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<h1>CSS Image Values and Replaced Content Module Level 4</h1>
<pre class='metadata'>
Status: ED
Work Status: Exploring
Shortname: css-images
Level: 4
Group: csswg
ED: https://drafts.csswg.org/css-images-4/
TR: https://www.w3.org/TR/css4-images/
Editor: Tab Atkins Jr., Google, http://xanthir.com/contact/
Editor: Elika J. Etemad / fantasai, Invited Expert, http://fantasai.inkedblade.net/contact, w3cid 35400
Editor: Lea Verou, Invited Expert, http://lea.verou.me/about, w3cid 52258
Abstract: This module contains the features of CSS level 4 relating to the <<image>> type and replaced elements.
It includes and extends the functionality of CSS level 2 [[CSS21]] and in the previous level of this specification [[css3-images]].
The main extensions compared to "CSS Image Values and Replaced Content Module Level 3" [[css3-images]] are several additions to the <<image>> type, such as the ''image()'' notation, the ''element()'' notation, and conic gradients.
Issue Tracking: Tracker http://www.w3.org/Style/CSS/Tracker/products/27
Previous Version: https://www.w3.org/TR/2012/WD-css4-images-20120911/
Ignored Terms: <offset>, background positioning area, border image area, <meetorslice>, <ending-shape>, Map, <image>, invalid image, invalid images, concrete object size, linear-gradient(), radial-gradient(), intrinsic dimensions, default object size, CSS
Ignored Vars: H, P
Include Can I Use Panels: yes
</pre>
<pre class=link-defaults>
spec: css-images-3;
type: function;
text: image-set();
text: cross-fade();
</pre>
Introduction {#intro}
=====================
<em>This section is not normative.</em>
This module introduces additional ways of representing 2D images,
for example as <a section href="#image-notation">a URL with color fallback</a>,
as <a href="#conic-gradients">conic gradients</a>,
or as the <a href="#element-notation">rendering of another element in the document</a>.
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Image Values: the <<image>> type {#image-values}
=================================================
The <<image>> value type denotes a 2D image. It can be a
<a section href="#url-notation">url reference</a>,
<a section href="#image-notation">image notation</a>,
or <a section href="#gradients">gradient notation</a>.
Its syntax is:
<pre class="prod"><dfn><image></dfn> = <<url>> | <<image()>> | <<image-set()>> | <<cross-fade()>> | <<gradient>></pre>
An <<image>> can be used in many CSS properties,
including the 'background-image', 'list-style-image', 'cursor' properties [[!CSS21]]
(where it replaces the <<url>> component in the property's value).
In some cases, an image is invalid,
such as a <<url>> pointing to a resource that is not a valid image format.
An <dfn export lt="invalid image|valid image">invalid image</dfn> is rendered as a solid-color ''transparent'' image with no intrinsic dimensions.
However, <a>invalid images</a> have special behavior in some contexts,
such as the ''image()'' notation.
Image File Formats {#image-file-formats}
----------------------------------------
At minimum, the UA must support the following image file formats
when referenced from an <<image>> value,
for all the properties in which using <<image>> is valid:
<ul>
<li>PNG, as specified in [[!PNG]]
<li>SVG, as specified in [[!SVG11]],
using the <a href="https://www.w3.org/TR/svg-integration/#secure-static-mode">secure static mode</a> (See [[!SVG-INTEGRATION]])
<li>If the UA supports animated <<image>>s,
SVG, as specified in [[!SVG11]],
using the <a href="https://www.w3.org/TR/svg-integration/#secure-animated-mode">secure animated mode</a> (See [[!SVG-INTEGRATION]])
</ul>
The UA may support other file formats as well.
Image References: the ''url()'' notation {#url-notation}
------------------------------------------------------------------------------------------
Note: No change from [[css3-images]].
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Resolution Negotiation: the ''image-set()'' notation {#image-set-notation}
--------------------------------------------------------------------------
Delivering the most appropriate image resolution for a user's device can be a difficult task.
