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<pre class='metadata'>
Title: CSS Images Module Level 3
Status: ED
Shortname: css-images
Level: 3
Group: csswg
ED: https://drafts.csswg.org/css-images-3/
TR: https://www.w3.org/TR/css3-images/
Work Status: Testing
Editor: Tab Atkins Jr., Google, http://xanthir.com/contact/, w3cid 42199
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 3 relating to the <<image>> type and some replaced elements.
It includes and extends the functionality of CSS level 2 [[CSS2]].
The main extensions compared to CSS2.1 are the generalization of the <<url>> type to the <<image>> type,
several additions to the <<image>> type, a generic sizing algorithm for images and other replaced content in CSS,
definitions for interpolating several <<image>> types,
and several properties controlling the interaction of replaced elements and CSS's layout models.
At Risk: 'image-orientation'
Issue Tracking: Tracker http://www.w3.org/Style/CSS/Tracker/products/27
Previous Version: https://www.w3.org/TR/2012/CR-css3-images-20120417/
Previous Version: https://www.w3.org/TR/2012/WD-css3-images-20120112/
Ignored Terms: <offset>, background positioning area, border image area, <meetorslice>, <ending-shape>, Map, center, content
Ignored Vars: start image, end image
Link Defaults: css2 (property) display
Include Can I Use Panels: yes
</pre>
Introduction {#intro}
=====================
In CSS Levels 1 and 2,
image values, such as those used in the 'background-image' property,
could only be given by a single URL value.
This module introduces additional ways of representing 2D images,
for example as <a section href="#gradients">a gradient</a>.
This module also defines several properties for <a section href="#image-processing">manipulating raster images</a>
and for <a section href="#the-object-fit">sizing</a>
or <a section href="#the-object-position">positioning</a>
replaced elements such as images within the box determined by the CSS layout algorithms.
It also defines in a generic way CSS's <a section href="#sizing">sizing algorithm</a> for images and other similar replaced elements.
<em>This subsection (above) is not normative.</em>
Module Interactions {#placement}
--------------------------------
This module defines and extends the <<image>> value type
defined in [[!CSS-VALUES-3]].
It also replaces the <<url>> type with <<image>>
in the 'background-image', 'cursor', and 'list-style-image' definitions in CSS1 and CSS2
and adds <<image>> as an alternative to <<url>>
in the 'content' property's value.
It is presumed that CSS specifications beyond CSS2.1
will use the <<image>> notation in place of <<url>>
where 2D images are expected.
(See e.g. [[CSS3BG]].)
None of the properties defined in this module, only 'image-rendering'
applies to <code>::first-line</code> and <code>::first-letter</code>.
Values {#values}
----------------
This specification follows
the <a href="https://www.w3.org/TR/CSS2/about.html#property-defs">CSS property definition conventions</a> from [[!CSS2]].
Value types not defined in this specification are defined in CSS Level 2 Revision 1 [[!CSS2]].
Other CSS modules may expand the definitions of these value types:
for example [[CSS-VALUES-3]], when combined with this module,
adds the ''initial'' keyword as a possible property value.
In addition to the property-specific values listed in their definitions,
all properties defined in this specification also accept the
<a href="https://www.w3.org/TR/CSS2/cascade.html#value-def-inherit">inherit</a>
keyword as their property value.
For readability it has not been repeated explicitly.
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Image Values: the <<image>> type {#image-values}
================================================
The <<image>> value type denotes a 2D image.
It can be a
<a href="#url-notation">url reference</a>
or a <a href="#gradients">color gradient</a>.
Its syntax is:
<pre class="prod"><dfn><image></dfn> = <<url>> | <<cross-fade()>> | <<gradient>></pre>
An <<image>> can be used in many CSS properties,
including the 'background-image', 'list-style-image', 'cursor' properties [[!CSS2]]
(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> can trigger error-handling clauses
in some contexts.
