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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN">
<html lang="en">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>CSS Image Values and Replaced Content Module Level 3</title>
<link rel="stylesheet" type="text/css" href="default.css">
<link rel="stylesheet" type="text/css" href="http://www.w3.org/StyleSheets/TR/W3C-ED">
</head>
<body>
<div class="head">
<!--logo-->
<h1>CSS Image Values and Replaced Content Module Level 3</h1>
<h2 class="no-num no-toc">[LONGSTATUS] [DATE]</h2>
<dl>
<dt>Latest Version:</dt>
<dd><a href="http://dev.w3.org/csswg/css3-images/">http://dev.w3.org/csswg/css3-images/</a>
<dt>Latest Published Version:</dt>
<dd><a href="http://www.w3.org/TR/css3-images/">http://www.w3.org/TR/css3-images/</a></dd>
<dt>Previous Version:</dt>
<dd><a href="http://www.w3.org/TR/2011/WD-css3-images-20110217/">http://www.w3.org/TR/2011/WD-css3-images-20110217/</a></dd>
<dd><a href="http://www.w3.org/TR/2009/WD-css3-images-20090723/">http://www.w3.org/TR/2009/WD-css3-images-20090723/</a></dd>
<dt>Editor:</dt>
<dd><a href="http://fantasai.inkedblade.net/contact">Elika J. Etemad</a> (Invited Expert)</dd>
<dd><a href="http://www.xanthir.com/contact">Tab Atkins Jr.</a> (Google)</dd>
</dl>
<!--begin-copyright-->
<p>[Here will be included the file "../copyright.inc"]</p>
<!--end-copyright-->
<hr title="Separator for header">
</div>
<!-- ====================================================================== -->
<h2 class="no-num no-toc" id="abstract">
Abstract</h2>
<p>This CSS Image Values and Replaced Content module has two parts:
First, it defines the syntax for <i><image></i> values in CSS.
<i><image></i> values can be a single URI to an image, a list of
URIs denoting a series of fallbacks, a reference to an element in the document, or
gradients. Second, it defines properties used to control the
interaction of replaced content and the CSS layout algorithms.
These properties can affect the used image resolution for bitmaps,
the replaced object's orientation, and whether and how to preserve
the object's aspect ratio.</p>
<!-- ====================================================================== -->
<h2 class="no-num no-toc" id="status">
Status of this document</h2>
<!--status-->
<!-- ====================================================================== -->
<h2 class="no-num no-toc" id="contents">
Table of contents</h2>
<!--toc-->
<!-- ====================================================================== -->
<h2 id="intro">
Introduction</h2>
<p><em>This section is non-normative.</em></p>
<p>In CSS Levels 1 and 2, image values, such as those used in the
'background-image' property, could only be given by a single URI
value. This module introduces additional notations that allow a
2D image to be given as a list of URIs denoting fallbacks, as a
reference to an element in the document, and as a gradient.</p>
<!-- ====================================================================== -->
<h2 id="conformance">
Conformance</h2>
<p>A document or implementation cannot conform to CSS Image Values & Replaced Content Level 3
alone, but can claim conformance to CSS Image Values & Replaced Content Level 3 if it satisfies
the conformance requirements in this specification when implementing CSS or
another host language that normatively references this specification.</p>
<p>Conformance to CSS Image Values & Replaced Content Level 3 is defined for three classes:
<dl>
<dt><dfn>minimal</dfn></dt>
<dd>A device that does not implement CSS Transitions, CSS Animations, nor
CSSOM may ignore the chapters on Serializing and Interpolating values for
the purpose of claiming conformance.</dd>
<dt><dfn>transition-capable</dfn></dt>
<dd>A device that implements CSS Transitions or CSS Animations must conform
to the <i>minimal</i> class, and additionally must implement the chapter
on Interpolation.</dd>
<dt><dfn>CSSOM-capable</dfn></dt>
<dd>A device that implements CSSOM must conform to the <i>minimal</i> class,
and additionally must implement the chapter on Serialization.</dd>
</dl>
<p>The 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]]</p>
<p>Examples in this specification are introduced with the words "for example"
or are set apart from the normative text with <code>class="example"</code>,
like this:
<div class="example">
<p>This is an example of an informative example.</p>
</div>
<p>Informative notes begin with the word "Note" and are set apart from the
normative text with <code>class="note"</code>, like this:
<p class="note">Note, this is an informative note.</p>
<!-- ====================================================================== -->
<h2 id='resolution-units'>
Resolution Units: the <resolution> value type</h2>
<p>This specification defines the following units as part of the <dfn><resolution></dfn>
value type:</p>
<dl>
<dt><dfn>dpi</dfn></dt>
<dd>dots per inch</dd>
<dt><dfn>dpcm</dfn></dt>
<dd>dots per centimeter</dd>
<dt><dfn>dppx</dfn></dt>
<dd>dots per ''px'' unit</dd>
</dl>
<p class="note">The default resolution of raster images in CSS is ''1dppx''.</p>
<!-- ====================================================================== -->
<h2 id="image">
Image Values: the <image> value type</h2>
<p>The <image> value type denotes a 2D image. It is defined as
<pre class="prod"><dfn><image></dfn> = <url> | <image-list> | <element-reference> | <image-combination> | <gradient></pre>
<p>Image values can be used in many CSS properties, including the
'background-image', 'list-style-image', 'cursor' properties [[!CSS21]].
