used in a mask (e.g., when referencing a single-channel grayscale image file), the effect is as if the object were converted into a four-channel RGBA image, where the single channel from the referenced object is used to compute the three color channels and the alpha channel is uniformly set to 1.

Note that when referencing a grayscale image file, the transfer curve relating the encoded grayscale values to linear light values must be taken into account when computing the color channels.

Note that SVG graphics elements (e.g., <circle> or <text>) are all treated as four-channel RGBA images for the purposes of masking operations.

The effect of a mask is identical to what would have happened if there were no mask but instead the alpha channel of the given object were multiplied with the mask's resulting mask values.

6. Box-Image Masks

With ‘mask-box-image’ a mask can be split into nine pieces: four corners, four edges and the middle piece as demonstrated in the figure below.

Pieces of a mask box image.

These pieces may be sliced, scaled and stretched in various ways to fit the size of the mask image area. The usage of ‘mask-box-image’ corresponds to the ‘border-image’ property of CSS Background and Borders [CSS3BG].

The mask image in the following example is split into four corners with dimensions of 75 pixels, four edges and the middle piece that is stretched and scaled.

Example for ‘mask-box-image’. The object on the left is the object to mask. The second image is the alpha mask and the last image the masked object.

div {
  background: linear-gradient(bottom, #F27BAA 0%, #FCC8AD 100%);
  mask-box-image-slice: 25 fill;
  mask-box-image-repeat: stretch;
  mask-box-image-source: url(mask.png);
}

6.1. The ‘mask-box-image-source’ property

Specifies an image to be used as mask. If the value is ‘none’ or if the image cannot be displayed (or the property doesn't apply), the content won't get masked.

A computed value of other than ‘none’ results in the creation of a stacking context [CSS21] the same way that CSS ‘opacity’ [CSS3COLOR] does.

6.2. The ‘mask-box-image-slice’ property

This property specifies inward offsets from the top, right, bottom, and left edges of the mask image, dividing it into nine regions: four corners, four edges and a middle. The middle image part is discarded (treated as fully transparent black) unless the ‘fill’ keyword is present.

See Image Slicing: the ‘border-image-slice’ property [CSS3BG] for the definitions of the property values.

6.3. The ‘mask-box-image-width’ property

The mask image is drawn inside an area called the mask box image area. This is an area whose boundaries by default correspond to the mask box, see ‘mask-box-image-outset’.

See Drawing Areas: the ‘border-image-width’ property [CSS3BG] for the definitions of the property values.

Note that for SVG elements without an associated layout box the ‘border-width’ is considered to be ‘0’.

6.4. The ‘mask-box-image-outset’ property

The values specify the amount by which the mask box image area extends beyond the border box. If it has four values, they set the outsets on the top, right, bottom and left sides in that order. If the left is missing, it is the same as the right; if the bottom is missing, it is the same as the top; if the right is missing, it is the same as the top.

As with ‘mask-box-image-width’, a <number> represents a multiple of the corresponding ‘border-width’. Negative values are not allowed for any of the ‘mask-box-image-outset’ values.

Note that for SVG elements without associated layout box the ‘border-width’ is considered to be ‘0’.

6.5. The ‘mask-box-image-repeat’ property

This property specifies how the images for the sides and the middle part of the mask image are scaled and tiled. The first keyword applies to the horizontal sides, the second to the vertical ones. If the second keyword is absent, it is assumed to be the same as the first.

See Image Tiling: the ‘border-image-repeat’ property [CSS3BG] for the definitions of the property values.

The exact process for scaling and tiling the mask box image parts is given in the section Masking with the mask-box-image.

6.6. The ‘mask-box-image’ property

This is a shorthand property for setting ‘mask-box-image-source’, ‘mask-box-image-slice’, ‘mask-box-image-width’, ‘mask-box-image-outset’ and ‘mask-box-image-repeat’. Omitted values are set to their initial values.

6.7. Masking with the mask-box-image

After the mask-box-image given by ‘mask-box-image-source’ is sliced by the ‘mask-box-image-slice’ values, the resulting nine images are scaled, positioned, and tiled into their corresponding mask image regions in four steps as described in the section Drawing the Border Image [CSS3BG].

The application of the ‘mask-box-image-source’ property to an element formatted with the CSS box model establishes a stacking context in the same way that CSS ‘opacity’ [CSS3COLOR] does, and all the element's descendants are rendered together as a group with the masking applied to the group as a whole.

The ‘mask-box-image-source’ property has no effect on the geometry or hit-testing of any element's CSS boxes.