For example, an <a>invalid image</a> in 'list-style-image'
is treated as ''list-style-type/none'',
allowing the 'list-style-type' to render in its place. [[CSS2]]
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Image References: the ''url()'' notation {#url-notation}
------------------------------------------------------------------------------------------
The simplest way to indicate an image is to reference an image file by URL.
This can be done with the ''url()'' notation, defined in [[CSS-VALUES-3]].
<div class="example">
In the example below, a background image is specified with ''url()''syntax:
<pre>background-image: url(wavy.png);</pre>
</div>
If the UA cannot download, parse, or otherwise successfully display
the contents at the URL as an image
(i.e. if the image is not fully <a href="https://html.spec.whatwg.org/multipage/webappapis.html#concept-imagebitmap-good">fully decodable</a>)
it must be treated as an <a>invalid image</a>.
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Gradients {#gradients}
======================
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()>>
</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:
* <code>background: linear-gradient(white, gray);</code>
* <code>list-style-image: radial-gradient(circle, #006, #00a 90%, #0000af 100%, white 100%)</code>
</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 geometrically be 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}
------------------------------------------------------------------------
A linear gradient is created by specifying a straight <a>gradient line</a>,
and then several colors placed along that line.
The image is constructed by creating an infinite canvas
and painting it with lines perpendicular 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.
<h4 class='no-toc' id='linear-gradient-syntax'>linear-gradient() syntax</h4>
The linear gradient syntax is:
<pre class=prod>
<dfn>linear-gradient()</dfn> = linear-gradient(
[ <<angle>> | to <<side-or-corner>> ]? ,
<<color-stop-list>>
)
<dfn><side-or-corner></dfn> = [left | right] || [top | bottom]
</pre>
The first argument to the function specifies the <a>gradient line</a>,
which gives the gradient a direction and determines how color-stops are positioned.
It may be omitted;
if so, it defaults to ''to bottom''.
The <a>gradient line's</a> direction may be specified in two ways:
: using <<angle>>
:: For the purpose of this argument,
''0deg'' points upward,
and positive angles represent clockwise rotation,
so ''90deg'' point toward the right.
The unit identifier may be omitted if the <<angle>> is zero.
: using keywords
:: If the argument is ''to top'', ''to right'', ''to bottom'', or ''to left'',
the angle of the <a>gradient line</a> is ''0deg'', ''90deg'', ''180deg'', or ''270deg'', respectively.
If the argument instead specifies a corner of the box such as ''to top left'',
the <a>gradient line</a> must be angled such that it points into the same quadrant as the specified corner,
and is perpendicular to a line intersecting the two neighboring corners of the <a>gradient box</a>.
<span class='note'>This causes a color-stop at 50% to intersect the two neighboring corners (see <a href='#corner-gradient-example'>example</a>).</span>
Starting from the center of the <a>gradient box</a>,
extend a line at the specified angle in both directions.
The ending point is the point on the <a>gradient line</a>
where a line drawn perpendicular to the <a>gradient line</a>
would intersect the corner of the <a>gradient box</a> in the specified direction.
The starting point is determined identically, but in the opposite direction.
Note: It is expected that the next level of this module will provide the ability to define the gradient's direction relative to the current text direction and writing-mode.
<div class=example>
<div style="overflow: hidden">
<img style="float: right; margin-left: 1em;" src='images/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.]">
This example illustrates visually how to calculate the <a>gradient line</a> from the rules above.
This shows the starting and ending point of the <a>gradient line</a>,
along with the actual gradient,
produced by an element with ''background: linear-gradient(45deg, white, black);''.
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.
</div>
</div>
<div class='note'>
Given:
* <var>A</var> the angle (in any quadrant) defining the gradient line’s direction such that 0 degrees points upwards and positive angles represent clockwise rotation,
* <var>W</var> the width of the gradient box,
* <var>H</var> the height of the gradient box,
The length of the gradient line (between the <a>starting point</a> and <a>ending point</a>) is:
<code>abs(<var>W</var> * sin(<var>A</var>)) + abs(<var>H</var> * cos(<var>A</var>))</code>
</div>
The gradient's color stops are typically placed between
the <a>starting point</a> and <a>ending point</a> on the <a>gradient line</a>,
but this isn't required:
the <a>gradient line</a> extends infinitely in both directions.