Ideally, images should be in the same resolution as the device they're being viewed in,
which can vary between users.
However, other factors can factor into the decision of which image to send;
for example, if the user is on a slow mobile connection,
they may prefer to receive lower-res images
rather than waiting for a large proper-res image to load.
The ''image-set()'' function allows an author to ignore most of these issues,
simply providing multiple resolutions of an image
and letting the UA decide which is most appropriate in a given situation.
Issue: This solution assumes that resolution is a proxy for filesize,
and therefore doesn't appropriately handle multi-resolution sets of vector images,
or mixing vector images with raster ones (e.g. for icons).
For example, use a vector for high-res,
pixel-optimized bitmap for low-res,
and same vector again for low-bandwidth (because it's much smaller, even though it's higher resolution).
The syntax for ''image-set()'' is:
<pre class='prod'>
<dfn caniuse="css-image-set">image-set()</dfn> = image-set( <<image-set-option>># )
<dfn><image-set-option></dfn> = [ <<image>> | <<string>> ] <<resolution>>
</pre>
Issue: We should add "w" and "h" dimensions as a possibility, and a "format()" function,
to match the functionality of HTML's <a element>picture</a>.
The ''image-set()'' function can not be nested inside of itself,
either directly
or indirectly
(as an argument to another <<image>> type).
Issue: Is this restriction needed?
Each <<string>> inside ''image-set()'' represents a <<url>>.
Every <<image-set-option>> in a given ''image-set()'' must have a different <<resolution>>,
or else the function is invalid.
UAs must make a UA-specific choice of which <<image-set-option>> to load,
based on whatever criteria they find relevant
(such as the resolution of the display,
connection speed,
etc).
The ''image-set()'' then represents the image associated with the URL of that choice.
The image's <a spec=css-images-4>intrinsic resolution</a> is the resolution associated with that choice.
UAs <strong>may</strong> change which <<image-set-option>> they wish to use for a given ''image-set()''
over the lifetime of the page,
if the criteria used to determine which option to choose change significantly enough to make it worthwhile in the UA's estimation.
<div class='example'>
This example shows how to use ''image-set()'' to provide an image in three versions:
a "normal" version,
a "high-res" version,
and an extra-high resolution version for use in high-quality printing
(as printers can have <em>extremely</em> high resolution):
<pre>
background-image: image-set( "foo.png" 1x,
"foo-2x.png" 2x,
"foo-print.png" 600dpi );
</pre>
</div>
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Image Fallbacks and Annotations: the ''image()'' notation {#image-notation}
---------------------------------------------------------------------------
The ''image()'' function allows an author to:
* use <a href="https://www.w3.org/TR/media-frags/">media fragments</a> to clip out a portion of an image
* use a solid color as an image
* fallback to a solid-color image, when the image at the specified url can't be downloaded or decoded
* automatically respect the image orientation specified in the image's metadata
The ''image()'' notation is defined as:
<pre class='prod'>
<dfn>image()</dfn> = image( <<image-tags>>? [ <<image-src>>? , <<color>>? ]! )
<dfn><image-tags></dfn> = [ ltr | rtl ]
<dfn><image-src></dfn> = [ <<url>> | <<string>> ]
</pre>
A <<string>> used in ''image()'' represents a <<url>>.
As usual for URLs in CSS,
relative URLs are resolved to an absolute URL
(as described in Values & Units [[!css-values-3]])
when a specified ''image()'' value is computed.
If the image has an orientation specified in its metadata,
such as EXIF,
the UA must rotate or flip the image to correctly orient it
as the metadata specifies.
### Image Fallbacks ### {#image-fallbacks}
If both a URL and a <<color>> are specified in ''image()'',
then whenever the URL represents an <a>invalid image</a>,
the ''image()'' function renders as if the URL were not specified at all;
it generates a solid-color image as specified in [[#color-images]].
<div class='example'>
The fallback color can be used to ensure that text is still readable
even when the image fails to load.