<!-- ====================================================================== -->
<h3 id="url">
Image References and Image Slices: the ''url()'' notation</h3>
<p>The simplest way to indicate an image is to reference an image file
by URI. This is done with the
<a href="http://www.w3.org/TR/CSS21/syndata.html#uri">''url()'' notation</a>,
defined in [[!CSS21]].
<div class="example">
<p>In the example below, a background image is specified with ''url()''
syntax:</p>
<pre>background-image: url(wavy.png);</pre>
</div>
<p>A portion of an image may be referenced (clipped out and used as a
standalone image) by use of
<a href="http://www.w3.org/TR/media-frags/#naming-space">media fragment identifiers</a>.
[[!MEDIA-FRAGS]]
<div class="example">
<p>For example,</p>
<pre>background-image: url('sprites.png#xywh=10,30,60,20')</pre>
<p>uses the 60 pixel by 20 pixel rectangle of <code>sprites.png</code> beginning
at the point 10 pixels in from the left, 30 pixels down from the top.
</div>
<p class="note">Note however that a UA that doesn't understand the media
fragments notation will display the entirety an image specified with ''url''.
However, since URLs with media fragment identifiers can also be used in the
''image()'' notation defined below, authors can take advantage of CSS's
forward-compatible parsing rules to provide a fallback when using an image
fragment URL:
<div class="example">
<p>In the example below, the ''image()'' notation is used together with
the media fragment syntax, so that UAs that don't support media fragments
fail to parse the second declaration and use the first.
<pre>
<!-- -->background-image: url('swirl.png'); /* old UAs */
<!-- -->background-image: image('sprites.png#xywh=10,30,60,20'); /* new UAs */</pre>
</div>
</div>
<!-- ====================================================================== -->
<h3 id="image-notation">
Image Annotations and Fallbacks: the ''image()'' notation</h3>
<p>The ''image()'' notation allows an author to tag an image with a few types
of useful processing instructions which can affect the rendering of the image
and to declare fallback images to be used if the original image can't be
decoded or is a type that the browser doesn't recognize.
The author can specify the desired resolution the image should be rendered at,
declare the directionality of an image so that it can be automatically be
reversed if used in text with a different directionality.</p>
<p>So that authors can take advantage of CSS's forwards-compatible parsing
rules to provide a fallback for image slices, implementations that support
the ''image()'' notation <em>must</em> support the <code>xywh=#,#,#,#</code>
form of media fragment identifiers for images. [[!MEDIA-FRAGS]]
<p>The ''image()'' notation is defined as:
<pre class='prod'><dfn><image-list></dfn> =
image( [ <image-decl> , ]* [ <image-decl> | <element-reference> | <gradient> | <color> ] )</pre>
<p>where <image-decl> is given by:</p>
<pre class='prod'><dfn><image-decl></dfn> =
[ <string> | <url> ] [ snap? && <resolution> ]? && [ ltr | rtl ]?</pre>
<p class="issue">Is there a point to allowing ''url()''?
<p>Each <i><image-decl></i> represents an external image. If a <string>
is provided, it represents the same image that it would if the the string
were given to the ''url()'' function.</p>
<p>If a <resolution> is given, the image must be rendered at that resolution.
<span class='note'>Recall that the default resolution of images is ''1dppx'',
so that one image pixel corresponds to one CSS ''px'' unit.</span> If the
''snap'' keyword is also specified, and the specified resolution would make
one image pixel larger than one device pixel, the image must be rendered at
the specified resolution, rounded to the nearest value that would map one image
pixel to an integer number of device pixels; if the specified resolution would
make one image pixel smaller than one device pixel, the image must be rendered
at the specified resolution, rounded to the nearest value that would map an
integer number of image pixels to one device pixel.</p>
<p>If a directional keyword (''ltr'' or ''rtl'') is given, the image itself
gains that directionality. If the image is used in a property on an element
with opposite directionality, is must be mirrored in the inline dimension
when rendered.</p>
<p>Multiple arguments can be given separated by commas, in which case the
function represents the first <image-decl> representing an image that
the browser can successfully load and display.
The final argument can be a <color>,
<a href="#gradients"><gradient></a>, or
<a href="#element-reference"><element-reference></a>,
which serves as the ultimate fallback if none of the preceding <image-decl>s
can be used. If the final argument is a <color>, it represents a solid-color
image of the given color with no <i>intrinsic dimensions</i>.