The starting point and ending point are merely arbitrary location markers,
the starting point defining 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%.
The color of a linear gradient at any point is determined
by finding the unique line passing through that point that is perpendicular to the <a>gradient line</a>.
The point's color is the color of the <a>gradient line</a>
at the point where this line intersects it.
<h4 class='no-toc' id='linear-gradient-examples'>Linear Gradient Examples</h4>
All of the following ''linear-gradient()'' examples are presumed to be backgrounds applied to a box that is 200px wide and 100px tall.
<div class=example>
Below are various ways of specifying a basic vertical gradient:
<pre>
linear-gradient(yellow, blue);
linear-gradient(to bottom, yellow, blue);
linear-gradient(180deg, yellow, blue);
linear-gradient(to top, blue, yellow);
linear-gradient(to bottom, yellow 0%, blue 100%);
</pre>
<img src="images/linear1.png" alt="" >
</div>
<div class=example>
This demonstrates the use of an angle in the gradient.
Note that, though the angle is not exactly the same as the angle between the corners,
the <a>gradient line</a> is still sized so as to make the gradient yellow exactly at the upper-left corner,
and blue exactly at the lower-right corner.
<pre>
linear-gradient(135deg, yellow, blue);
linear-gradient(-45deg, blue, yellow);
</pre>
<img src="images/linear3.png" alt="" >
</div>
<div class=example>
This demonstrates a 3-color gradient, and how to specify the location of a stop explicitly:
<pre>linear-gradient(yellow, blue 20%, #0f0);</pre>
<img src="images/linear4.png" alt="" >
</div>
<div class=example id='corner-gradient-example'>
This demonstrates a corner-to-corner gradient specified with keywords.
Note how the gradient is red and blue exactly in the bottom-left and top-right corners, respectively,
exactly like the second example.
Additionally, the angle of the gradient is automatically computed so that the color at 50% (in this case, white)
stretches across the top-left and bottom-right corners.
<pre>linear-gradient(to top right, red, white, blue)</pre>
<object data="images/gradient1.svg" width="200" height="100">(Image requires SVG)</object>
</div>
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Radial Gradients: the ''radial-gradient()'' notation {#radial-gradients}
------------------------------------------------------------------------
In a radial gradient,
rather than colors smoothly fading from one side of the <a>gradient box</a> to the other
as with linear gradients,
they instead emerge from a single point
and smoothly spread outward in a circular or elliptical shape.
A radial gradient is specified by indicating the center of the gradient
(where the 0% ellipse will be)
and the size and shape of the <dfn>ending shape</dfn>
(the 100% ellipse).
Color stops are given as a list,
just as for ''linear-gradient()''.
Starting from the <a>gradient center</a>
and progressing towards (and potentially beyond) the <a>ending shape</a>,
uniformly-scaled concentric ellipses are drawn and colored
according to the specified color stops.
<h4 class='no-toc' id='radial-gradient-syntax'>radial-gradient() Syntax</h4>
The radial gradient syntax is:
<pre>
<dfn>radial-gradient()</dfn> = radial-gradient(
[ <<ending-shape>> || <<size>> ]? [ at <<position>> ]? ,
<<color-stop-list>>
)
</pre>
<div class=example>
Here is an example of a circular radial gradient 5em wide
and positioned with its center in the top left corner:
<pre>radial-gradient(5em circle at top left, yellow, blue)</pre>
</div>
Note: A future level may add the ability to move the focus of the gradient,
as in the original -webkit-gradient() function.