7. Clipping paths

The clipping path restricts the region to which paint can be applied, the so-called clipping region. Conceptually, any parts of the drawing that lie outside of this region are not drawn. This includes any content, background, borders, text decoration, outline and visible scrolling mechanism of the element to which the clipping path is applied, and those of its descendants.

An element's ancestors may also clip portions of their content (e.g., via their own ‘clip’ or ‘clip-path’ properties and/or if their ‘overflow’ property is not ‘visible’). What is rendered is the cumulative intersection.

If the clipping region exceeds the bounds of the UA's document window, content may be clipped to that window by the native operating environment.

7.1. The ‘clip-path’ property

Specifies a basic shape or references a <clipPath> element to create a clipping path.

<clip-source> = <url> | child | <child-selector>

<basic-shape>

A basic shape function as defined in CSS3 Exclusions. The origin of clipping shapes is on the top left of the bounding client rect. Percentage values are resolved against the bounding client rect width and height. (See [CSS3-EXCLUSIONS].)

‘child’

A keyword to indicate that the last direct child <clipPath> element of the element the ‘clip-path’ property is applied should be used as the clip source. It is equivalent to ‘select(clipPath:last-of-type)’.

<child-selector>

A functional notation [CSS3VAL] accepting a comma-separated list of compound selectors that represents first matching child <clipPath> element in tree order (as defined in [DOM]).

‘none’

No clipping path gets created.

A computed value of other than ‘none’ results in the creation of a stacking context [CSS21] the same way that CSS ‘opacity’ [CSS3COLOR] does.

If the URI reference is not valid (e.g it points to an object that doesn't exist or the object is not a <clipPath> element), no clipping is applied.

7.2. The ‘clip-rule’ property

‘nonzero’

See description of ‘fill-rule’ property [SVG11].

‘evenodd’

See description of ‘fill-rule’ property [SVG11].

The ‘clip-rule’ property only applies to graphics elements that are contained within a <clipPath> element.

<g clip-rule="nonzero">
  <clipPath id="MyClip">
    <path d="..." clip-rule="evenodd" />
  </clipPath>
  <rect clip-path="url(#MyClip)" ... />
</g>

<g clip-rule="nonzero">
  <clipPath id="MyClip">
    <path d="..." />
  </clipPath>
  <rect clip-path="url(#MyClip)" clip-rule="evenodd" ... />
</g>

7.3. The ‘clip’ property

With this specification the ‘clip’ property is deprecated. Authors are encouraged to use the ‘clip-path’ property instead. UAs must support the ‘clip’ property.

The ‘clip’ property applies only to absolutely positioned elements. In SVG, it applies to elements which establish a new viewport, <pattern> elements and <marker> elements. Values have the following meanings:

‘auto’

The element does not clip.

<shape>

In CSS 2.1, the only valid <shape> value is: rect(<top>, <right>, <bottom>, <left>) where <top> and <bottom> specify offsets from the top border edge of the box, and <right>, and <left> specify offsets from the left border edge of the box. Authors should separate offset values with commas. User agents must support separation with commas, but may also support separation without commas (but not a combination), because a previous revision of this specification was ambiguous in this respect.

<top>, <right>, <bottom>, and <left> may either have a <length> value or ‘auto’. Negative lengths are permitted. The value ‘auto’ means that a given edge of the clipping region will be the same as the edge of the element's generated border box (i.e., ‘auto’ means the same as ‘0’ for <top> and <left>, the same as the used value of the height plus the sum of vertical padding and border widths for <bottom>, and the same as the used value of the width plus the sum of the horizontal padding and border widths for <right>, such that four ‘auto’ values result in the clipping region being the same as the element's border box).

When coordinates are rounded to pixel coordinates, care should be taken that no pixels remain visible when <left> and <right> have the same value (or <top> and <bottom> have the same value), and conversely that no pixels within the element's border box remain hidden when these values are ‘auto’.

Example: The following two rules:

p#one { clip: rect(5px, 40px, 45px, 5px); }
p#two { clip: rect(5px, 55px, 45px, 5px); }

and assuming both Ps are 50 by 55 pixel, will create, respectively, the rectangular clipping regions delimited by the dashed lines in the following illustrations:

This diagram illustrates two block boxes, one above the other, with rectangular clipping regions of different dimensions. (See long description.)

7.4. The <clipPath> element

CSS properties inherit into the <clipPath> element from its ancestors; properties do not inherit from the element referencing the <clipPath> element.

