<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/css-images-3/

Work Status: Testing

Editor: Tab Atkins Jr., Google, http://xanthir.com/contact/, w3cid 42199

Editor: Elika J. Etemad / fantasai, Apple, 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'

Implementation Report: https://wpt.fyi/results/css/css-images?label=experimental&label=master&aligned

Previous Version: https://www.w3.org/TR/2019/CR-css-images-3-20191010/

Previous Version: https://www.w3.org/TR/2012/CR-css3-images-20120417/

Ignored Terms: <offset>, background positioning area, border image area, <meetorslice>, <ending-shape>

Ignored Vars: start image, end image

Include Can I Use Panels: yes

WPT Path Prefix: css/css-images/

WPT Display: open

</pre>

<pre class=link-defaults>

spec:css-backgrounds-3;

type:property;

text:border-image

text:border-image-repeat

type:value; text:center

</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>.

Value Definitions {#values}

---------------------------

This specification follows

the <a href="https://www.w3.org/TR/CSS2/about.html#property-defs">CSS property definition conventions</a> from [[!CSS2]].

using the <a href="https://www.w3.org/TR/css-values-3/#value-defs">value definition syntax</a> from [[!CSS-VALUES-3]].

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>CSS-wide keywords</a> as their property value.

For readability they have 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>&lt;image></dfn> = <<url>> | <<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

or that has failed to load.

An <dfn export lt="invalid image|valid image">invalid image</dfn> is rendered

as a solid-color ''transparent'' image with no [=natural 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'

it is treated as ''list-style-type/none'',

allowing the 'list-style-type' to render in its place. [[CSS2]]

While an image is loading,

is a <dfn export>loading image</dfn>.

[=Loading images=] are <em>not</em> [=invalid images=],

but have similar behavior:

they are rendered as a solid-color ''transparent'' image with no [=natural dimensions=],

and may trigger fallback rendering in contexts that offer it,

but must not trigger loading of fallback resources.

Alternately, if a <a>loading image</a> happens to be replacing

an already-loaded image

(for example due to changes in the document or style sheet)

and the UA is tracking this information,

it may continue to render the already-loaded image

in place of the <a>loading image</a>.

Partially-loaded images (whose [=natural dimensions=] are known, but whose image data is not fully loaded)

may be either treated as [=loading images=]

or as loaded images rendered with partial data.

For example, a UA may render an interlaced GIF in place

as soon as its first pass of pixel data has loaded

or even as soon as the image header (which contains sizing data) has parsed

and refresh the rendering as more data loads;

or it may wait until the entire image has loaded before using it.

A <dfn export>[=computed value|computed=] <<image>></dfn> value

is the [=specified value=]

with any <<url>>s, <<color>>s, and <<length>>s computed.

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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>.

### Ambiguous Reference-or-Image URLs ### {#ambiguous-urls}

URLs are used in many contexts for many types of resources,

and therefore can be interpreted in many ways.

Usually the context the URL appears in

makes it clear how to interpret the resource,

but in some instances

it can be ambiguous.

For example,

a 'mask-image' <<url>> value pointing to an SVG file

could be interpreted as a reference to an element in the file

or as an <<image>>.

An <dfn export for=CSS>ambiguous image URL</dfn> is

a <<url>> value that can be interpreted

as either an <<image>> or an element reference.

If an [=ambiguous image URL=] is a [[CSS-VALUES-3#local-urls|fragment-only URL]],

then it must be treated as an element reference.

Otherwise, if the [=ambiguous image URL=] has a fragment

that references an element in the resource

that is an appropriate type of element for the context in which the <<url>> appears

(such as a <{mask}> element for the 'mask-image' property),

it is interpreted as an element reference.

Otherwise, it is treated as an <<image>>.

Specs using the [=ambiguous image URL=] concept

must define what elements are valid references for the URL,

and any additional conditions that might apply.

<div class=example>

For example,

a reference like ''mask-image: url(icon.svg#foo)''

might be pointing to a <code>&lt;mask id="foo"></code> element in the SVG document,

<em>or</em> be pointing to a <code>&lt;g id="foo"></code> element

and depending on the '':target'' pseudo-class to change how it renders as an image.

When this occurs,

the "icon.svg" file is loaded up and examined;

if the #foo element is indeed a <{mask}>,

the ''url()'' is treated as a reference to that element;

otherwise,

it's interpreted as an image.

</div>

### Image Metadata ### {#url-metadata}

Images can contain metadata

such as color space, resolution and orientation

which specifies how to render the image.