See <a href="http://lists.w3.org/Archives/Public/www-style/2011Nov/0210.html">proposal</a>
tracked in <a href="https://github.com/w3c/csswg-drafts/issues/1575">Issue 1575</a>
for "from <<position>>" and "from offset <<offset>>".
The arguments are defined as follows:
<dl dfn-type=value dfn-for="radial-gradient(), repeating-radial-gradient()">
<dt><dfn><<position>></dfn>
<dd>
Determines the <dfn dfn lt="gradient center">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 ''<position>/center''.
<dt><dfn><<ending-shape>></dfn>
<dd>
Can be either <dfn value for="<ending-shape>">circle</dfn> or <dfn value for="<ending-shape>">ellipse</dfn>;
determines whether the gradient's <a>ending shape</a> is a circle or an ellipse, respectively.
If <<ending-shape>> is omitted,
the <a>ending shape</a> defaults to a circle
if the <<size>> is a single <<length>>,
and to an ellipse otherwise.
<dt><dfn><<size>></dfn>
<dd>
Determines the size of the gradient's <a>ending shape</a>.
If omitted it defaults to ''farthest-corner''.
It can be given explicitly or by keyword.
For the purpose of the keyword definitions,
consider the <a>gradient box</a> edges as extending infinitely in both directions,
rather than being finite line segments.
If the ending-shape is an ellipse,
its axises are aligned with the horizontal and vertical axises.
Both ''circle'' and ''ellipse'' gradients accept the following keywords as their <dfn type for=''><size></dfn>:
<dl dfn-for="<size>">
<dt><dfn>closest-side</dfn>
<dd>
The <a>ending shape</a> is sized so that
it exactly meets the side of the <a>gradient box</a> closest to the gradient's center.
If the shape is an ellipse,
it exactly meets the closest side in each dimension.
<dt><dfn>farthest-side</dfn>
<dd>
Same as ''closest-side'',
except the <a>ending shape</a> is sized based on the farthest side(s).
<dt><dfn>closest-corner</dfn>
<dd>
The <a>ending shape</a> is sized so that
it passes through the corner of the <a>gradient box</a> closest to the gradient's center.
If the shape is an ellipse,
the <a>ending shape</a> is given the same aspect-ratio it would have if ''closest-side'' were specified.
<dt><dfn>farthest-corner</dfn>
<dd>
Same as ''closest-corner'',
except the <a>ending shape</a> is sized based on the farthest corner.
If the shape is an ellipse,
the <a>ending shape</a> is given the same aspect ratio it would have if ''farthest-side'' were specified.
</dl>
If <<ending-shape>> is specified as ''circle'' or is omitted,
the <<size>> may be given explicitly as:
<dl>
<dt><dfn for="<size>"><length></dfn>
<dd>
Gives the radius of the circle explicitly.
Negative values are invalid.
Note: Percentages are <em>not</em> allowed here;
they can only be used to specify the size of an elliptical gradient,
not a circular one.
This restriction exists because there is are multiple reasonable answers as to which dimension the percentage should be relative to.
A future level of this module may provide the ability to size circles with percentages,
perhaps with more explicit controls over which dimension is used.
</dl>
If <<ending-shape>> is specified as ''ellipse'' or is omitted,
<<size>> may instead be given explicitly as:
<dl>
<dt><dfn for="<size>"><<length-percentage>>{2}</dfn>
<dd>
Gives the size of the ellipse explicitly.
The first value represents the horizontal radius,
the second the vertical radius.
Percentages values are relative to the corresponding dimension of the <a>gradient box</a>.
Negative values are invalid.
</dl>
</dl>
<div class='note'>
Expanded with the above definitions,
the grammar becomes:
<pre>
radial-gradient() = radial-gradient(
[ [ circle || <<length>> ] [ at <<position>> ]? , |
[ ellipse || <<length-percentage>>{2} ] [ at <<position>> ]? , |
[ [ circle | ellipse ] || <<extent-keyword>> ] [ at <<position>> ]? , |
at <<position>></span> ,
]?
