CSS Shapes Module Level 1

Editor’s Draft, 30 April 2014

This version:
http://dev.w3.org/csswg/css-shapes/
Latest version:
http://www.w3.org/TR/css-shapes/
Editor’s Draft:
http://dev.w3.org/csswg/css-shapes/
Feedback:
www-style@w3.org with subject line “[css-shapes] … message topic …”(archives)
Test Suite:
http://test.csswg.org/suites/css3-shapes/nightly-unstable/
Editors:
(Adobe Systems, Inc.)
(Microsoft Corporation)
(Adobe Systems, Inc.)
Issues List:
In Bugzilla

Abstract

CSS Shapes describe geometric shapes for use in CSS. For Level 1, CSS Shapes can be applied to floats. A circle shape on a float will cause inline content to wrap around the circle shape instead of the float’s bounding box. CSS is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, in speech, etc.

Status of this document

This is a public copy of the editors’ draft. It is provided for discussion only and may change at any moment. Its publication here does not imply endorsement of its contents by W3C. Don’t cite this document other than as work in progress.

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

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

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes 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

This section is not normative.

Shapes define arbitrary geometries that can be used as CSS values. This specification defines properties to control the geometry of an element’s float area. The shape-outside property uses shape values to define the float area for a float.

Note: Future levels of CSS Shapes will allow use of shapes on elements other than floats. Other CSS modules can make use of shapes as well, such as CSS Masking [CSS-MASKING] and CSS Exclusions [CSS3-EXCLUSIONS].

Note: If a user agent implements both CSS Shapes and CSS Exclusions, the shape-outside property defines the exclusion area for an exclusion.

Note: A future level of CSS Shapes will define a shape-inside property, which will define a shape to wrap content within the element.

1.1 Module Interactions

This module extends the float features defined in [CSS21] chapter 9.

1.2 Values

This specification follows the CSS property definition conventions from [CSS21]. 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.

1.3 Animated Values

It is expected that CSS will include ways to animate transitions between styles. (The section "Animation of property types" of the CSS Transitions module [CSS3-TRANSITIONS] is expected to define how different kinds of values are interpolated during a transition.) In anticipation of that, this module includes a line "Animatable" for each property, which specifies whether and how values of the property can be animated.

1.4 Terminology

Wrap

This specification uses the term wrap to refer to flowing content around the sides of a float area, defined in [CSS21] chapter 9. Content wraps around the right side of a left-floated box, and content wraps around the left side of a right-floated box. One result of this wrapping is that line boxes next to a float are shortened as necessary to avoid intersections with the float area.

Float area

The area used for wrapping content around a float element. The rules for float behavior use the sides of the float area to determine where content flows. By default, the float area is the float element’s margin box (note this can be different than the float area produced by the margin-box value, which includes border-radius curvature). This specification’s shape-outside property can be used to define an arbitrary, non-rectangular float area.

2 Relation to the box model and float behavior

While the boundaries used for wrapping inline flow content outside a float can be defined using shapes, the actual box model does not change. If the element has specified margins, borders or padding they will be computed and rendered according to the [CSS3BOX] module. Also, float positioning and stacking are not affected by defining a float area with a shape.

When a shape is used to define a float area, the shape is clipped to the float’s margin box. In other words, a shape can only ever reduce a float area, not increase it. A reduced float area may have no effect on some line boxes that would normally be affected by the float. An empty float area (where the shape encloses no area) has no effect on line boxes.

A float area defined by a shape may reduce the normal float area on all sides, but this does not allow content to wrap on both sides of a float. Left floats with a shape-outside still only allow content wrapping on the right side, and right floats only allow wrapping on the left.

In the following example the left and right floating img elements specify a triangular shape using the shape-outside property.