For example, the following legacy code works fine if the image is rectangular and has no transparency:
<pre class="lang-css">
body { color: black; background: white; }
p.special { color: white; background: url("dark.png") black; }
</pre>
When the image doesn't load,
the background color is still there to ensure that the white text is readable.
However, if the image has some transparency,
the black will be visible behind it,
which is probably not desired.
The ''image()'' function addresses this:
<pre class="lang-css">
body { color: black; background: white; }
p.special { color: white; background: image("dark.png", black); }
</pre>
Now, the black won't show at all if the image loads,
but if for whatever reason the image fails,
it'll pop in and prevent the white text from being set against a white background.
</div>
<!-- Good example for the fallback() function
<div class='example'>
For example, if a future specification defined a way to refer to a specific frame of an animated GIF with a fragment identifier,
an author could write the following to get newer browsers to use the GIF's frame,
and older browsers to instead download the fallback image:
<pre class="lang-css">background-image: image("cat_meme.gif#frame=5", "lolcat.png");</pre>
</div>
-->
### Image Fragments ### {#image-fragments}
When a URL specified in ''image()'' represents a portion of a resource
(e.g. by the use of <a href="https://www.w3.org/TR/media-frags/#naming-space">media fragment identifiers</a>)
that portion is clipped out of its context and used as a standalone image.
<div class="example">
For example, given the following image and CSS:
<a href="images/sprites.svg">
<img src="images/sprites.svg" height="20" width="180" alt="[9 circles, with 0 to 8 eighths filled in]">
</a>
<pre class="lang-css">background-image: image('sprites.svg#xywh=40,0,20,20')</pre>
...the background of the element will be the portion of the image that starts at (40px,0px) and is 20px wide and tall,
which is just the circle with a quarter filled in.
</div>
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 specified via ''image()''. [[!MEDIA-FRAGS]]
<div class='example'>
Note that image fragments can also be used with the ''url()'' notation.
However, a legacy UA that doesn't understand the media fragments notation
will ignore the fragment and simply display the entirety of the image.
Since the ''image()'' notation requires UAs to support media fragments,
authors can take advantage of CSS's forward-compatible parsing rules
to provide a fallback when using an image fragment URL:
<pre class="lang-css">
background-image: url('swirl.png'); /* old UAs */
background-image: image('sprites.png#xywh=10,30,60,20'); /* new UAs */
</pre>
</div>
If a URL uses a fragment identifier syntax that the implementation does not understand,
or does not consider valid for that type of image,
the URL must be treated as representing an <a>invalid image</a>.
Note: This error-handling is limited to ''image()'',
and not in the definition of URL,
for legacy compat reasons.
### Solid-color Images ### {#color-images}
If the ''image()'' function is specified with only a <<color>> argument (no URL),
it represents a solid-color image of the specified color with no intrinsic dimensions.
<div class='example'>
For example,
one can use this as a simple way to "tint" a background image,
by overlaying a partially-transparent color over the top of the other image:
<pre class="lang-css">background-image: image(rgba(0,0,255,.5)), url("bg-image.png");</pre>
'background-color' does not work for this,
as the solid color it generates always lies <em>beneath</em> all the background images.
</div>
### Bidi-sensitive Images ### {#bidi-images}
Before listing any <code><image-src>s</code>,
the author may specify a directionality for the image,
similar to adding a <code>dir</code> attribute to an element in HTML.
If a directional image is used on or in an element with opposite <a href="https://www.w3.org/TR/CSS21/visuren.html#propdef-direction">direction</a>,
the image must be flipped in the inline direction
(as if it was transformed by, e.g., <code>scaleX(-1)</code>, if the inline direction is the X axis).
Note: Absent this declaration,
images default to no directionality at all,
and thus don't care about the directionality of the surrounding element.
<div class='example'>
A list may use an arrow for a bullet that points into the content.