<<color-stop>> [ , <<color-stop>> ]+
)
<dfn noexport><extent-keyword></dfn> = closest-corner | closest-side | farthest-corner | farthest-side
</pre>
&
1C6B
lt;/div>
<h4 class="no-toc" id="radial-color-stops">Placing Color Stops</h4>
Color-stops are placed on a <a>gradient line</a> shaped like a ray
(a line that starts at one point, and extends infinitely in a one direction),
similar to the <a>gradient line</a> of linear gradients.
The <a>gradient line's</a> <a>starting point</a> is at the center of the gradient,
and it extends toward the right,
with the <a>ending point</a> on the point where the <a>gradient line</a> intersects the <a>ending shape</a>.
A color-stop can be placed at a location before 0%;
though the negative region of the <a>gradient line</a> is never directly consulted for rendering,
color stops placed there can affect the color of non-negative locations on the <a>gradient line</a> through interpolation or repetition
(see <a section href='#repeating-gradients'>repeating gradients</a>).
For example, ''radial-gradient(red -50px, yellow 100px)'' produces an elliptical gradient
that starts with a reddish-orange color in the center
(specifically, #f50)
and transitions to yellow.
Locations greater than 100% simply specify a location a correspondingly greater distance from the center of the gradient.
The color of the gradient at any point is determined by
first finding the unique ellipse passing through that point
with the same center, orientation, and ratio between major and minor axises as the ending-shape.
The point's color is then the color of the positive section of the <a>gradient line</a> at the location where this ellipse intersects it.
<h4 class="no-toc" id="degenerate-radials">Degenerate Radial Gradients</h4>
Some combinations of position, size, and shape
will produce a circle or ellipse with a radius of 0.
This will occur, for example,
if the center is on a <a>gradient box</a> edge and ''closest-side'' or ''closest-corner'' is specified
or if the size and shape are given explicitly and either of the radiuses is zero.
In these degenerate cases,
the gradient must be be rendered as follows:
: If the <a>ending shape</a> is a circle with zero radius:
:: Render as if the <a>ending shape</a> was
a circle whose radius was an arbitrary very small number greater than zero.
<span class='note'>This will make the gradient continue to look like a circle.</span>
: If the <a>ending shape</a> has zero width (regardless of the height):
:: Render as if the <a>ending shape</a> was
an ellipse whose height was an arbitrary very large number
and whose width was an arbitrary very small number greater than zero.
<span class='note'>This will make the gradient look similar to a horizontal linear gradient
that is mirrored across the center of the ellipse.
It also means that all color-stop positions specified with a percentage resolve to ''0px''.</span>
: Otherwise, if the <a>ending shape</a> has zero height:
:: Render as if the <a>ending shape</a> was
an ellipse whose width was an arbitrary very large number
and whose height was an arbitrary very small number greater than zero.
<span class='note'>This will make the gradient look like a solid-color image
equal to the color of the last color-stop,
or equal to the average color of the gradient if it's repeating.</span>
<h4 class='no-toc' id='radial-gradient-examples'>Radial Gradient Examples</h4>
All of the following examples are applied to a box that is 200px wide and 100px tall.
<div class=example>
These examples demonstrate different ways to write the basic syntax for radial gradients:
<pre>
radial-gradient(yellow, green);
radial-gradient(ellipse at center, yellow 0%, green 100%);
radial-gradient(farthest-corner at 50% 50%, yellow, green);
</pre>
<img src="images/radial1.png" alt="" >
<pre>radial-gradient(circle, yellow, green);</pre>
<img src="images/radial2.png" alt="" >
<pre>radial-gradient(red, yellow, green);</pre>
<img src="images/radial3.png" alt="" >
</div>
<div class=example>
This image shows a gradient originating from somewhere other than the center of the box:
<pre>radial-gradient(farthest-side at left bottom, red, yellow 50px, green);</pre>
<img src="images/radial4.png" alt="" >
</div>
<div class=example>
Here we illustrate a ''closest-side'' gradient.