<clipPath> elements are never rendered directly; their only usage is as something that can be referenced using the ‘clip-path’ property. The ‘display’ property does not apply to the <clipPath> element; thus, <clipPath> elements are not directly rendered even if the ‘display’ property is set to a value other than none, and <clipPath> elements are available for referencing even when the ‘display’ property on the <clipPath> element or any of its ancestors is set to none.

A <clipPath> element can contain <path> elements, <text> elements, basic shapes (such as <circle>) or a <use> element. If a <use> element is a child of a <clipPath> element, it must directly reference <path>, <text> or basic shapes elements. Indirect references are an error and the <clipPath> element must be ignored.

The raw geometry of each child element exclusive of rendering properties such as ‘fill’, ‘stroke’, ‘stroke-width’ within a <clipPath> conceptually defines a 1-bit mask (with the possible exception of anti-aliasing along the edge of the geometry) which represents the silhouette of the graphics associated with that element. Anything outside the outline of the object is masked out. If a child element is made invisible by ‘display’ or ‘visibility’ it does not contribute to the clipping path. When the <clipPath> element contains multiple child elements, the silhouettes of the child elements are logically OR'd together to create a single silhouette which is then used to restrict the region onto which paint can be applied. Thus, a point is inside the clipping path if it is inside any of the children of the <clipPath>.

For a given graphics element, the actual clipping path used will be the intersection of the clipping path specified by its ‘clip-path’ property (if any) with any clipping paths on its ancestors, as specified by the ‘clip-path’ property on the elements which establish a new viewport. (See [SVG11])

A couple of additions:

• The <clipPath> element itself and its child elements do not inherit clipping paths from the ancestors of the <clipPath> element.
• The <clipPath> element or any of its children can specify property ‘clip-path’.
  If a valid ‘clip-path’ reference is placed on a <clipPath> element, the resulting clipping path is the intersection of the contents of the <clipPath> element with the referenced clipping path.
  If a valid ‘clip-path’ reference is placed on one of the children of a <clipPath> element, then the given child element is clipped by the referenced clipping path before OR'ing the silhouette of the child element with the silhouettes of the other child elements.
• An empty clipping path will completely clip away the element that had the ‘clip-path’ property applied.

7.5. Clipping paths, geometry, and pointer events

A clipping path is conceptually equivalent to a custom viewport for the referencing element. Thus, it affects the rendering of an element, but not the element's inherent geometry. The bounding client rect of a clipped element (that is, an element which references a <clipPath> element via a ‘clip-path’ property, or a child of the referencing element) must remain the same as if it were not clipped.

By default, pointer events must not be dispatched on the clipped (non-visible) regions of a shape. For example, a circle with a radius of 10 which is clipped to a circle with a radius of 5 will not receive ‘click’ events outside the smaller radius. Later versions of this specification may define new properties to enable fine-grained control over the interactions between hit testing and clipping.

8. Security

The following description of the security model is not sufficient and detailed enough. See public-fx mailing list.

8.1. Origin Restrictions

External references to resources like <mask> or <clipPath> must not violate the same origin restrictions [CORS].

This restriction includes:

• Any images, either as content or via styling, that are not exposed with CORS
• Any tainted canvas
• Any cross-origin content referenced by ‘xlink:href’ or <use>

If content falls under this restriction, the resource must be rejected. The element is drawn as if the resource was not specified (i.e. the ‘clip-path’ or ‘mask’ property were set to ‘none’).

9. DOM interfaces

9.1. Interface SVGClipPathElement

The SVGClipPathElement interface corresponds to the <clipPath> element.

interface SVGClipPathElement : SVGElement {
  readonly attribute SVGAnimatedEnumeration clipPathUnits;
  readonly attribute SVGAnimatedTransformList transform;
};

SVGClipPathElement implements SVGUnitTypes;

Attributes:

clipPathUnits (readonly SVGAnimatedEnumeration)

Corresponds to attribute ‘clipPathUnits’ on the given <clipPath> element. Takes one of the constants defined in SVGUnitTypes.

transform (readonly SVGAnimatedTransformList)

Corresponds to attribute ‘transform’ on the given element.