If the list can contain both LTR and RTL text,
though, the bullet may be on the left or the right,
and an image designed to point into the text on one side will point out of the text on the other side.
This can be fixed with code like:
<pre class="lang-html">
<ul style="list-style-image: image(ltr 'arrow.png');">
<li dir='ltr'>My bullet is on the left!</li>
<li dir='rtl'>MY BULLET IS ON THE RIGHT!</li>
</ul>
</pre>
This should render something like:
<pre class="lang-html">
⇒ My bullet is on the left!
!THGIR EHT NO SI TELLUB YM ⇐
</pre>
In LTR list items, the image will be used as-is.
In the RTL list items, however,
it will be flipped in the inline direction,
so it still points into the content.
</div>
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Resolution Negotiation: the ''image-set()'' notation {#image-set-notation}
--------------------------------------------------------------------------
Note: No change from [[css3-images]].
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Combining images: the ''cross-fade()'' notation {#cross-fade-function}
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Note: No change from [[css3-images]].
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<h3 id='element-notation' caniuse="css-element-function">Using Elements as Images: the ''element()'' notation</h3>
The ''element()'' function allows an author to use an element in the document as an image.
As the referenced element changes appearance,
the image changes as well.
This can be used, for example,
to create live previews of the next/previous slide in a slideshow,
or to reference a canvas element for a fancy generated gradient or even an animated background.
Note: The ''element()'' function only reproduces the <em>appearance</em> of the referenced element,
not the actual content and its structure.
Authors should only use this for decorative purposes,
and must not use ''element()'' to reproduce an element with significant content across the page.
Instead, just insert multiple copies of the element into the document.
The syntax for ''element()'' is:
<pre class=prod><dfn>element()</dfn> = element( <<id-selector>> )</pre>
where <<id-selector>> is an ID selector [[!SELECT]].
<p class='issue'>
Do we need to be able to refer to elements in external documents
(such as SVG paint servers)?
Or is it enough to just use url() for this?
<p class='issue'>
This name conflicts with a somewhat similar function in GCPM.
This needs to be resolved somehow.
<p class='issue'>
Want the ability to do "reflections" of an element,
either as a background-image on the element or in a pseudo-element.
This needs to be specially-handled to avoid triggering the cycle-detection.
<p class='issue'>
When we have overflow:paged,
how can we address a single page in the view?
The ''element()'' function references the element matched by its argument.
The ID is first looked up in the <a idl>elementSources</a> map,
as described in that section.
If it's not found,
it's then matched against the document.
If multiple elements are matched,
the function references the first such element.
The image represented by the ''element()'' function can vary based on whether the element is visible in the document:
<dl>
<dt>
an <a lt="element-not-rendered">element that is rendered</a>,
is not a descendant of a replaced element,
and generates a <a spec=css21>stacking context</a>
<dd>
The function represents an image with its intrinsic size equal to the <dfn export>decorated bounding box</dfn> of the referenced element:
<ul>
<li>
for an element rendered using a CSS rendering model,
the <a>decorated bounding box</a> is the smallest axis-aligned rectangle
that contains the <a href="https://www.w3.org/TR/2011/CR-css3-background-20110215/#border-image-area">border image areas</a> of all the fragments of the principal box
<li>
for an element rendered using the SVG rendering model,
<a href="https://www.w3.org/TR/SVGTiny12/intro.html#TermDecoratedBoundingBox">the decorated bounding box is defined by SVG</a>
</ul>
Note: Because images clip anything outside their bounds by default,
this means that decorations that extend outside the <a>decorated bounding box</a>,
like box shadows,
may be clipped.
The image is constructed by rendering the referenced element and its descendants
(at the same size that they would be in the document)
over an infinite ''transparent'' canvas,
positioned so that the edges of the <a>decorated bounding box</a> are flush with the edges of the image.
<p class='issue'>
Requiring some degree of stacking context on the element appears to be required for an efficient implementation.
Do we need a full stacking context, or just a pseudo-stacking context?