<div class="example">
<p>The rule below would tell the UA to load ''wavy.svg'' if
it can; failing that to load ''wavy.png'' and display it at 150dpi;
failing that to display ''wavy.gif''; and finally, if none of the images
can be loaded and displayed, to use the color ''rgba(0,0,255,0.5)'' to create a
dimensionless background image. For example, the browser may not understand
how to render SVG images, the PNG may be malformed, and the GIF may not
exist on the server, so requesting it returns an HTML 404 page instead of
an image.</p>
<pre>background-image: image(url(wavy.svg), 'wavy.png' 150dpi, "wavy.gif", rgba(0,0,255,0.5));</pre>
<p>The 'background-image' property specifies that dimensionless images
must stretch to cover the entire background positioning area
[[CSS3BG]], so if none of the specified images can be displayed
the background will be painted semi-transparent blue. As with any image,
this fallback will be painted over the 'background-color' (if any).</p>
</div>
<!-- ====================================================================== -->
<h3 id='element-reference'>
Using Elements as Images: the ''element()'' notation</h3>
<p>The ''element()'' function allows an author to reference an element in
the document that should be used as an image. As the referenced element
changes, for example, by the user typing into a <textarea> element or
a script drawing into a <canvas> element in HTML, the image produced by
the ''element()'' function stays in sync, allowing dynamic effects such as
script-animated background images or previews of the next slide in a slideshow.
The syntax for ''element()'' is defined as:</p>
<pre class=prod><dfn><element-reference></dfn> = element( [<id-selector> | <identifier> ] )</pre>
<p>where <id-selector> is an ID selector [[!SELECT]], and <identifier>
is an identifer [[!CSS3VAL]].</p>
<p>If the argument to the ''element()'' function is an ID selector, the
function references the element matched by the selector. If it's an identifier,
the function references the element whose <dfn>CSS element reference identifier</dfn>
is the given identifier. (CSS does not define how an element acquires a
<i>CSS element reference identifier</i>; that is determined by the host language.)
If no element in the document matches the selector, or no element has the
identifier as its <i>CSS element reference identifier</i>, the function
represents a fully transparent image with no intrinsic dimensions, equivalent
to <code>image(transparent)</code>. If the document changes so that which
element is matched, or whether an element is matched at all, changes, the
image represented by the function must change accordingly.</p>
<p>If the ''element()'' function refers to an element, then it represents
an image with width and height equal to the width and height of the margin
box of the referenced element. The image must be constructed by rendering
the referenced element and its descendants at the same size that the element
would be in its document, over an infinite transparent black background,
positioned so that the edges of the margin box of the element is flush with
the edges of the image. <span class=note>If the element has decorations
or descendants that extend outside the margin box, these will be clipped
to the margin box in the generated image by default. ''background-repeat:extend''
may allow the author to override this behavior so that decorations and
descendants outside the margin box are still painted.</span> If the
referenced element or an ancestor of the referenced element has a transform
applied to it, the transform must be ignored for the purpose of constructing
this image (transforms on descendants must be unaffected).</p>
<p>If the argument passed to ''element()'' isn't an ID selector or an ident,
it is a syntax error.</p>
<p>Implementations may either re-use existing bitmap data generated for the
referenced element or regenerate the display of the element to maximize quality
at the image's size (for example, if the implementation detects that the referenced
element is an SVG fragment); in the latter case, the layout of the referenced
element in the image must not be changed by the regeneration process. That is,
the image must look identical to the referenced element, modulo rasterization
quality.</p>
<div class=example>
TODO: copy an example from the MozHacks article
</div>
<!-- ====================================================================== -->
<h4 class="no-num no-toc" id='element-cycles'>
Detecting and Resolving Circular Relationships Introduced by ''element()''</h4>
<p>The ''element()'' function can produce nonsensical circular relationships,
such as an element using itself as its own background. These relationships
can be easily and reliably detected and resolved, however, by keeping
track of a dependency graph and using common cycle-detection algorithms.</p>
<p>Populate the dependency graph initially by having every element depend
on each of its children. Then, whenever a property on an element A uses
the ''element()'' function to refer to an element B, add an edge to the
graph by having A depend on B. If a dependency cycle is detected, any
''element()'' functions that produced a dependency in the cycle represent
a fully transparent image with no intrinsic dimensions.</p>
<p class=issue>Someone else needs to review this and make sure that I'm
not missing any cycles.</p>
</div>
<!-- ====================================================================== -->
<h3 id='cross-fade-function'>
Combining images: the ''cross-fade()'' notation</h3>
<p>When transitioning between images, CSS requires a way to explicitly refer
to the intermediate image that is a combination of the start and end images.
This is accomplished with the ''cross-fade()'' function, which indicates
the two images to be combined and how far along in the transition the
combination is.
<p class="note">Authors can also use the ''cross-fade()'' function for many
simple image manipulations, such as tinting an image with a solid color or
highlighting a particular area of the page by combining an image with a
radial gradient.
<p>The syntax for ''cross-fade()'' is defined as:</p>
<pre class=prod><dfn><image-combination></dfn> = cross-fade( <image>, <image>, <percentage> )</pre>
<p>The function represents an image generated by combining the first and
second image (referred to in this section as the "start" and "end" images,
respectively). The percentage represents how far along the transformation
is, with 0% representing the start image, 100% representing the end image,
and percentages between that representing corresponding combinations of the
two images. The <code><percentage></code> must be between 0% and 100% inclusive;
any other value is invalid.</p>
<p>Given the percentage <var>p</var>, the combined image represented by
the ''cross-fade()'' function has a width equal to
<code><var>start image width</var> × (1-<var>p</var>) +
<var>end image width</var> × <var>p</var></code>
and a height equal to
<code><var>start image height</var> × (1-<var>p</var>) +
<var>end image height</var> × <var>p</var></code>.