<pre>
radial-gradient(closest-side at 20px 30px, red, yellow, green);
radial-gradient(20px 30px at 20px 30px, red, yellow, green);
</pre>
<img src="images/radial6.png" alt="" >
<pre>
radial-gradient(closest-side circle at 20px 30px, red, yellow, green);
radial-gradient(20px 20px at 20px 30px, red, yellow, green);
</pre>
<img src="images/radial7.png" alt="" >
</div>
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Repeating Gradients: the ''repeating-linear-gradie
A80D
nt()'' and ''repeating-radial-gradient()'' notations {#repeating-gradients}
------------------------------------------------------------------------------------------------------
In addition to ''linear-gradient()'' and ''radial-gradient()'',
this specification defines <dfn caniuse="css-repeating-gradients">repeating-linear-gradient()</dfn>
and <dfn>repeating-radial-gradient()</dfn> values.
These notations take the same values
and are interpreted the same
as their respective non-repeating siblings defined previously.
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.
<div class=example>
Repeating gradient syntax is identical to that of non-repeating gradients:
<pre>repeating-linear-gradient(red, blue 20px, red 40px)</pre>
<img src="images/repeating1.png" alt="">
<pre>repeating-radial-gradient(red, blue 20px, red 40px)</pre>
<img src="images/repeating2.png" alt="">
<pre>repeating-radial-gradient(circle closest-side at 20px 30px, red, yellow, green 100%, yellow 150%, red 200%)</pre>
<img src="images/repeating3.png" alt="">
</div>
If the distance between the first and last color-stops is non-zero,
but is small enough that the implementation knows that the physical resolution of the output device is insufficient to faithfully render the gradient,
the implementation must <a lt="gradient-average-color">find the average color of the gradient</a>
and render the gradient as a solid-color image equal to the average color.
If the distance between the first and last color-stops is zero
(or rounds to zero due to implementation limitations),
the implementation must <a lt="gradient-average-color">find the average color of a gradient</a>
with the same number and color of color-stops,
but with the first and last color-stop an arbitrary non-zero distance apart,
and the remaining color-stops equally spaced between them.
Then it must render the gradient as a solid-color image equal to that average color.
If the width of the ending shape of a repeating radial gradient is non-zero
and the height is zero,
or is close enough to zero that the implementation knows that the physical resolution of the output device is insufficient to faithfully render the gradient,
the implementation must <a lt="gradient-average-color">find the average color of the gradient</a>
and render the gradient as a solid-color image equal to the average color.
Note: The <a section href="#degenerate-radials">Degenerate Radial Gradients</a> section
describes how the ending shape is adjusted when its width is zero.
To <dfn lt="gradient-average-color">find the average color of a gradient</dfn>,
run these steps:
1. Define <var>list</var> as an initially-empty list of premultiplied RGBA colors,
and <var>total-length</var> as the distance between first and last color stops.
2. For each adjacent pair of color-stops,
define <var>weight</var> as half the distance between the two color-stops,
divided by <var>total-length</var>.
Add two entries to <var>list</var>,
the first obtained by representing the color of the first color-stop in premultiplied sRGBA
and scaling all of the components by <var>weight</var>,
and the second obtained in the same way with the second color-stop.
3. Sum the entries of <var>list</var> component-wise to produce the average color,
and return it.
Note: As usual, implementations may use whatever algorithm they wish,
so long as it produces the same result as the above.