Should it need to be a stacking context normally,
or can we just render it as a stacking context when rendering it to element()?
If the referenced element has a transform applied to it or an ancestor,
the transform must be ignored when rendering the element as an image. [[!CSS3-TRANSFORMS]]
If the referenced element is broken across pages,
the element is displayed as if the page content areas were joined flush in the pagination direction,
with pages' edges corresponding to the initial containing block's start edge aligned.
<span class='note'>Elements broken across lines or columns are just rendered with their <a>decorated bounding box</a>.</span>
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.
<div class='example'>
As a somewhat silly example, a <code><p></code> element can be reused as a background elsewhere in the document:
<pre class="lang-html">
<style>
#src { color: white; background: lime; width: 300px; height: 40px; position: relative; }
#dst { color: black; background: element(#src); padding: 20px; margin: 20px 0; }
</style>
<p id='src'>I'm an ordinary element!</p>
<p id='dst'>I'm using the previous element as my background!</p>
</pre>
<img src="images/element-function.png" alt="">
</div>
<dt>an <a lt='element-not-rendered'>element that is not rendered</a>, but which provides a <a>paint source</a>
<dd>
The function represents an image with the intrinsic size and appearance of the <a>paint source</a>.
The host language defines the size and appearance of paint sources.
<div class='example'>
For example, the ''element()'' function can reference an SVG <code><pattern></code> element in an HTML document:
<pre class=lang-html>
<!DOCTYPE html>
<svg>
<defs>
<pattern id='pattern1'>
<path d='...'>
</pattern>
</defs>
</svg>
<p style="background: element(#pattern1)">
I'm using the pattern as a background!
If the pattern is changed or animated,
my background will be updated too!
</p>
</pre>
HTML also defines that a handful of elements,
such as <code><canvas></code>, <code><img></code>, and <code><video></code>,
provide a paint source.
This means that CSS can, for example,
reference a canvas that's being drawn into,
but not displayed in the page:
<pre class=lang-html>
<!DOCTYPE html>
<script>
var canvas = document.querySelector('#animated-bullet');
canvas.width = 20; canvas.height = 20;
drawAnimation(canvas);
</script>
<canvas id='animated-bullet' style='display:none'></canvas>
<ul style="list-style-image: element(#animated-bullet);">
<li>I'm using the canvas as a bullet!</li>
<li>So am I!</li>
<li>As the canvas is changed over time with Javascript,
we'll all update our bullet image with it!</li>
</ul>
</pre>
</div>
<dt>anything else
<dd>
The function represents an <a>invalid image</a>.
<div class='example'>
For example, all of the following ''element()'' uses will result in a transparent background:
<pre class=lang-html>
<!DOCTYPE html>
<p id='one' style="display:none; position: relative;">one</p>
<iframe src="http://example.com">
<p id='two' style="position: relative;">I'm fallback content!</p>
</iframe>
<ul>
<li style="background: element(#one);">
A display:none element isn't rendered, and a P element
doesn't provide a paint source.
</li>
<li style="background: element(#two);">
The descendants of a replaced element like an IFRAME
can't be used in element() either.
</li>
<li style="background: element(#three);">
There's no element with an id of "three", so this also
gets rendered as a transparent image.
</li>
</ul>
</pre>
</div>
</dl>
An element is <dfn export id='element-not-rendered' lt='element-not-rendered'>not rendered</dfn> if it does not have an associated box.
This can happen, for example,
if the element or an ancestor is ''display:none''.
Host languages may define additional ways in which an element can be considered not rendered;
for example, in SVG,
any descendant of a <code><defs></code> element is considered to be not rendered.
<div class='example'>
The ''element()'' function can be put to many uses.
For example, it can be used to show a preview of the previous or next slide in a slideshow:
<pre class=lang-html>
<!DOCTYPE html>
<script>
function navigateSlides() {
var currentSlide = ...;
document.querySelector('#prev-slide').id = '';
document.querySelector('#next-slide').id = '';
currentSlide.previousElementSibling.id = 'prev-slide';
currentSlide.nextElementSibling.id = 'next-slide';
}
</script>
<style>
.slide {
/* Need to be a stacking context to be element()-able. */
position: relative;
}
#prev-preview, #next-preview {
position: fixed;
...
