CSS Images Module Level 3

1. Introduction

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 gradient.

This module also defines several properties for manipulating raster images and for sizing or positioning replaced elements such as images within the box determined by the CSS layout algorithms. It also defines in a generic way CSS’s sizing algorithm for images and other similar replaced elements.

This subsection (above) is not normative.

1.1. Module Interactions

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 ::first-line and ::first-letter.

1.2. Value Definitions

This specification follows the CSS property definition conventions from [CSS2]. using the value definition syntax 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 CSS-wide keywords as their property value. For readability they have not been repeated explicitly.

2. Image Values: the <image> type

The <image> value type denotes a 2D image. It can be a url reference or a color gradient. Its syntax is:

<image> = <url> | <gradient>

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 invalid image is rendered as a solid-color transparent image with no natural dimensions. However, invalid images can trigger error-handling clauses in some contexts. For example, an invalid image in list-style-image it is treated as none, allowing the list-style-type to render in its place. [CSS2]

While an image is loading, is a loading image. Loading images are not 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 loading image 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 loading image.

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 computed <image> value is the specified value with any <url>s, <color>s, and <length>s computed.

2.1. Image References: the 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].

In the example below, a background image is specified with url()syntax:

background-image: url(wavy.png);

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 fully decodable) it must be treated as an invalid image.

2.1.1. Ambiguous Reference-or-Image 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 ambiguous image URL is a <url> value that can be interpreted as either an <image> or an element reference. If an ambiguous image URL is a 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.

For example, a reference like mask-image: url(icon.svg#foo) might be pointing to a <mask id="foo"> element in the SVG document, or be pointing to a <g id="foo"> 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.

2.1.2. Image 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 after 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 must 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 should 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 must 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.)

3. 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 functions described in this section allow an author to specify such an image in a terse syntax, so that the UA can generate the image automatically when rendering the page. The syntax of a <gradient> is:

<gradient> =
  <linear-gradient()> | <repeating-linear-gradient()> |
  <radial-gradient()> | <repeating-radial-gradient()>

As with the other <image> types defined in this specification, gradients can be used in any property that accepts images. For example:

background: linear-gradient(white, gray);
list-style-image: radial-gradient(circle, #006, #00a 90%, #0000af 100%, white 100%)

A gradient is drawn into a box with the dimensions of the concrete object size, referred to as the gradient box. However, the gradient itself has no natural dimensions.

For example, if you use a gradient as a background, by default the gradient will draw into a gradient box the size of the element’s padding box. If background-size is explicitly set to a value such as 100px 200px, then the gradient box 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 default object size for that property.

Gradients are specified by defining the starting point and ending point of a gradient line (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 gradient line to produce the actual gradient.

3.1. Linear Gradients: the linear-gradient() notation

A linear gradient is created by specifying a straight gradient line, 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.

3.1.1. linear-gradient() syntax

The linear-gradient() notation specifies a linear gradient in CSS. Its syntax is as follows:

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

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

<side-or-corner> = [left | right] || [top | bottom]

The first argument to the function specifies the gradient line, 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 gradient line’s 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 gradient line is 0deg, 90deg, 180deg, or 270deg, respectively.

If the argument instead specifies a corner of the box such as to top left, the gradient line 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 gradient box. This causes a color-stop at 50% to intersect the two neighboring corners (see example).

Starting from the center of the gradient box, extend a line at the specified angle in both directions. The ending point is the point on the gradient line where a line drawn perpendicular to the gradient line would intersect the corner of the gradient box 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.

[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 gradient line from the rules above. This shows the starting and ending point of the gradient line, 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 exactly at the corner, and pure black exactly at the opposite corner. That’s intentional, and will always be true for linear gradients.

Given:

A the angle (in any quadrant) defining the gradient line’s direction such that 0 degrees points upwards and positive angles represent clockwise rotation,
W the width of the gradient box,
H the height of the gradient box,

The length of the gradient line (between the starting point and ending point) is:

abs(W * sin(A)) + abs(H * cos(A))

The gradient’s color stops are typically placed between the starting point and ending point on the gradient line, but this isn’t required: the gradient line 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 gradient line. The point’s color is the color of the gradient line at the point where this line intersects it.

3.1.2. Linear Gradient Examples

All of the following linear-gradient() examples are presumed to be backgrounds applied to a box that is 200px wide and 100px tall.

Below are various ways of specifying a basic vertical gradient:

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%);

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 gradient line is still sized so as to make the gradient yellow exactly at the upper-left corner, and blue exactly at the lower-right corner.

linear-gradient(135deg, yellow, blue);
linear-gradient(-45deg, blue, yellow);

This demonstrates a 3-color gradient, and how to specify the location of a stop explicitly:

linear-gradient(yellow, blue 20%, #0f0);

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.

linear-gradient(to top right, red, white, blue)

3.2. Radial Gradients: the radial-gradient() notation

In a radial gradient, rather than colors smoothly fading from one side of the gradient box to the other as with linear gradients, they instead emerge from a single point and smoothly spread outward in a circular or elliptical shape.

The radial-gradient() 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 ending shape (the 100% ellipse). Color stops are given as a list, just as for linear-gradient(). Starting from the gradient center and progressing towards (and potentially beyond) the ending shape, uniformly-scaled concentric ellipses are drawn and colored according to the specified color stops.