<div class='example'>
For example, the following gradient is rendered as a solid light-purple image (equal to <code>rgb(75%,50%,75%)</code>):
<pre class="css">repeating-linear-gradient(red 0px, white 0px, blue 0px);</pre>
The following gradient would render the same as the previous under normal circumstances
(because desktop monitors can't faithfully render color-stops 1/10th of a pixel apart),
but would render as a normal repeating gradient if, for example,
the author applied "zoom:100;" to the element on which the gradient appears:
<pre class="css">repeating-linear-gradient(red 0px, white .1px, blue .2px);</pre>
</div>
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Gradient Color-Stops {#color-stop-syntax}
-----------------------------------------
<pre class=prod>
<dfn><color-stop-list></dfn> = <<color-stop>>#{2,}
<dfn><color-stop></dfn> = <<color>> <<length-percentage>>?
</pre>
The colors in gradients are specified using <a>color stops</a>.
A <dfn export>color stop</dfn> is a combination of a color and a position.
<!-- UNCOMMENT FOR L4
Depending on the type of gradient, that position can be a length, angle, or percentage.
-->
While every color stop conceptually has a position,
the position can be omitted in the syntax,
in which case it gets automatically filled in by the user agent;
see below for details.
The color stops for a gradient are specified
in a <dfn export>color stop list</dfn>,
which is a list of two or more <a>color stops</a>,
specified in geometric order.
<a>Color stops</a> are placed on a <a>gradient line</a>,
which defines the colors at every point of a gradient.
The gradient function defines the shape and length of the <a>gradient line</a>,
along with its <a>starting point</a> and <a>ending point</a>.
Percentages refer to the length of the <a>gradient line</a>
between the <a>starting point</a> and <a>ending point</a>,
with 0% being at the starting point
and 100% being at the ending point.
Lengths are measured along the <a>gradient line</a>
from the <a>starting point</a>
in the direction of the <a>ending point</a>.
<!-- UNCOMMENT FOR L4
Angles are measured with 0deg pointing up,
and positive angles corresponding to clockwise rotations from there.
-->
<a>Color stops</a> are usually placed between
the <a>starting point</a> and <a>ending point</a>,
but that's not required:
the gradient line extends infinitely in both directions,
and a <a>color stop</a> can be placed at any position on the <a>gradient line</a>.
When the position of a <a>color stop</a> is omitted,
it is positioned automatically
halfway between the two surrounding stops.
If multiple stops in a row lack a position,
they space themselves out equally.
The following steps must be applied <em>in order</em> to process the list of <a>color stops</a>.
After applying these rules,
all <a>color stops</a> will have a definite position and color
and they will be in ascending order:
1. If the first <a>color stop</a> does not have a position,
set its position to 0%.
If the last <a>color stop</a> does not have a position,
set its position to 100%.
2. If a <a>color stop</a> has a position that is less than
the specified position of any <a>color stop</a> before it in the list,
set its position to be equal to the largest specified position of any <a>color stop</a> before it.
3. If any <a>color stop</a> still does not have a position,
then, for each run of adjacent <a>color stops</a> without positions,
set their positions so that they are evenly spaced between the preceding and following <a>color stops</a> with positions.
At each <a>color stop</a> position,
the line is the color of the <a>color stop</a>.
Between two <a>color stops</a>,
the line's color is linearly interpolated between the colors of the two <a>color stops</a>,
with the interpolation taking place in premultiplied RGBA space.
Before the first <a>color stop</a>,
the line is the color of the first <a>color stop</a>.
After the last <a>color stop</a>, the line is the color of the last <a>color stop</a>.
If multiple <a>color stops</a> 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.
<div class=example>
Below are several pairs of gradients.
The latter of each pair is a manually "fixed-up" version of the former,
obtained by applying the above rules.
For each pair, both gradients will render identically.