}
#prev-preview { background: element(#prev-slide); }
#next-preview { background: element(#next-slide); }
</style>
<a id='prev-preview'>Previous Slide</a>
<a id='next-preview'>Next Slide</a>
<section class='slide'>...</section>
<section class='slide current-slide'>...</section>
...
</pre>
In this example, the <code>navigateSlides</code> function updates the ids of the next and previous slides,
which are then displayed in small floating boxes alongside the slides.
Since you can't interact with the slides through the ''element()'' function (it's just an image),
you could even use <code>click</code> handlers on the preview boxes to help navigate through the page.
</div>
<h4 id='paint-sources'>
Paint Sources</h4>
Host languages may define that some elements provide a <dfn export>paint source</dfn>.
Paint sources have an intrinsic appearance and can obtain a <a>concrete object size</a>
without having to do layout or rendering,
and so may be used as images even when they're <a lt='element-not-rendered'>not rendered</a>.
In HTML, the <code><img></code>, <code><video></code>, and <code><canvas></code> elements provide paint sources
(defined in each element's section in <a href='http://www.whatwg.org/specs/web-apps/current-work/multipage/'>HTML5</a>).
In SVG, any element that provides a <a href='https://www.w3.org/TR/SVG/pservers.html'>paint server</a> provides a paint source.
<span class='note'>Note: In SVG1.1,
the <code><linearGradient></code>,
<code><radialGradient></code>,
and <code><pattern></code> elements
provide paint sources.</span>
They are drawn as described in the spec,
with the coordinate systems defined as follows:
<dl>
<dt>objectBoundingBox
<dd>
The coordinate system has its origin at the top left corner of the rectangle defined by the <a>concrete object size</a> that it's being drawn into,
and the same width and height as the <a>concrete object size</a>.
A single <a href="https://www.w3.org/TR/SVG/coords.html#Units">user coordinate</a> is the width and height of the <a>concrete object size</a>.
<dt>userSpaceOnUse
<dd>
The coordinate system has its origin at the top left corner of the rectangle defined by the <a>concrete object size</a> that it's being drawn into,
and the same width and height as the <a>concrete object size</a>.
<a href="https://www.w3.org/TR/SVG/coords.html#Units">User coordinates</a> are sized equivalently to the CSS ''px'' unit.
</dl>
Note: It is expected that a future version of this module will define ways to refer to paint sources in external documents,
or ones that are created solely by script and never inserted into a document at all.
<h4 id='elementsources'>
Using Out-Of-Document Sources: the <code>ElementSources</code> interface</h4>
The ''element()'' function normally selects elements within a document,
but elements that provide a <a>paint source</a> don't necessarily need to be in-document.
For example, an HTML <code><canvas></code> element can be created, maintained, and drawn into entirely in script,
with no need for it to be inserted into the document directly.
All that's needed is a way to refer to the element,
as an ID selector cannot select elements outside of the document.
The <a idl>elementSources</a> Map object provides this.
<pre class='idl'>
partial interface CSS {
[SameObject] readonly attribute Map elementSources;
};
</pre>
Any entries in the <a idl>elementSources</a> map with a string key
and a value that is an object providing a <a>paint source</a>
are made available to the ''element()'' function.
Whenever ''element()'' uses an <<id-selector>>,
the ID's value (without the leading <code>#</code> character)
is first looked up in the <a idl>elementSources</a> map:
<ul>
<li>
If it's found,
and the object associated with it provides a <a>paint source</a>,
the ''element()'' function represents that paint source.
<li>
If it's found,
but the object associated with it <em>doesn't</em> provide a <a>paint source</a>,
the ''element()'' function represent an <a>invalid image</a>.