  <img class="left" src="hand.svg"/>
  <img class="right" src="hand.svg"/>
  <p>
    Sometimes a web page’s text content appears to be
    funneling your attention towards a spot on the page
    to drive you to follow a particular link. Sometimes
    you don’t notice.
  </p>

  <style type="text/css">
    .left {
      shape-outside: polygon(0 0, 100% 100%, 0 100%);
      float: left;
      width: 40%;
      height: 12ex;
      transform: scaleX(-1);
    }

    .right {
      shape-outside: polygon(100% 0, 100% 100%, 0 100%);
      float: right;
      width: 40%;
      height: 12ex;
    }

    p {
      text-align: center;
    }
  </style>

Using the shape-outside property with floats

Since shapes are clipped to the float’s margin box, adding this shape to the left float above would result in the same rendering.


  shape-outside: polygon(0 0, 500% 500%, 0 500%);

A shape with no extent will create a float area with no extent. Because wrapping only considers the float area, either shape below applied to a float will allow inline content to flow through all of the float’s box.


  shape-outside: inset(50% 50% 50% 50%);
  shape-outside: inset(150% 150% 0% 0%);

A shape-outside can create open areas on both the left and right of a float area. Content still wraps only on one side of a float in this case. In the picture, the shape is rendered in blue, and the content area outside the shape in mauve.


  shape-outside: polygon(50px 0px, 100px 100px, 0px 100px);

wrapping around right side of a left-float float area

The following styling creates a shape much smaller than the float’s content area, and adds a margin-top to the float. In the picture, the shape is rendered in blue, the content area outside the shape in mauve, and the margin area of the float box in yellow. The inline content only wraps around the shape, and otherwise overlays the rest of the float margin box.


  .float-left {
    shape-outside: polygon(0% 50%, 50% 100%, 0 100%);
    float: left;
    width: 100px;
    height: 100px;
    margin-top: 20px;
  }

Adding margin-top to a float with a small shape-outside

The next picture shows a possible result if two of these floats were stacked next to each other. Note that the floats are positioned using their margin boxes, not the float area.

Stacking two floats with a small shape-outside

3 Basic Shapes

The <basic-shape> type can be specified using basic shape functions. When using this syntax to define shapes, the reference box is defined by each property that uses <basic-shape> values. The coordinate system for the shape has its origin on the top-left corner of the reference box with the x-axis running to the right and the y-axis running downwards. All the lengths expressed in percentages are resolved from the used dimensions of the reference box.

3.1 Supported Shapes

The following shapes are supported. All <basic-shape> values use functional notation and are defined here using the Value Definition Syntax.

inset() = inset( <shape-arg>{1,4} [round <border-radius>]? )

Defines an inset rectangle.

A pair of insets in either dimension that add up to more than the used dimension (such as left and right insets of 75% apiece) define a shape enclosing no area. For this specification, this results in an empty float area.

circle() = circle( [<shape-radius>]? [at <position>]? )

ellipse() = ellipse( [<shape-radius>{2}]? [at <position>]? )

polygon() = polygon( [<fill-rule>,]? [<shape-arg> <shape-arg>]# )

The UA must close a polygon by connecting the last vertex with the first vertex of the list.

At least three vertices are required to define a polygon with an area. This means that (for this specification) polygons with less than three vertices (or with three or more vertices arranged to enclose no area) result in an empty float area.

The arguments not defined above are defined as follows:

<shape-arg> = <length> | <percentage>
<shape-radius> = <length> | <percentage> | closest-side | farthest-side

Defines a radius for a circle or ellipse. If omitted it defaults to closest-side.

3.2 Computed Values of Basic Shapes

The values in a <basic-shape> function are computed as specified, with these exceptions:

3.3 Serialization of Basic Shapes

To serialize the <basic-shape> functions, serialize as per their individual grammars, in the order the grammars are written in, avoiding calc() expressions where possible, avoiding calc() transformations, omitting components when possible without changing the meaning, joining space-separated tokens with a single space, and following each serialized comma with a single space.

The <position> values in ellipse() and circle() serialize to their 2- and 4-value forms only, preferring the 2-value form when it can be expressed without calc(), preferring left and top origins, and preferring 0% over a zero length.