Some image formats are flexible in where this metadata can be placed in the file;

however, if the metadata occurs <em>after</em> the actual image data,

it harms the UA's ability to “progressively decode” the image

and display it as the image's data streams in.

To reduce the impact of this issue:

* If the choice exists for a given image format,

authors <em>must</em> produce their images

so that such metadata occurs before the image data in the image.

(Note: This is the default for most images already.)

* User agents <em>should</em> ignore any layout-impacting metadata (such as orientation or resolution)

that occurs after the image data begins.

If a user agent cannot ignore the metadata based its location in the image

(for example, if the decoder being used

does not report where in the image the metadata was located),

it <em>must</em> use the metadata in all cases.

(In particular, it is not valid

to use the metadata only when the image is "small"

and the entire image is downloaded quickly,

but to ignore it if the image is large

and the metadata isn't downloaded until well after the image starts being displayed.)

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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 <dfn export lt="gradient function">gradient functions</dfn> 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>&lt;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 [=natural dimensions=].

<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 <dfn>linear-gradient()</dfn> notation

specifies a linear gradient in CSS.

Its syntax is as follows:

<pre class=prod>

<<linear-gradient()>> = linear-gradient( [ <<linear-gradient-syntax>> ] )

<dfn>&lt;linear-gradient-syntax></dfn> = [ <<angle>> | <<zero>> | to <<side-or-corner>> ]? , <<color-stop-list>>

<dfn>&lt;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 src="images/gradient-diagram.png" width="454" height="455" style="float: right; margin-left: 1em;"

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" width="200" height="100" 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" width="200" height="100" 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" width="200" height="100" 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>

<img src="images/gradient1.svg" width="200" height="100" alt="">

</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.

The <dfn>radial-gradient()</dfn> notation specifies a radial gradient

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 class=prod>

<<radial-gradient()>> = radial-gradient( [ <<radial-gradient-syntax>> ] )

<dfn>&lt;radial-gradient-syntax></dfn> =

[ <<radial-shape>> || <<radial-size>> ]? [ at <<position>> ]? ,

<<color-stop-list>>

<dfn>&lt;radial-size></dfn> = <<radial-extent>> | <<length [0,∞]>> | <<length-percentage [0,∞]>>{2}

<dfn>&lt;radial-extent></dfn> = closest-corner | closest-side | farthest-corner | farthest-side

<dfn>&lt;radial-shape></dfn> = circle | ellipse

</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><<radial-shape>></dfn>

<dd>

Can be either <dfn value for="<radial-shape>">circle</dfn> or <dfn value for="<radial-shape>">ellipse</dfn>;

determines whether the gradient's <a>ending shape</a> is a circle or an ellipse, respectively.

If <<radial-shape>> is omitted,

the <a>ending shape</a> defaults to a circle

if the <<radial-size>> is a single <<length>>,

and to an ellipse otherwise.

<dt><dfn><<radial-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 <<radial-extent>> values:

<dl dfn-for="<radial-extent>, radial-gradient(), repeating-radial-gradient()">

<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 <<radial-shape>> is specified as ''circle'' or is omitted,

the <<radial-size>> may be given explicitly as:

<dl>

<dt><dfn for="<radial-size>, radial-gradient(), repeating-radial-gradient()">&lt;length [0,∞]></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 <<radial-shape>> is specified as ''ellipse'' or is omitted,

<<radial-size>> may instead be given explicitly as:

<dl>

<dt><dfn for="<radial-size>, radial-gradient(), repeating-radial-gradient()"><<length-percentage [0,∞]>>{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 [0,∞]>> ] [ at <<position>> ]? , |

[ ellipse || <<length-percentage [0,∞]>>{2} ] [ at <<position>> ]? , |

[ [ circle | ellipse ] || <<radial-extent>> ] [ at <<position>> ]? , |

at <<position>>,

]?

<<color-stop-list>>

)

</pre>

</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 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" width="200" height="100" alt="">

<pre>radial-gradient(circle, yellow, green);</pre>

<img src="images/radial2.png" width="200" height="100" alt="">

<pre>radial-gradient(red, yellow, green);</pre>

<img src="images/radial3.png" width="200" height="100" 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" width="200" height="100" 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" width="200" height="100" 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" width="200" height="100" alt="">

</div>

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Repeating Gradients: the ''repeating-linear-gradient()'' 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.