<li>
If the ID isn't found in the map at all,
it's then looked for in the document as normal.
</ul>
<p class='issue'>
This reuse of the ID selector matches Moz behavior.
I'm trying to avoid slapping a <<custom-ident>> right in the beginning of the grammar,
as that eats too much syntax-space.
Another possibility, though, is to start the value with a language-defined keyword
<em>followed by</em> a <<custom-ident>>,
like ''element(external fancy)'' or something.
Naming suggestions welcome.
<div class='example'>
For example, fancy animating backgrounds can be done with an external canvas:
<pre class=lang-html>
<script>
var bg = document.createElement('canvas');
bg.height = 200;
bg.width = 1000;
drawFancyBackground(bg);
CSS.elementSources.set('fancy', bg);
</script>
<style>
h1 {
background-image: element(#fancy);
}
</style>
</pre>
As the "fancy" canvas is drawn into and animated,
the backgrounds of all the H1 elements will automatically update in tandem.
Note that the <a idl>elementSources</a> map is consulted <em>before</em> the document
to match the ID selector,
so even if there's an element in the document that would match ''#fancy'',
the backgrounds will still predictably come from the <a idl>elementSources</a> value instead.
</div>
<h4 id='element-cycles'>
Cycle Detection</h4>
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.
The dependency graph consists of edges such that:
<ul>
<li>
every element depends on its children
<li>
for any element A with a property using the ''element()'' function pointing to an element B,
A depends on B
<li>
if a host language defines a way for elements to refer to the rendering of other elements,
the referencing element depends on the referenced element.
For example, in SVG,
a <code><use></code> element depends on the element it referenced.
</ul>
If the graph contains a cycle,
any ''element()'' functions participating in the cycle are <a>invalid images</a>.
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<h2 id="gradients">
Gradients</h2>
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 gradient notations 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.
The syntax of a <<gradient>> is:
<pre class=prod>
<dfn><gradient></dfn> = [
<<linear-gradient()>> | <<repeating-linear-gradient()>> |
<<radial-gradient()>> | <<repeating-radial-gradient()>> |
<<conic-gradient()>> | <<repeating-conic-gradient()>> ]
</pre>
<div class=example>
As with the other <<image>> types defined in this specification,
gradients can be used in any property that accepts images.
For example:
<ul class=lang-css>
<li><code>background: linear-gradient(white, gray);</code>
<li><code>list-style-image: radial-gradient(circle, #006, #00a 90%, #0000af 100%, white 100%)</code>
</ul>
</div>
A gradient is drawn into a box with the dimensions of the <a>concrete object size</a>,
referred to as the <dfn export>gradient box</dfn>.
However, the gradient itself has no <a>intrinsic dimensions</a>.
<div class='example'>
For example, if you use a gradient as a background,
by default the gradient will draw into a <a>gradient box</a> the size of the element's padding box.
If 'background-size' is explicitly set to a value such as ''100px 200px'',
then the <a>gradient box</a> will be 100px wide and 200px tall.
Similarly, for a gradient used as a 'list-style-image',
the box would be a 1em square,
which is the <a>default object size</a> for that property.
</div>
Gradients are specified by defining the <dfn>starting point</dfn> and <dfn>ending point</dfn>
of a <dfn export>gradient line</dfn>
(which, depending on the type of gradient,
may be technically a line, or a ray, or a spiral),
and then specifying colors at points along this line.
The colors are smoothly blended to fill in the rest of the line,
and then each type of gradient defines how to use the color of the <a>gradient line</a> to produce the actual gradient.
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Linear Gradients: the ''linear-gradient()'' notation {#linear-gradients}
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Radial Gradients: the ''radial-gradient()'' notation {#radial-gradients}
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Note: No change from [[css3-images]].
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<h3 id='conic-gradients'>
Conic Gradients: the ''conic-gradient()'' notation</h3>
A conic gradient starts by specifying the center of a circle,
similar to radial gradients,
except that conic gradient color-stops are placed <em>around</em> the circumference of the circle,
rather than on a line emerging from the center,
causing the color to smoothly transition as you spin around the center,
rather than as you progress outward from the center.