Since <position> keywords stand in for percentages, keywords without an offset turn into percentages.


  circle(at left bottom) 

  serializes as "circle(at 0% 100%)"

Omitting components means that some default values do not show up in the serialization. But since <position> always uses the 2- or 4-value form, a default <position> is not omitted.


  circle(closest-side at center) 

  serializes as "circle(at 50% 50%)"

Using grammar order means that <position> values always give horizontal components first, then vertical.


  circle(at bottom left) 

  serializes as "circle(at 0% 100%)"

Avoiding calc() expressions means that some <position> values that could be simplified to the 2-value form must be serialized in 4-value form instead.


  circle(at right 5px bottom 10px) 

  serializes as "circle(at right 5px bottom 10px)"

  not as "circle(at calc(100% - 5px) calc(100% - 10px))"

Avoiding calc() transformations means that if a specified (or computed) calc() must stay in calc() form, it will be used as-is, not reformulated with a different origin or reduced.


  bottom calc(10% + 5px)

  serializes as "bottom calc(10% + 5px)"

  not as "top calc(90% - 5px)" or "calc(90% - 5px)"

Preferring 0% over a zero length comes up when you must supply an omitted offset.


  circle(at right 5px top) 

  serializes as "circle(at right 5px top 0%)"

Preferring left and top origins means that some percentage offsets will normalize to those origins (when calc can be avoided).


  circle(at right 5% top 0px) 

  serializes as "circle(at 95% 0%)"

3.4 Interpolation of Basic Shapes

For interpolating between one basic shape and a second, the rules below are applied. The values in the shape functions interpolate as a simple list. The list values interpolate as length, percentage, or calc where possible. If list values are not one of those types but are identical (such as finding nonzero in the same list position in both lists) those values do interpolate.

4 Shapes from Image

Another way of defining shapes is by specifying a source <image> whose alpha channel is used to compute the shape. The shape is computed to be the path or paths that enclose the area(s) where the opacity of the specified image is greater than the shape-image-threshold value. The absence of any pixels with an alpha value greater than the specified threshold results in an empty float area. If the shape-image-threshold is not specified, the initial value to be considered is 0.0.

The image is sized and positioned as if it were a replaced element whose specified width and height are the same as the element’s used content box size.

For animated raster image formats (such as GIF), the first frame of the animation sequence is used.

An image is floating to the left of a paragraph. The image shows the 3D version of the CSS logo over a transparent background. The logo has a shadow using an alpha-channel.

The image defines its float area through the shape-outside property.

  
    <p>
      <img id="CSSlogo" src="CSS-logo1s.png"/>
      blah blah blah blah...
    </p>

    <style>
      #CSSlogo {
        float: left;
        shape-outside: attr(src url);
        shape-image-threshold: 0.1;
      }
    </style>
  

The shape-outside property re-uses the url from the src attribute of the img element.

It is perfectly possible to display an image and use a different image for its float area.

In the figure below, the alpha-channel threshold is represented by the dotted line around the CSS logo.

It’s then possible to affect where the lines of the paragraph start in three ways:

  1. Modifying the alpha channel in the image
  2. Changing the value of the shape-image-threshold property
  3. Changing the value of the shape-margin property (see example 8)
A float shape around an image using its alpha-channel
A float shape around an image using its alpha-channel.

5 Shapes from Box Values

Shapes can be defined by reference to edges in the CSS Box Model. These edges include border-radius curvature [CSS3BG] from the used border-radius values. The <shape-box> value extends the <box> value to include margin-box. Its syntax is:

  <shape-box> = <box> | margin-box

The definitions of the values are:

The margin-box value defines the shape enclosed by the outside margin edge. The corner radii of this shape are determined by the corresponding border-radius and margin values. If the ratio of border-radius/margin is 1 or more, then the margin box corner radius is border-radius + margin. If the ratio of border-radius/margin is less than 1, then the margin box corner radius is border-radius + (margin * (1 + (ratio-1)^3)).