9.2. Interface SVGMaskElement

The SVGMaskElement interface corresponds to the <mask> element.

interface SVGMaskElement : SVGElement {
  readonly attribute SVGAnimatedEnumeration maskUnits;
  readonly attribute SVGAnimatedEnumeration maskContentUnits;
  readonly attribute SVGAnimatedLength x;
  readonly attribute SVGAnimatedLength y;
  readonly attribute SVGAnimatedLength width;
  readonly attribute SVGAnimatedLength height;
};

SVGMaskElement implements SVGUnitTypes;

Attributes:

maskUnits (readonly SVGAnimatedEnumeration)

Corresponds to attribute ‘maskUnits’ on the given <mask> element. Takes one of the constants defined in SVGUnitTypes.

maskContentUnits (readonly SVGAnimatedEnumeration)

Corresponds to attribute ‘maskContentUnits’ on the given <mask> element. Takes one of the constants defined in SVGUnitTypes.

x (readonly SVGAnimatedLength)

Corresponds to attribute ‘x’ on the given <mask> element.

y (readonly SVGAnimatedLength)

Corresponds to attribute ‘y’ on the given <mask> element.

width (readonly SVGAnimatedLength)

Corresponds to attribute ‘width’ on the given <mask> element.

height (readonly SVGAnimatedLength)

Corresponds to attribute ‘height’ on the given <mask> element.

10. Conformance

10.1. Conformance to CSS Background and Borders

Some property and element definitions in this specification require an SVG 1.1 implementation [SVG11]. UAs without support for SVG must not implement any of the following properties and elements: ‘mask-type’, ‘clip-rule’, <clipPath> element, <mask> element. For UAs without support for SVG, the ‘mask’ and ‘clip-path’ properties must neither support references to <mask> elements nor to <clipPath> elements.

Document conventions

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]

Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class="example", like this:

Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:

Note, this is an informative note.

Conformance classes

Conformance to this is defined for three conformance classes:

style sheet

A CSS style sheet.

renderer

A UA that interprets the semantics of a style sheet and renders documents that use them.

authoring tool

A UA that writes a style sheet.

A style sheet is conformant to this specification if all of its statements that use syntax defined in this module are valid according to the generic CSS grammar and the individual grammars of each feature defined in this module.

A renderer is conformant to this specification if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the features defined by this specification by parsing them correctly and rendering the document accordingly. However, the inability of a UA to correctly render a document due to limitations of the device does not make the UA non-conformant. (For example, a UA is not required to render color on a monochrome monitor.)

An authoring tool is conformant to this specification if it writes style sheets that are syntactically correct according to the generic CSS grammar and the individual grammars of each feature in this module, and meet all other conformance requirements of style sheets as described in this module.

Partial implementations

So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid (and ignore as appropriate) any at-rules, properties, property values, keywords, and other syntactic constructs for which they have no usable level of support. In particular, user agents must not selectively ignore unsupported component values and honor supported values in a single multi-value property declaration: if any value is considered invalid (as unsupported values must be), CSS requires that the entire declaration be ignored.

Experimental implementations

To avoid clashes with future CSS features, the CSS 2.1 specification reserves a prefixed syntax for proprietary and experimental extensions to CSS.

Prior to a specification reaching the Candidate Recommendation stage in the W3C process, all implementations of a CSS feature are considered experimental. The CSS Working Group recommends that implementations use a vendor-prefixed syntax for such features, including those in W3C Working Drafts. This avoids incompatibilities with future changes in the draft.

Non-experimental implementations

Once a specification reaches the Candidate Recommendation stage, non-experimental implementations are possible, and implementors should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec.

To establish and maintain the interoperability of CSS across implementations, the CSS Working Group requests that non-experimental CSS renderers submit an implementation report (and, if necessary, the testcases used for that implementation report) to the W3C before releasing an unprefixed implementation of any CSS features. Testcases submitted to W3C are subject to review and correction by the CSS Working Group.

Further information on submitting testcases and implementation reports can be found from on the CSS Working Group's website at http://www.w3.org/Style/CSS/Test/. Questions should be directed to the public-css-testsuite@w3.org mailing list.

Acknowledgments

Thanks to Elika J. Etemad, Liam R. E. Quin, Björn Höhrmann for their careful reviews, comments, and corrections.

References

Normative references

[CORS]

Anne van Kesteren. Cross-Origin Resource Sharing. 29 January 2013. W3C Candidate Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2013/CR-cors-20130129/

[CSS21]

Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. 7 June 2011. W3C Recommendation. URL: http://www.w3.org/TR/2011/REC-CSS2-20110607

[CSS3-EXCLUSIONS]

Vincent Hardy; Rossen Atanassov; Alan Stearns. CSS Exclusions and Shapes Module Level 3. 3 May 2012. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2012/WD-css3-exclusions-20120503/

[CSS3-IMAGES]

Elika J. Etemad; Tab Atkins Jr. CSS Image Values and Replaced Content Module Level 3. 17 April 2012. W3C Candidate Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2012/CR-css3-images-20120417/

[CSS3BG]

Bert Bos; Elika J. Etemad; Brad Kemper. CSS Backgrounds and Borders Module Level 3. 24 July 2012. W3C Candidate Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2012/CR-css3-background-20120724/