The image itself is generated by first scaling both the start and end images
to the size of the combined image.
Then, the start image has a global alpha applied to it equal to <code>(1-<var>p</var>)</code>,
the end image has a global alpha applied to it equal to <var>p</var>, and the end image
is then composited over the start image with the plus operation.
[[PORTERDUFF]]</p>
<!-- ====================================================================== -->
<h2 id="gradients">
Gradients</h2>
<p>A gradient is an image that smoothly fades from one color to another. These
are commonly used for subtle shading in background images, buttons, and many
other things. The two functions described in this section allow an author to
specify such an image in a terse syntax, so that the UA can generate the image
automatically when rendering the page. Gradients are a type of image, and can
be used anywhere an image can, such as in the 'background-image' or
'list-style-image' properties. Gradients have no <i>intrinsic dimensions</i>.
The syntax of a <i><gradient></i> is:</p>
<pre class=prod><dfn><gradient></dfn> = [ <linear-gradient> | <radial-gradient> | <repeating-linear-gradient> | <repeating-radial-gradient> ]</pre>
<p>where <i><linear-gradient></i>, <i><radial-gradient></i>,
<i><repeating-linear-gradient></i>, and <i><repeating-radial-gradient></i> are
defined in their applicable sections below.</p>
<div class=example>
<p>As with the other <i><image></i> types defined in this specification,
gradients can be used in any property that accepts images. For example:</p>
<ul>
<li><code>background: linear-gradient(white, gray);</code></li>
<li><code>list-style-image: radial-gradient(circle, #006, #00a 90%, #0000af 100%, white 100%)</code></li>
</ul>
</div>
<p class=note>In many places this section references a box, such as "the box's
top-left corner" or "the box's right side". In all of these circumstances,
the "box" refers to the 'CSS View Box' (see the "Sizing Images and Objects in
CSS" section of this spec for clarification). A gradient has no intrinsic
dimensions. This means that, for example, if you use a gradient in a
'background-image', the "box" will simply be the size of the background sizing
area. If you use a gradient in a list-style-image, the "box" will be a 1em
square.</p>
<p class=issue>It has been suggested that several of the controls offered by gradients are
unnecessary. Repeating gradients could potentially be done by hooking into
'background-repeat', sizing and positioning radial gradients could be done by
hooking into 'background-size' and 'background-position', etc.</p>
<p class=issue>Angles in gradients denote directions and match the behavior of polar coordinates,
where 0deg is East, 90deg is North, and in general a larger angle corresponds
to an angle further CCW. Other CSS properties that use angles to denote rotations
use the convention that larger angles are further CW. It has been suggested
that gradients be changed so that larger angles are more CW, and 0deg either
remain East (matching a polar coordinate system with the Y axis flipped) or
changed to North (matching bearings).</p>
<!-- ====================================================================== -->
<h3 id='linear-gradients'>
Linear Gradients</h3>
<p>A linear gradient is created by specifying a gradient-line and then several
colors placed along that line. The image is constructed by creating an
infinite canvas and painting it with lines perdendicular to the gradient-line,
with the color of the painted line being the color of the gradient-line
where the two intersect. This produces a smooth fade from each color to
the next, progressing in the specified direction.</p>
<!-- ====================================================================== -->
<h4 class='no-toc' id='linear-gradient-syntax'>
linear-gradient() syntax</h4>
<pre class=prod><code><dfn><linear-gradient></dfn> = linear-gradient(
[[
[ [top | bottom] || [left | right] ]
|
<angle>
],]?
<color-stop>[, <color-stop>]+
);</code></pre>
<p>The first argument to the function specifies the <dfn>gradient-line</dfn>,
which gives the gradient a direction and determines how color-stops are
positioned. It may be omitted; if so, it defaults to ''top''.</p>
<p>The <i>gradient-line</i> may be specified in two different ways. The
first is by specifying the angle the <i>gradient-line</i> should assume;
this uses the standard algebraic notation for angles where 0deg points
to the right, 90deg points up, and positive angles go counterclockwise.
The starting-point and ending-point of the <i>gradient-line</i> are
determined by extending a line in both direction from the center of the
box at the angle specified. In the direction of the angle, the ending-point
is the point on the <i>gradient-line</i> where a line drawn perpendicular
to the <i>gradient-line</i> would intersect the corner of the box in that
direction. The starting-point is determined identically, except in the
opposite direction of the angle.</p>
<p>The second way is to simply provide a side or corner of the box that
the gradient should start at; the gradient will then automatically angle
itself to extend from the specified side or corner to the opposite side
or corner in a straight line. To be precise, the gradient is converted
to the angle form described in the previous paragraph at used-value time.