The border-box value defines the shape enclosed by the outside border edge. This shape follows all of the normal border radius shaping rules for the outside of the border.

The padding-box value defines the shape enclosed by the outside padding edge. This shape follows all of the normal border radius shaping rules for the inside of the border.

The content-box value defines the shape enclosed by the outside content edge. Each corner radius of this box is the larger of 0 or border-radius - border-width - padding.

Given the 100px square below with 10px padding, border and margins, the box values define these shapes:

Colored boxes representing simple box edges
Simple CSS Box Model Edges

And the same definitions apply to a more complex example with the same 100px square, but with these border, padding and margin properties:

  
    border-radius: 20px 20px 20px 40px;
    border-width: 30px 10px 20px 10px;
    padding: 10px 20px 10px 10px;
    margin: 20px 10px 10px 10px;
  
Colored boxes representing complex box edges
Complex CSS Box Model Edges

The difference between normal float wrapping and wrapping around the shape defined by the margin-box value is that the margin-box shape includes corner shaping. Take the 100px square with 10px padding, border and margins, but with a border-radius of 60px. If you make a left float out of it, content normally wraps in this manner:

Text wrapping around float with no shape
Normal float wrapping

If you add a margin-box shape to the float, then content wraps around the rounded margin-box corners.

  
    shape-outside: margin-box;
  
Text wrapping around float with margin-box shape
Float wrapping with margin-box

6 Declaring Shapes

Shapes are declared with the shape-outside property, with possible modifications from the shape-margin property. The shape defined by the shape-outside and shape-margin properties changes the geometry of a float element’s float area.

6.1 Float Area Shape: the shape-outside property

Name:shape-outside
Value:none | [ <basic-shape> || <shape-box> ] | <image>
Initial:none
Applies to:floats
Inherited:no
Media:visual
Computed value:as defined for <basic-shape> (with <shape-box> following, if supplied), the <image> with its URI made absolute, otherwise as specified.
Animatable:as specified for <basic-shape>, otherwise no

The values of this property have the following meanings:

none

The float area is unaffected.

<shape-box>

If one of these values is specified by itself the shape is computed based on one of margin-box, border-box, padding-box or content-box which use their respective boxes including curvature from border-radius, similar to background-clip [CSS3BG].

<basic-shape>

The shape is computed based on the values of one of inset(), circle(), ellipse() or polygon(). If a <shape-box> is also supplied, this defines the reference box for the <basic-shape> function. If <shape-box> is not supplied, then the reference box defaults to margin-box.

<image>

The shape is extracted and computed based on the alpha channel of the specified <image> as defined by shape-image-threshold.

User agents must use the potentially CORS-enabled fetch method defined by the [HTML5] specification for all URLs in a shape-outside value. When fetching, user agents must use "Anonymous" mode, set the referrer source to the stylesheet’s URL and set the origin to the URL of the containing document. If this results in network errors such that there is no valid fallback image, the effect is as if the value none had been specified.

6.2 Choosing Image Pixels: the shape-image-threshold property

The shape-image-threshold defines the alpha channel threshold used to extract the shape using an image. A value of 0.5 means that the shape will enclose all the pixels that are more than 50% opaque.

Name:shape-image-threshold
Value:<number>
Initial:0.0
Applies to:floats
Inherited:no
Media:visual
Computed value:The same as the specified value after clipping the <number> to the range [0.0,1.0]
Animatable:as number

The values of this property have the following meanings:

<number>

Sets the threshold used for extracting a shape from an image. The shape is defined by the pixels whose alpha value is greater than the threshold. A threshold value outside the range 0.0 (fully transparent) to 1.0 (fully opaque) will be clamped to this range.

Note: A future level of CSS Shapes may define a switch to use the luminance data from an image instead of the alpha data. When this happens, shape-image-threshold will be extended to apply its threshold to either alpha or luminance, depending on the switch state.