<span class='note'>The numbers in each arrow specify which fixup steps are invoked in the transformation.</span>
<pre>
1. linear-gradient(red, white 20%, blue)
=1=>
linear-gradient(red 0%, white 20%, blue 100%)
2. linear-gradient(red 40%, white, black, blue)
=13=>
linear-gradient(red 40%, white 60%, black 80%, blue 100%)
3. linear-gradient(red -50%, white, blue)
=13=>
linear-gradient(red -50%, white 25%, blue 100%)
4. linear-gradient(red -50px, white, blue)
=13=>
linear-gradient(red -50px, white calc(-25px + 50%), blue 100%)
5. linear-gradient(red 20px, white 0px, blue 40px)
=2=>
linear-gradient(red 20px, white 20px, blue 40px)
6. linear-gradient(red, white -50%, black 150%, blue)
=12=>
linear-gradient(red 0%, white 0%, black 150%, blue 150%)
7. linear-gradient(red 80px, white 0px, black, blue 100px)
=23=>
linear-gradient(red 80px, white 80px, black 90px, blue 100px)
</pre>
</div>
<div class=example>
The following example illustrates the difference between
a gradient transitioning in pre-multiplied sRGBA
and one transitioning (incorrectly) in non-premultiplied.
In both of these example,
the gradient is drawn over a white background.
Both gradients could be written with the following value:
<pre>linear-gradient(90deg, red, transparent, blue)</pre>
In premultiplied space,
transitions to or from "transparent" always look nice:
<object data="images/gradient2.svg" width="200"height="100">(Image requires SVG)</object>
On the other hand,
if a gradient were to incorrectly transition in non-premultiplied space,
the colors near "transparent" would noticeably darken to a grayish color,
because "transparent" is actually a shorthand for ''rgba(0,0,0,0)'', or transparent black:
<object data="images/gradient3.svg" width="200"height="100">(Image requires SVG)</object>
</div>
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 <a>color stop</a> to unintentionally try to move before an earlier one.
For example, the rule ''background-image: linear-gradient(yellow 100px, blue 50%)''
wouldn't require any fix-up as long as the background area is at least 200px tall.
8C50
If it was 150px tall, however,
the blue <a>color stop's</a> position would be equivalent to "75px",
which precedes the yellow <a>color stop</a>,
and would be corrected to a position of 100px.
Additionally, since the relative ordering of such color stops
cannot be determined without performing layout,
they will not interpolate smoothly in
<a href="http://www.w3.org/TR/css-animations/">animations</a>
or <a href="http://www.w3.org/TR/css-transitions/">transitions</a>.
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.
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.
Note that transitions where either the transparency or the color are held constant
(for example, transitioning between rgba(255,0,0,100%) and rgba(0,0,255,100%),
or rgba(255,0,0,100%) and rgba(255,0,0,0%))
have identical results whether the color interpolation is done in premultiplied or non-premultiplied color-space.
Differences only arise when both the color and transparency differ between the two endpoints.
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Sizing Images and Objects in CSS {#sizing}
==========================================
Images used in CSS may come from a number of sources:
from binary 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 noexport local-lt=object>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.
Object-Sizing Terminology {#sizing-terms}
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In order to define this handling,
we define a few terms,
to make it easier to refer to various concepts:
<dl export>
<dt><dfn>intrinsic dimensions</dfn>
<dd>
The term intrinsic dimensions refers to the set of
the <dfn>intrinsic height</dfn>,
<dfn>intrinsic width</dfn>,
and <dfn>intrinsic aspect ratio</dfn>
(the ratio between the width and height),
each of which may or may not exist for a given [=object=].
These intrinsic dimensions represent a preferred or natural size of the object itself;
that is, they are not a function of the context in which the object is used.
CSS does not define how the intrinsic dimensions are found in general.
Raster images are an example of an [=object=] with all three intrinsic dimensions.
SVG images designed to scale might have only an <a>intrinsic aspect ratio</a>;
SVG images can also be created with only an <a>intrinsic width</a> or <a lt="intrinsic height">height</a>.
CSS gradients, defined in this specification, are an example of an object with no intrinsic dimensions at all.
Another example of this is embedded documents,
such as the <{iframe}> element in HTML.
In general, [=objects=] cannot have only two intrinsic dimensions,
as any two automatically define the third.
However some types of replaced elements,
such as form controls,
can have an <a>intrinsic width</a> and an <a>intrinsic height</a>,
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