If a ''left'', ''bottom'', ''right'', or ''top'' is given, the used value
of the gradient is 0deg, 90deg, 180deg, or 270 deg, respectively. If
a corner is given, the used value of the gradient is the angle necessary
to place the starting-point of the gradient in that corner of the box.</p>
<div class=example>
<div style="overflow: hidden">
<img style="float: right; margin-left: 1em;" src='gradient-diagram.png' alt="[An image showing a box with a background shading gradually from white in the bottom-left corner to black in the top-right corner. There is a line, illustrating the gradient-line, angled at 45 degrees and passing through the center of the box. The starting-point and ending-point of the gradient-line are indicated by the intersection of the gradient-line with two additional lines that pass through the bottom-left and top-right corners of the box.]">
<p>This example illustrates visually how to calculate the
<i>gradient-line</i> from the rules above. This shows the starting
and ending-point of the <i>gradient-line</i>, along with the actual
gradient, produced by an element with
''background: linear-gradient(45deg, white, black);''.</p>
<p>Notice how, though the starting-point and ending-point are outside
of the box, they're positioned precisely right so that the gradient
is pure white <em>exactly</em> at the corner, and pure black
<em>exactly</em> at the opposite corner. That's intentional, and
will always be true for linear gradients.</p>
</div>
</div>
<p>The gradient's color stops are typically placed between the starting-point
and ending-point on the <i>gradient-line</i>, but this isn't required - the
<i>gradient-line</i> extends infinitely in both directions. The starting-point
and ending-point are merely arbitrary distance markers - the starting-point
defines where 0%, 0px, etc are located when specifying color-stops, and
the ending-point defines where 100% is located. Color-stops are allowed
to have positions before 0% or after 100%.</p>
<!-- ====================================================================== -->
<h4 class='no-toc' id='linear-gradient-examples'>
Linear Gradient Examples</h4>
<p>All of the following ''linear-gradient()'' examples are presumed to be
backgrounds applied to a box that is 200px wide and 100px tall.</p>
<div class=example>
<p>Below are various ways of specifying a basic vertical gradient:</p>
<pre><code>linear-gradient(yellow, blue);
linear-gradient(top, yellow, blue);
linear-gradient(bottom, blue, yellow);
linear-gradient(-90deg, yellow, blue);
linear-gradient(270deg, yellow, blue);
linear-gradient(top, yellow 0%, blue 100%);</code></pre>
<p><img src="linear1.png" alt="" ></p>
</div>
<div class=example>
<p>This gradient goes from the upper-left to the lower-right corner.</p>
<pre><code>linear-gradient(top left, yellow, blue);</code></pre>
<p><img src="linear2.png" alt="" ></p>
</div>
<div class=example>
<p>This demonstrates the use of an angle in the gradient. Compare this
image with the previous example. In both gradients, the top-left of the
box is pure yellow, and the bottom-right of the box is pure blue. The
difference is in the angle that the gradient follows.</p>
<pre><code>linear-gradient(-45deg, yellow, blue);
linear-gradient(315deg, yellow, blue);</code></pre>
<p><img src="linear3.png" alt="" ></p>
</div>
<div class=example>
<p>This demonstrates a 3-color gradient, and how to specify the location of a stop explicitly:</p>
<pre><code>linear-gradient(yellow, blue 20%, #0f0);</code></pre>
<p><img src="linear4.png" alt="" ></p>
</div>
<!-- ====================================================================== -->
<h3 id='radial-gradients'>
Radial Gradients</h3>
<p>In a radial gradient, rather than colors smoothly fading from one side
of the box to the other as with linear gradients, they instead emerge from
a single point and smoothly spread outward in a circular or elliptical shape.</p>
<p>A radial gradient is specified by first pinpointing the center of the
gradient, where the 0% ellipse will be, then specifying the size and shape
of the 100% ellipse, ending with a list of color-stops just like a linear-gradient.
Between the center and the ending-ellipse, and past the ending-ellipse,
concentric ellipses are drawn and colored according to the specified color-stops.</p>
<!-- ====================================================================== -->
<h4 class='no-toc' id='radial-gradient-syntax'>
radial-gradient() Syntax</h4>
<pre class=prod>
<dfn><radial-gradient></dfn> = radial-gradient(
[<bg-position>,]?
[[
[<shape> || <size>]
|
[<length> | <percentage>]{2}
],]?
<color-stop>[, <color-stop>]+
)
</pre>
<p>The first argument to the function specifies the center of the ellipse.
