CSS Shapes Module Level 1

Status: ED
Work Status: Testing
Shortname: css-shapes
Level: 1
Group: csswg
TR: http://www.w3.org/TR/css-shapes/
ED: http://dev.w3.org/csswg/css-shapes/
Editor: Vincent Hardy, Adobe Systems, Inc., vhardy@adobe.com
Editor: Rossen Atanassov, Microsoft Corporation, ratan@microsoft.com
Editor: Alan Stearns, Adobe Systems, Inc., stearns@adobe.com
!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.
Link Defaults: css21 (property) margin

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.

Module Interactions

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

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.

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.

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.

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

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 <> 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.

Supported Shapes

The following shapes are supported. All <> values use functional notation and are defined here using the Value Definition Syntax.
inset() = inset( <>{1,4} [round <'border-radius'>]? )
Defines an inset rectangle.
  • When all of the first four arguments are supplied they represent the top, right, bottom and left offsets from the reference box inward that define the positions of the edges of the inset rectangle. These arguments follow the syntax of the 'margin' shorthand, that let you set all four insets with one, two or four values.
  • The optional <<'border-radius'>> argument(s) define rounded corners for the inset rectangle using the 'border-radius' shorthand syntax.
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( [<>]? [at <>]? )
  • The shape-radius argument represents r, the radius of the circle. Negative values are invalid. A percentage value here is resolved from the used width and height of the reference box as
    sqrt(width2+height2)/sqrt(2).
  • The position argument defines the center of the circle. This defaults to center if omitted.
ellipse() = ellipse( [<>{2}]? [at <>]? )
  • The shape-radius arguments represent rx and ry, the x-axis and y-axis radii of the ellipse, in that order. Negative values for either radius are invalid. Percentage values here are resolved against the used width (for the rx value) and the used height (for the ry value) of the reference box.
  • The position argument defines the center of the ellipse. This defaults to center if omitted.
polygon() = polygon( [<>,]? [<> <>]# )
  • <> - The filling rule used to determine the interior of the polygon. See fill-rule property in SVG for details. Possible values are ''nonzero'' or ''evenodd''. Default value when omitted is ''nonzero''.
  • Each pair argument in the list represents xi and yi - the x and y axis coordinates of the i-th vertex of the polygon.
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:
<> = <> | <>
<> = <> | <> | closest-side | farthest-side
Defines a radius for a circle or ellipse. If omitted it defaults to closest-side.
  • closest-side uses the length from the center of the shape to the closest side of the reference box. For circles, this is the closest side in any dimension. For ellipses, this is the closest side in the radius dimension.
  • farthest-side uses the length from the center of the shape to the farthest side of the reference box. For circles, this is the farthest side in any dimension. For ellipses, this is the farthest side in the radius dimension.

Computed Values of Basic Shapes

The values in a <> function are computed as specified, with these exceptions:
  • Omitted values are included and compute to their defaults.
  • A <> value in ''circle()'' or ''ellipse()'' is computed as a pair of offsets (horizontal then vertical) from the top left origin, each given as a combination of an absolute length and a percentage.
  • A <'border-radius'> value in ''inset()'' is computed as an expanded list of all eight <> or percentage values.

Serialization of Basic Shapes

To serialize the <> 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 <> 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 <> 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 <> always uses the 2- or 4-value form, a default <> is not omitted.

		circle(closest-side at center)

		serializes as "circle(at 50% 50%)"
	
Using grammar order means that <> values always give horizontal components first, then vertical.

		circle(at bottom left)

		serializes as "circle(at 0% 100%)"
	
Avoiding calc() expressions means that some <> 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%)"
	

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.
  • Both shapes must use the same reference box.
  • If both shapes are the same type, that type is ''ellipse()'' or ''circle()'', and none of the radii use the ''closest-side'' or ''farthest-side'' keywords, interpolate between each value in the shape functions.
  • If both shapes are of type ''inset()'', interpolate between each value in the shape functions.
  • If both shapes are of type ''polygon()'', both polygons have the same number of vertices, and use the same <>, interpolate between each value in the shape functions.
  • In all other cases no interpolation is specified.

Shapes from Image

Another way of defining shapes is by specifying a source <> 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.

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 <> value extends the <> value to include ''margin-box''. Its syntax is:
		<> = <> | ''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:
  • ''margin-box'': the shape containing all of the yellow pixels
  • ''border-box'': the shape containing all of the black pixels
  • ''padding-box'': the shape containing all of the mauve pixels
  • ''content-box'': the shape containing all of the blue pixels
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

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.

Float Area Shape: the 'shape-outside' property

		Name: shape-outside
		Value: none | [ <> || <> ] | <>
		Initial: none
		Applies to: floats
		Inherited: no
		Computed value: as defined for <> (with <> following, if supplied), the <> with its URI made absolute, otherwise as specified.
		Media: visual
		Animatable: as specified for <>, otherwise no
	
The values of this property have the following meanings:
none
The float area is unaffected.
<>
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]].
<>
The shape is computed based on the values of one of ''inset()'', ''circle()'', ''ellipse()'' or ''polygon()''. If a <> is also supplied, this defines the reference box for the <> function. If <> is not supplied, then the reference box defaults to ''margin-box''.
<>
The shape is extracted and computed based on the alpha channel of the specified <> 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.

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: <>
		Initial: 0.0
		Applies to: floats
		Inherited: no
		Computed value: The same as the specified value after clipping the <> to the range [0.0,1.0]
		Media: visual
		Animatable: as number
	
The values of this property have the following meanings:
<>
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.

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

  • Clarified shape-margin computed value

Since February 11th 2014

  • Replaced divs with images in the first example
  • Add 0px to last serialization example

Since December 3rd 2013

  • Updated computed value and serialization of basic shapes
  • Added a margin-box example
  • Change auto to none for shape-outside
  • Defined shape-box instead of redefining box
  • Clarified that shape from image may produce more than one path

Since June 20th 2013

  • Added shape from box value section
  • Updated basic-shape interpolation
  • Allow negative insets, disallow negative radii
  • Changed relevant to reference
  • Remove box-sizing dependency, add relevant box keywords
  • Changed circle() and ellipse() to use radial gradient syntax
  • Postponed rectangle() to level 2
  • Clarified shape-from-image sizing and positioning
  • Change inset-rectangle() to inset()
  • Future-proof shape-image-threshold to possibly apply to luminance
  • Added CORS fetching to shape-outside URLs
  • Changed shape-outside value from <uri> to <image>
  • Remove 'percentages based on auto-sizing resolve to 0'
  • Change initial value of shape-image-threshold to 0.0
  • Change float positioning to be unaffected by shape-outside
  • Shapes on floats clipped to float's margin box

Since May 3rd 2012

  • Postpone shapes from SVG elements to a future Shapes level
  • Postpone shape-inside to a future Shapes level
  • split exclusions from shapes into separate modules
  • added inset-rectangle() to basic shapes
  • Changed shape-inside overflow diagrams to show exclusion behavior
  • Changed shape-inside to contribute to the wrapping context
  • Defined exclusion edges relative to wrapping content's writing mode
  • Made use of start, end, before and after consistent
  • Added interpolation for basic shapes
  • Changed basic shapes to depend on box specified with box-sizing
  • Added overflow behavior for shape-inside.
  • Added wrap-flow:minimum.
  • Clarified processing model.
  • Changed wrap-margin and wrap-padding to shape-margin and shape-padding.
  • Removed wrap shorthand.

Since December 13th 2011

  • Clarified processing model.
  • Clarified interaction with floats.
  • Clarified that an exclusion element establishes a new block formatting context.