[CSS3VAL]

Håkon Wium Lie; Tab Atkins; Elika J. Etemad. CSS Values and Units Module Level 3. 4 April 2013. W3C Candidate Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2013/CR-css3-values-20130404/

[DOM]

Anne van Kesteren; et al. DOM4. 6 December 2012. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2012/WD-dom-20121206/

[RFC2119]

S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. Internet RFC 2119. URL: http://www.ietf.org/rfc/rfc2119.txt

[SELECTORS4]

Elika J. Etemad; Tab Atkins Jr. Selectors Level 4. 2 May 2013. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2013/WD-selectors4-20130502/

[SVG11]

Erik Dahlström; et al. Scalable Vector Graphics (SVG) 1.1 (Second Edition). 16 August 2011. W3C Recommendation. URL: http://www.w3.org/TR/2011/REC-SVG11-20110816/

Other references

[CSS3-TRANSFORMS]

Simon Fraser; et al. CSS Transforms. 11 September 2012. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2012/WD-css3-transforms-20120911/

[CSS3COLOR]

Tantek Çelik; Chris Lilley; L. David Baron. CSS Color Module Level 3. 7 June 2011. W3C Recommendation. URL: http://www.w3.org/TR/2011/REC-css3-color-20110607

[CSSOM-VIEW]

Anne van Kesteren. CSSOM View Module. 4 August 2011. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2011/WD-cssom-view-20110804/

[FILTER-EFFECTS]

Vincent Hardy; et al. Filter Effects 1.0. 25 October 2012. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2012/WD-filter-effects-20121025/

Changes since last publication

The following significant changes were made since the 15 November 2012 Working Draft.

• Better integration with terms and definitions of CSS Backgrounds and Borders module.
• Syntax changes on ‘mask’ shorthand property to be conform with ‘background’ shorthand property.
• Define how the implementation can differ between an SVG resource (<mask>, <clipPath>) and an image resource.
• Added ‘mask-source-type’ property to alter between luminance and alpha mask on ‘mask-image’.
• Adapt IDL definition of SVGMaskElement and SVGClipPathElement to WebIDL.
• Further editorial changes.

See detailed list of changes in the ChangeLog.

Property index

Index

• ‘alpha’, 5.2.
• authoring tool, 10.1.
• ‘auto’, 5.2.
• <basic-shape>, 7.1.
• ''border-box''
  • 'mask-clip' value, 5.5.
  • 'mask-origin' value, 5.6.
• <bottom>, 7.3.
• bounding client rect, 4., 7.1., 7.1., 7.5.
• <child-selector>, 5.1.
• clip, 7.3.
• clipPath, 7.4.
• clip-path, 7.1.
• clip-rule, 7.2.
• <clip-source>, 7.1.
• <compound-selector>, 5.1.
• ''content-box''
  • 'mask-clip' value, 5.5.
• ‘content-box’, 5.6.
• ‘evenodd’, 7.2.
• <image>, 5.1.
• layers, 5.11.1.
• <left>, 7.3.
• local coordinate system, 4.
• ‘luminance’, 5.2.
• mask, 5.8., 5.10., 4.
• mask-box-image, 6.6.
• mask box image area, 4., 6.4.
• mask-box-image-outset, 6.4.
• mask-box-image-repeat, 6.5.
• mask-box-image-slice, 6.2.
• mask-box-image-source, 6.1.
• mask-box-image-width, 6.3.
• mask-clip, 5.5.
• <mask-image>, 5.1.
• mask-image, 5.1.
• mask image area, 6.
• mask images, 4.
• <mask-layer>, 5.8.
• mask-origin, 5.6.
• mask painting area, 5.5., 4., 5.3., 5.4.
• mask-position, 4., 5.4.
• mask positioning area, 5.6., 4.
• <mask-reference>, 5.1.
• mask-repeat, 5.3.
• mask-size, 4., 5.7.
• <mask-source>, 5.1.
• mask source, 4.
• mask-source-type, 5.2.
• mask-type, 5.9.
• ''no-clip''
  • 'mask-clip' value, 5.5.
• ‘nonzero’, 7.2.
• object bounding box units, 4., 7.4.
• ''padding-box''
  • 'mask-clip' value, 5.5.
  • 'mask-origin' value, 5.6.
• renderer, 10.1.
• <right>, 7.3.
• <source-type>, 5.2.
• style sheet
  • as conformance class, 10.1.
• <top>, 7.3.
• <url>, 5.1.
• user coordinate system, 4.