<i><bg-position></i> is taken from the Backgrounds and Borders Module, and
has the same definition. It specifies the center of the gradient. If omitted,
it defaults to ''center''. Color-stop positions are measured along an
imaginary line extending from the center of the gradient to the right.</p>
<p>The second argument to the function specifies the size and shape of the
ending-ellipse. This can be specified in two ways, with different characteristics:</p>
<dl>
<dt>Implicitly</dt>
<dd>
<p>The size and shape of the ending-ellipse can be defined
<em>implicitly</em> with a size and shape keyword. The <i><shape></i>
is defined as </p>
<pre><code><dfn><shape></dfn> = [ circle | ellipse ]</code></pre>
<p>''circle'' indicates that the ending-ellipse will be a circle with
a constant radius. ''ellipse'' indicates that the gradient-shape
will be an axis-aligned ellipse (that is, its major and minor radiuses
will be horizontal and vertical, not necessarily in that order).</p>
<p>The <i><size></i> keyword is defined as </p>
<pre><code><dfn><size></dfn> = [ closest-side | closest-corner | farthest-side | farthest-corner | contain | cover ]</code></pre>
<p>If <i><shape></i> is ''circle'' and <i><size></i> is ''closest-side'',
the ending-shape is a circle sized so that it exactly meets the side
of the box closest to its center. For example, if the box was 100px
wide and 200px tall, and the center of the gradient was ''10% 10%'',
then the closest side is the left side of the box (it is 10px from
the starting-point, while the top is 20px from it, and the right and
bottom sides are much further). The gradient-shape would thus be a
circle with a radius of 10px. If <i><shape></i> is ''ellipse'' and
<i><size></i> is ''closest-side'', the gradient-shape is an ellipse
sized so that it exactly meets the vertical and horizontal sides of
the box closest to its center. Using the same box and starting-point
as the previous example, the gradient-shape would be an ellipse with
a 20px vertical radius and a 10px horizontal radius. (If necessary,
such as if the starting-point is outside of the box, extend the sides
of the box so that there is a line the ellipse can meet.)</p>
<p>''farthest-side'' is identical to ''closest-side'', except that
the gradient-shape is sized to meet the side of the box that is farthest
from its center (or the farthest vertical and horizontal sides, if
the shape is ''ellipse''). ''closest-corner'' and ''farthest-corner''
size the gradient-shape so that it exactly meets the closest or farthest
corner of the box from its center, respectively. If <i><shape></i>
is ''ellipse'', the gradient-shape has the same ratio of width to height
that it would if ''closest-side'' or ''farthest-side'' were specified,
as appropriate. ''contain'' is a synonym for ''closest-side'', and
''cover'' is a synonym for ''farthest-corner''.</p>
<p>If this implicit form is used, then it is converted to an equivalent
explicit form (described below) at used-value time.</p>
</dd>
<dt>Explicitly</dt>
<dd>
<p>Alternately, the ending-shape's size and shape can be defined
explicitly, by providing two lengths or percentages. These measure
the length of the horizontal and vertical axes of the ellipse,
respectively. (The axis length is the length from the center of the
ellipse to the edge, similar to the radius of a circle, not the
diameter.)</p>
<p>Percentages used in the first value are relative to the width of
the box, while percentages used in the second value are relative to
the height of the box.</p>
<p>Both of the values must be positive - specifying either as zero
or negative is a syntax error.</p>
</dd>
</dt>
<p>If this argument is omitted, it defaults to ''ellipse cover''.</p>
<p>If only one argument is provided before the color-stops, and it could
be interpreted as either a position or an explicit size (for example, in
''radial-gradient(10% 10%, red, blue)''), it must be interpreted as a position.</p>
<p>In certain circumstances the given parameters may define a degenerate
shape - a circle or ellipse with a radius of 0. In these instances the
gradient image is just a solid color equal to the color of the last color-stop
in the rule. The following combinations of values will trigger this:
''closest-side'' if the starting-point is on a box edge, ''closest-corner''
if the starting-point is on a box corner, and ''ellipse closest-corner''
if the starting-point is on a box edge.</p>
<p>Color-stops are placed on an imaginary line extending from the center
of the gradient toward the right, with the 0% point at the center of the
gradient, and 100% at the point where the line intersects the ending-ellipse.
The color of each ellipse is equal to the color of the line where the
ellipse intersects it. Distances past 100% can be specified, and simply
indicate a color-stop placed on the line a corresponding distance from the
center. Negative distances are allowed in a radial gradient and work the
same as in linear gradients with respect to setting the color of the
<i>gradient-line</i>, but colors before the starting-point of the <i>gradient-line</i>
are not displayed. For example, ''radial-gradient(red -50px, yellow 100px)''
would produce an elliptical gradient which starts with a reddish-orange color
in the center (the color 1/3 between red and yellow) and transitions to
yellow at 100px wide.</p>
<!-- ====================================================================== -->
<h4 class='no-toc' id='radial-gradient-examples'>
Radial Gradient Examples</h4>
<p>All of the following examples are applied to a box that is 200px wide
and 100px tall.</p>
<div class=example>
<p>These examples demonstrate the basic syntax for radial gradients:</p>
<pre><code>radial-gradient(yellow, green);
radial-gradient(center, ellipse cover, yellow 0%, green 100%);
radial-gradient(50% 50%, farthest-corner, yellow, green);</code></pre>
<p><img src="radial1.png" alt="" ></p>
<pre><code>radial-gradient(circle, yellow, green);</code></pre>
<p><img src="radial2.png" alt="" ></p>
<pre><code>radial-gradient(red, yellow, green);</code></pre>
<p><img src="radial3.png" alt="" ></p>
</div>
<div class=example>
<p>This image shows a gradient originating from somewhere other than the center of the box:</p>
<pre><code>radial-gradient(bottom left, farthest-side, red, yellow 50px, green);</code></pre>
<p><img src="radial4.png" alt="" ></p>
</div>
<div class=example>
<p>Here we illustrate a 'contain' gradient.</p>
<pre><code>radial-gradient(20px 30px, contain, red, yellow, green);
radial-gradient(20px 30px, 20px 30px, red, yellow, green);</code></pre>
<p><img src="radial6.png" alt="" ></p>
<pre><code>radial-gradient(20px 30px, circle contain, red, yellow, green);
radial-gradient(20px 30px, 20px 20px, red, yellow, green);</code></pre>
<p><img src="radial7.png" alt="" ></p>
</div>
<!-- ====================================================================== -->
<h3 id='repeating-gradients'>
Repeating Gradients</h3>
<p>In addition to the ''linear-gradient()'' and ''radial-gradient()'' functions,
this specification defines ''repeating-linear-gradient()'' and
''repeating-radial-gradient()'' functions. These two functions take the
same values and are interpreted the same as their respective non-repeating
siblings defined previously:</p>
<pre class=prod><code><dfn><repeating-linear-gradient></dfn> = repeating-linear-gradient(
[[
[ [top | bottom] || [left | right] ]
||
<angle>
],]?