6.3 Embiggening a Shape: the shape-margin property

The shape-margin property adds a margin to a shape-outside. This defines a new shape that is the smallest contour (in the shrink-wrap sense) that includes all the points that are the shape-margin distance outward in the perpendicular direction from a point on the underlying shape. Note that at points where a perpendicular is not defined (e.g. sharp points) take all points on the circle centered at the point and with a radius of shape-margin. This property takes only non-negative values.

Name:shape-margin
Value:<length> | <percentage>
Initial:0
Applies to:floats
Inherited:no
Media:visual
Computed value:as specified, but with lengths made absolute
Animatable:as length, percentage, or calc.
<length>

Sets the margin of the shape to the <length>.

<percentage>

Sets the margin of the shape to a percentage of the width of the element’s containing block.

A shape-margin creating an offset from a polygonal shape-outside. The lighter blue area shows the shape in a 100x100px float, and the darker blue area shows the 10px offset.

  
    .float {
      shape-outside: polygon(10px 10px, 90px 50px, 40px 50px, 90px 90px, 10px 90px);
      shape-margin: 10px;
    }
  

Example of a shape-margin offset

If shape-margin is added to the CSS logo from example 6, the line boxes wrapping around the shape are shortened further.

  
    #CSSlogo {
      shape-margin: 35px;
    }
  
A float shape around an image using its alpha-channel with a 35 pixels shape-margin
A float shape around an image using its alpha-channel with a 35-pixel shape-margin

Acknowledgments

This specification is made possible by input from Andrei Bucur, Alexandru Chiculita, Elika Etemad, Arron Eicholz, Sylvain Galineau, Daniel Glazman, Arno Gourdol, Zoltan Horvath, Chris Jones, Bem Jones-Bey, Marcus Mielke, Alex Mogilevsky, Hans Muller, Mihnea Ovidenie, Virgil Palanciuc, Robert Sanderson, Dirk Schulze, Peter Sorotokin, Bear Travis, Eugene Veselov, Stephen Zilles and the CSS Working Group members.

Change Log

Since March 20th 2014

Since February 11th 2014

Since December 3rd 2013

Since June 20th 2013

Since May 3rd 2012

Since December 13th 2011

Conformance

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:

This is an example of an informative example.

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 specification 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 CSS2.1 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.

References

Normative References

[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
[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/
[CSS3BOX]
Bert Bos. CSS basic box model. 9 August 2007. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2007/WD-css3-box-20070809
[CSS3VAL]
Håkon Wium Lie; Tab Atkins; Elika J. Etemad. CSS Values and Units Module Level 3. 30 July 2013. W3C Candidate Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2013/CR-css3-values-20130730/
[HTML5]
Robin Berjon; et al. HTML5. 6 August 2013. W3C Candidate Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2013/CR-html5-20130806/
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. URL: http://www.ietf.org/rfc/rfc2119.txt

Informative References

[CSS-MASKING]
Dirk Schulze; Brian Birtles; Tab Atkins Jr.. CSS Masking Level 1. 20 June 2013. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2013/WD-css-masking-20130620/
[CSS3-EXCLUSIONS]
Vincent Hardy; Rossen Atanassov; Alan Stearns. CSS Exclusions Module Level 1. 28 May 2013. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2013/WD-css3-exclusions-20130528/
[CSS3-TRANSITIONS]
Dean Jackson; et al. CSS Transitions. 12 February 2013. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2013/WD-css3-transitions-20130212/

Index

Property index

NameValueInitialApplies toInh.%agesMediaAnimatableComputed value
shape-outsidenone | [ <basic-shape> || <shape-box> ] | <image>nonefloatsnovisualas specified for <basic-shape>, otherwise noas defined for <basic-shape> (with <shape-box> following, if supplied), the <image> with its URI made absolute, otherwise as specified.
shape-image-threshold<number>0.0floatsnovisualas numberThe same as the specified value after clipping the <number> to the range [0.0,1.0]
shape-margin<length> | <percentage>0floatsnovisualas length, percentage, or calc.as specified, but with lengths made absolute