Title: CSS Shapes Module Level 1
Status: ED
Work Status: Testing
Shortname: css-shapes
Level: 1
Group: csswg
TR: https://www.w3.org/TR/css-shapes/
ED: https://drafts.csswg.org/css-shapes/
Previous Version: http://www.w3.org/TR/2014/CR-css-shapes-1-20140320/
Editor: Vincent Hardy, Adobe Systems, Inc., vhardy@adobe.com
Editor: Rossen Atanassov, Microsoft Corporation, ratan@microsoft.com, w3cid 49885
Editor: Alan Stearns, Adobe Systems, Inc., stearns@adobe.com, w3cid 46659
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: css2 (property) margin
spec:css-masking-1; type: value
text: nonzero
text: evenodd
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 [[!CSS2]] chapter 9.
Values
This specification follows the CSS property definition conventions from [[!CSS2]].
Value types not defined in this specification are defined in CSS Values & Units [[!CSS-VALUES-3]].
Other CSS modules may expand the definitions of these value types.
In addition to the property-specific values listed in their definitions,
all properties defined in this specification
also accept the CSS-wide keywords keywords as their property value.
For readability they have 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 [[!CSS2]] 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>
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.
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.
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.
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.
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.
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 radius of zero defines a shape
enclosing no area,
which results in an empty float area.
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.
If either radius is zero this defines a shape
enclosing no area,
which results in an empty float area.
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
(including the defaults)
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.
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.
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:
Modifying the alpha channel in the image
Changing the value of the 'shape-image-threshold' property
Changing the value of the 'shape-margin' property (see example 8)
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
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;
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:
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;
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.
Animation type: as defined for <>, otherwise discrete
The values of this property have the following meanings:
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
Applies to: floats
Inherited: no
Computed value: specified number, clamped to the range [0,1]
Animation type: by computed value
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
Percentages: refer to the inline size of the containing block
Computed value: computed <> value
Animation type: by computed value
<>
Sets the margin of the shape to the specified value.
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.
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-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.
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 