<color-stop>[, <color-stop>]+
)
<dfn><repeating-radial-gradient></dfn> = repeating-radial-gradient(
[<bg-position>,]?
[[
[<shape> || <size>]
|
[<length> | <percentage>]{2}
],]?
<color-stop>[, <color-stop>]+
)</code></pre>
<p>When rendered, however, the color-stops are repeated infinitely in both
directions, with their positions shifted by multiples of the difference
between the last specified color-stop's position and the first specified
color-stop's position. For example, ''repeating-linear-gradient(red 10px, blue 50px)''
is equivalent to ''linear-gradient(..., red -30px, blue 10px, red 10px, blue 50px, red 50px, blue 90px, ...)''.
Note that the last color-stop and first color-stop will always coincide at
the boundaries of each group, which will produce sharp transitions if the
gradient does not start and end with the same color.</p>
<div class=example>
<p>Repeating gradient syntax is basically identical to that of non-repeating gradients:</p>
<pre><code>repeating-linear-gradient(red, blue 20px, red 40px)</code></pre>
<p><img src="repeating1.png" alt=""></p>
<pre><code>repeating-radial-gradient(red, blue 20px, red 40px)</code></pre>
<p><img src="repeating2.png" alt=""></p>
<pre><code>repeating-radial-gradient(20px 30px, circle contain, red, yellow, green 100%, yellow 150%, red 200%)</code></pre>
<p><img src="repeating3.png" alt=""></p>
</div>
<p>If the difference in the positions of the first and last color-stops is
0 (for example, in the value ''linear-gradient(red 10px, blue 10px)''), the
gradient defines a solid-color image with the color of the last color-stop
in the rule (in this case, then, it would simply define a blue image).</p>
<!-- ====================================================================== -->
<h3 id='color-stop-syntax'>
Gradient Color-Stops</h3>
<pre class=prod><code><dfn><color-stop></dfn> = <color> [ <percentage> | <length> ]?</code></pre>
<p>Color-stops are points placed along the line defined by the <i>gradient-line</i>
at the beginning of the rule. Color-stops must be specified in order.
Percentages refer to the length of the gradient-line, with 0% being at the
starting point and 100% being at the ending point. Lengths are measured
from the starting-point in the direction of the ending-point. Color-stops
are usually placed between the starting-point and ending-point, but that's
not required; the gradient-line extends infinitely in both directions, and
a color-stop can be placed at any position on the line.</p>
<p>At each color-stop, the line is the color of the color-stop. Between
two color-stops, the line's color is linearly interpolated between the colors
of the two color-stops, with the interpolation taking place in premultiplied
RGBA space. Before the first color-stop, the line is the color of the first
color-stop. After the last color-stop, the line is the color of the last
color-stop.</p>
<p>The following steps must be applied <em>in order</em> to process the list
of color-stops. After applying these rules, all color-stops will have a
definite position and they will be in ascending order:</p>
<ol>
<li>If the first color-stop does not have a position, its position defaults
to 0%. If the last color-stop does not have a position, its position defaults
to 100%.</li>
<li>If a color-stop has a position that is less than the specified position
of any color-stop before it in the list, its position is changed to be
equal to the largest specified position of any color-stop before it.</li>
<li>If any color-stop still does not have a position, then, for each run
of adjacent color-stops without positions, set their positions so that
they are evenly spaced between the preceding and following color-stops
with positions.</li>
</ol>
<p>If multiple color-stops have the same position, they produce an infinitesimal
transition from the one specified first in the rule to the one specified
last. In effect, the color suddenly changes at that position rather than
smoothly transitioning.</p>
<p class=note>It is recommended that authors not mix different types of units,
such as px, em, or %, in a single rule, as this can cause a color-stop to
unintentionally try to move before an earlier one. For example, the rule
''background-image: linear-gradient(red, yellow 100px, blue 50%)'' would
work as expected as long as the background area is at least 200px tall.