<pre class=prod>

<<repeating-linear-gradient()>> = repeating-linear-gradient( [ <<linear-gradient-syntax>> ] )

<<repeating-radial-gradient()>> = repeating-radial-gradient( [ <<radial-gradient-syntax>> ] )

</pre>

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" width="200" height="100" alt="">

<pre>repeating-radial-gradient(red, blue 20px, red 40px)</pre>

<img src="images/repeating2.png" width="200" height="100" 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" width="200" height="100" 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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Defining Gradient Color {#gradient-colors}

-------------------------

The colors in gradients are specified using <dfn export lt="color stop">color stops</dfn>

(a <<color>> and a corresponding position on the [=gradient line=])

and <dfn export lt="color transition hint" local-lt="transition hint">color transition hints</dfn>

(a position between two [=color stops=]

representing the halfway point in the color transition)

which are placed on the <a>gradient line</a>,

defining the color at every point of the line.

(Each [=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>;

see above.)

Colors throughout the gradient field are then determined

by tying them to specific points along the [=gradient line=]

as specified by the gradient function.

UAs may “dither” gradient colors slightly

(randomly alternate individual pixels with nearby colors on the gradient line)

to effect a smoother gradient.

<h4 id=color-stop-syntax>

Color Stop Lists</h4>

<a>Color stops</a> and [=transition hints=] are specified

in a <dfn export>color stop list</dfn>,

which is a list of one or more [=color stops=]

interleaved with optional [=transition hints=]:

<pre class=prod>

<dfn>&lt;color-stop-list></dfn> =

<<linear-color-stop>> , [ <<linear-color-hint>>? , <<linear-color-stop>> ]#?

<dfn>&lt;linear-color-stop></dfn> = <<color>> <<length-percentage>>?

<dfn>&lt;linear-color-hint></dfn> = <<length-percentage>>

</pre>

Percentages are resolved against 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>.

[=Color stop=] and [=transition hint=] positions

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 positions can be specified anywhere

on the <a>gradient line</a>.

When the position of a [=color stop=] is omitted,

it is automatically assigned a position.

The first or last [=color stop=] in the [=color stop list=]

is assigned

the [=gradient line’s=] [=starting point=] or [=ending point=]

(respectively).

Otherwise,

it's assigned the position halfway between the two surrounding stops.

If multiple stops in a row lack a position,

they space themselves out equally

between the surrounding positioned stops.

See [[#color-stop-fixup]] for details.

<h4 id=coloring-gradient-line>

Coloring the Gradient Line</h4>

At each <a>color stop</a> position,

the [=gradient line=] is the color of the <a>color stop</a>.

Before the first <a>color stop</a>,

the [=gradient line=] is the color of the first <a>color stop</a>,

and after the last <a>color stop</a>,

the [=gradient line=] is the color of the last <a>color stop</a>.

Between two <a>color stops</a>,

the [=gradient line’s=] color is interpolated between the colors of the two <a>color stops</a>,

with the interpolation taking place in <a href="#premultiplied">premultiplied RGBA space</a>.

By default,

this interpolation is linear--

at 25%, 50%, or 75% of the distance between two <a>color stops</a>,

the color is a 25%, 50%, or 75% blend of the colors of the two stops.

However, if a [=transition hint=] was provided between two <a>color stops</a>,

the interpolation is non-linear,

and controlled by the hint:

<div algorithm="interpolate with a color hint">

<ol>

<li>

Determine the location of the [=transition hint=] as a percentage of the distance between the two <a>color stops</a>,

denoted as a number between 0 and 1,

where 0 indicates the hint is placed right on the first <a>color stop</a>,

and 1 indicates the hint is placed right on the second <a>color stop</a>.

Let this percentage be <var>H</var>.

<li>

For any given point between the two color stops,

determine the point's location as a percentage of the distance between the two <a>color stops</a>,

in the same way as the previous step.

Let this percentage be <var>P</var>.

<li>

Let <var>C</var>, the color weighting at that point,

be equal to <code><var>P</var>^{log_<var>H</var>(.5)}</code>.

<li>

The color at that point is then a linear blend between the colors of the two <a>color stops</a>,

blending <code>(1 - <var>C</var>)</code> of the first stop and <var>C</var> of the second stop.

</ol>

</div>

Note: The [=transition hint=] specifies where the “halfway color”--

the 50% blend between the colors of the two surrounding color stops--

should be placed.

When the hint is exactly halfway between the two surrounding color stops,

the above interpolation algorithm

happens to produce the ordinary linear interpolation.

If the hint is placed anywhere else,

it produces a smooth exponential curve

between the surrounding color stops,

with the “halfway color” occurring exactly where the hint specifies.

Issue: Add a visual example of a color hint being used.