A conic gradient is specified by indicating a rotation angle, the center of the gradient,
and then specifying a list of color-stops.
Unlike linear and radial gradients,
whose color-stops are placed by specifying a <<length>>,
the color-stops of a conic gradient are specified with an <<angle>>.
Rays are then drawn emerging from the center and pointing in all directions,
with the color of each ray equal to the color of the gradient-line where they intersect it.
Note: These gradients are called "conic" or "conical"
because, if the color stops are chosen to be significantly lighter on one side than the other,
it produces a pattern that looks like a cone observed from above.
They are also known as "angle" gradients in some contexts,
since they are produced by varying the rotation angle of a ray.
<div class=example>
<div style="overflow: hidden">
<img style="float: right; margin-left: 1em;" src='images/conic-diagram.png' alt="[An image showing a box with a background shading gradually clockwise from white to black, starting from the top. A gradient circle is shown, and the colors at 0 and 216 degrees respectively.]">
This example visually illustrates how ''conic-gradient(at 25% 30%, white, black 60%)'' would be drawn. Note that since color stop positions always resolve to angles, the only effect of the center center is a 2D translation of the gradient, i.e. it does not affect how the gradient is drawn.
</div>
</div>
<h4 id='conic-gradient-syntax' class='no-toc'>
''conic-gradient()'' Syntax</h4>
The syntax for a conic gradient is:
<pre class='prod'>
<dfn>conic-gradient()</dfn> = conic-gradient(
[ from <<angle>> ]? [ at <<position>> ]?,
<<angular-color-stop-list>>
)
</pre>
The arguments are defined as follows:
<dl dfn-type=value dfn-for="conic-gradient(), repeating-conic-gradient()">
<dt><dfn><<angle>></dfn>
<dd>
The entire gradient is rotated by this angle.
If omitted, defaults to ''0deg''.
<dt><dfn><<position>></dfn>
<dd>
Determines the <dfn dfn>gradient center</dfn> of the gradient.
The <<position>> value type (which is also used for 'background-position')
is defined in [[css-values-3]],
and is resolved using the center-point as the object area
and the <a>gradient box</a> as the positioning area.
If this argument is omitted, it defaults to ''background-position/center''.
</dl>
<p class='issue'>
Usually in conic gradients the sharp transition at 0deg is undesirable, which is typically avoided by making sure the first and last color stops are the same color. Perhaps it would be useful to have a keyword for automatically achieving this.
<p class='issue'>
Would a radius (inner & outer) for clipping the gradient be useful? If so, we could also support lengths in color stop positions, since we now have a specific radius.
<p class='issue'>
Are elliptical conic gradients useful? Do graphics libraries support them?
<h4 id='conic-color-stops' class='no-toc'>
Placing Color Stops</h4>
Color stops are placed on a <a>gradient line</a> that curves around the center in a circle,
with both the 0% and 100% locations at 0deg.
Just like linear gradients,
0deg points to the top of the page,
and increasing angles correspond to clockwise movement around the circle.
Note: It may be more helpful to think of the gradient line as forming a spiral,
where only the segment from 0deg to 360deg is rendered.
This avoids any confusion about "overlap" when you have angles outside of the rendered region.
A color-stop can be placed at a location before 0% or after 100%;
though these regions are never directly consulted for rendering,
color stops placed there can affect the color of color-stops within the rendered region
through interpolation or repetition (see <a href="#repeating-gradients">repeating gradients</a>).
For example, ''conic-gradient(red -50%, yellow 150%)'' produces a conic gradient
that starts with a reddish-orange color at 0deg (specifically, #f50),
and transitions to an orangish-yellow color at 360deg (specifically, #fa0).
The color of the gradient at any point is determined by first finding the unique ray
anchored at the center of the gradient that passes through the given point.