If it was 150px tall, however, the blue color-stop's position would be
equivalent to "75px", which precedes the yellow color-stop, and would be
corrected to a position of 100px.</p>
<p class=note>The definition and implications of "premultiplied" color spaces
are given elsewhere in the technical literature, but a quick primer is given
here to illuminate the process. Given a color expressed as an rgba() 4-tuple,
one can convert this to a premultiplied representation by multiplying the
red, green, and blue components by the alpha component. For example, a
partially-transparent blue may be given as rgba(0,0,255,.5), which would
then be expressed as (0,0,127.5,.5) in its premultiplied representation.
Note that fully opaque colors have the same representation in rgba and
premultiplied-rgba (you multiply the components by 1), and all fully
transparent colors are expressed the same way in the premultiplied
representation (you multiply each component by 0, so no matter what the
source color was in rgba, the premultiplied representation is (0,0,0,0)).
Interpolating colors using the premultiplied representations rather than
the plain rgba representations tends to produce more attractive transitions,
particularly when transitioning from a fully opaque color to fully transparent.</p>
<!-- ====================================================================== -->
<h2 id="sizing">
Sizing Images and Objects in CSS</h2>
<p>Images used in CSS may come from a number of sources, from defined image
formats (such as gif, jpeg, etc), dedicated markup formats (such as SVG), and
CSS-specific formats (such as the linear-gradient() value type defined in this
specification). As well, a document may contain many other types of objects,
such as video, plugins, or nested documents. These images and objects (just
<dfn>objects</dfn> hereafter) may offer many types of sizing information to CSS, or none
at all. This section defines generically the size negotiation model between
the object and the CSS layout algorithms.</p>
<!-- ====================================================================== -->
<h3 id="sizing-terms">
Object-Sizing Terminology</h3>
<p>In order to define this handling, we define a few terms, to make it
easier to refer to various concepts:</p>
<dl>
<dt><dfn>intrinsic dimensions</dfn></dt>
<dd>
<p>An object's intrinsic dimensions are its preferred, natural width,
height, and aspect ratio, if they exist. There can be an <dfn>intrinsic height</dfn>
and <dfn>intrinsic width</dfn>, defining a definite rectangle. (Most bitmap
images fall into this category.) There can be an <dfn>intrinsic aspect ratio</dfn>
defining the relation of the width to the height, but no definite size.
(SVG images designed to scale may fall into this category.) There can be
just an intrinsic height or width. Or there can be no intrinsic dimensions
at all, implying that the object has no preferred size or aspect ratio.
(Embedded documents are often assumed to have no intrinsic size, as are
CSS gradients, defined in this specification.)</p>
<p>If an object (such as an icon) has multiple sizes, then the largest
size is used. If it has multiple aspect ratios of that size (or of no
size), then the aspect ratio closest to the aspect ratio of the
<i>default object size</i> is used.</p>
</dd>
<dt><dfn>specified size</dfn></dt>
<dd>The specified size of an object is given by CSS, such as through the
'object-fit' or 'background-size' properties. The specified size can be a
definite width and height, a set of constraints, or a combination thereof.</dd>
<dt><dfn>concrete object size</dfn></dt>
<dd>The <i>concrete object size</i> is the result of transforming the <i>intrinsic dimensions</i>
into a concrete size, based on the <i>specified size</i> and the
<i>default object size</i>. A <i>concrete object size</i> always has a definite height
and width.</dd>
<dt><dfn>default object size</dfn></dt>
<dd>
<p>The <i>default object size</i> is a rectangle with a definite height and
width used to determine the <i>concrete object size</i> when both the
<i>intrinsic dimensions</i> and <i>specified size</i> are missing dimensions.
It varies based on the context in which that the image is being laid out.</p>
<div class="example">
<p>Below are some examples of default object sizing areas:
<dl>
<dt>'background-image'</dt>
<dd>The <i>default object size</i> is the size of the element's
<a href="http://www.w3.org/TR/css3-background/#background-positioning-area">background
positioning area</a>. [[CSS3BG]]</dd>
<dt>'list-style-image'</dt>
<dd>The <i>default object size</i> is a 1em square. [[!CSS21]]</dd>
<dt>'border-image'</dt>
<dd>The <i>default object size</i> is the size of the element's
<a href="http://www.w3.org/TR/css3-background/#border-image-area">border image area</a>.
[[CSS3BG]]</dd>
<dt>'cursor'</dt>
<dd>The <i>default object size</i> is a UA-defined size that should
be based on the size of a typical cursor on the UA's operating system.
[[!CSS21]]</dd>
<dt>replaced elements</dt>
<dd>The <i>default object size</i> is a rectangle 300px wide and 150px
tall. [[!CSS21]]</dd>
</dl>
<p class=issue>The only reason these are examples is because the proper
place for the normative definitions of default object sizes is in the
definitions for the relevant properties. These are the correct values,
though.</p>
</div>
</dd>
</dl>
<!-- ====================================================================== -->
<h3 id="object-negotiation">
CSS⇋Object Negotiation</h3>
<p>Objects in CSS are sized and rendered as follows:</p>
<ol>
<li>When an image or object is specified in a document, such as through a url()
value in a 'background-image' property or a @src attribute on an <img> element,
CSS queries the object for its <i>intrinsic dimensions</i>.</li>