3.2.1. radial-gradient() Syntax

The radial gradient syntax is:

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

<radial-gradient-syntax> =
  [ <radial-shape> || <radial-size> ]? [ at <position> ]? ,
  <color-stop-list>

<radial-size> = <radial-extent> | <length [0,∞]> | <length-percentage [0,∞]>{2}

<radial-extent> = closest-corner | closest-side | farthest-corner | farthest-side

<radial-shape> = circle | ellipse

Here is an example of a circular radial gradient 5em wide and positioned with its center in the top left corner:

radial-gradient(5em circle at top left, yellow, blue)

Note: A future level may add the ability to move the focus of the gradient, as in the original -webkit-gradient() function. See proposal tracked in Issue 1575 for "from <position>" and "from offset <offset>".

The arguments are defined as follows:

<position>

Determines the center 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 gradient box as the positioning area. If this argument is omitted, it defaults to center.

<radial-shape>

Can be either circle or ellipse; determines whether the gradient’s ending shape is a circle or an ellipse, respectively. If <radial-shape> is omitted, the ending shape defaults to a circle if the <radial-size> is a single <length>, and to an ellipse otherwise.

<radial-size>

Determines the size of the gradient’s ending shape. If omitted it defaults to farthest-corner. It can be given explicitly or by keyword. For the purpose of the keyword definitions, consider the gradient box 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:

closest-side: The ending shape is sized so that it exactly meets the side of the gradient box closest to the gradient’s center. If the shape is an ellipse, it exactly meets the closest side in each dimension.
farthest-side: Same as closest-side, except the ending shape is sized based on the farthest side(s).
closest-corner: The ending shape is sized so that it passes through the corner of the gradient box closest to the gradient’s center. If the shape is an ellipse, the ending shape is given the same aspect-ratio it would have if closest-side were specified.
farthest-corner: Same as closest-corner, except the ending shape is sized based on the farthest corner. If the shape is an ellipse, the ending shape is given the same aspect ratio it would have if farthest-side were specified.

If <radial-shape> is specified as circle or is omitted, the <radial-size> may be given explicitly as:

<length [0,∞]>: Gives the radius of the circle explicitly. Negative values are invalid.
Note: Percentages are not 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.

If <radial-shape> is specified as ellipse or is omitted, <radial-size> may instead be given explicitly as:

<length-percentage [0,∞]>{2}: 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 gradient box. Negative values are invalid.

Expanded with the above definitions, the grammar becomes:

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>
)

3.2.2. Placing Color Stops

Color-stops are placed on a gradient line shaped like a ray (a line that starts at one point, and extends infinitely in a one direction), similar to the gradient line of linear gradients. The gradient line’s starting point is at the center of the gradient, and it extends toward the right, with the ending point on the point where the gradient line intersects the ending shape. A color-stop can be placed at a location before 0%; though the negative region of the gradient line is never directly consulted for rendering, color stops placed there can affect the color of non-negative locations on the gradient line through interpolation or repetition (see repeating gradients). 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 gradient line at the location where this ellipse intersects it.

3.2.3. Degenerate Radial Gradients

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 gradient box 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 ending shape is a circle with zero radius:: Render as if the ending shape was a circle whose radius was an arbitrary very small number greater than zero. This will make the gradient continue to look like a circle.
If the ending shape has zero width (regardless of the height):: Render as if the ending shape was an ellipse whose height was an arbitrary very large number and whose width was an arbitrary very small number greater than zero. 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.
Otherwise, if the ending shape has zero height:: Render as if the ending shape was an ellipse whose width was an arbitrary very large number and whose height was an arbitrary very small number greater than zero. 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.

3.2.4. Radial Gradient Examples

All of the following examples are applied to a box that is 200px wide and 100px tall.

These examples demonstrate different ways to write the basic syntax for radial gradients:

radial-gradient(yellow, green);
radial-gradient(ellipse at center, yellow 0%, green 100%);
radial-gradient(farthest-corner at 50% 50%, yellow, green);

radial-gradient(circle, yellow, green);

radial-gradient(red, yellow, green);

This image shows a gradient originating from somewhere other than the center of the box:

radial-gradient(farthest-side at left bottom, red, yellow 50px, green);

Here we illustrate a closest-side gradient.

radial-gradient(closest-side at 20px 30px, red, yellow, green);
radial-gradient(20px 30px at 20px 30px, red, yellow, green);

radial-gradient(closest-side circle at 20px 30px, red, yellow, green);
radial-gradient(20px 20px at 20px 30px, red, yellow, green);

3.3. Repeating Gradients: the repeating-linear-gradient() and repeating-radial-gradient() notations

In addition to linear-gradient() and radial-gradient(), this specification defines repeating-linear-gradient() and repeating-radial-gradient() values. These notations take the same values and are interpreted the same as their respective non-repeating siblings defined previously.

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

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.

Repeating gradient syntax is identical to that of non-repeating gradients:

repeating-linear-gradient(red, blue 20px, red 40px)

repeating-radial-gradient(red, blue 20px, red 40px)

repeating-radial-gradient(circle closest-side at 20px 30px, red, yellow, green 100%, yellow 150%, red 200%)

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 find the average color of the gradient 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 find the average color of a gradient 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 find the average color of the gradient and render the gradient as a solid-color image equal to the average color.

Note: The Degenerate Radial Gradients section describes how the ending shape is adjusted when its width is zero.

To find the average color of a gradient, run these steps:

Define list as an initially-empty list of premultiplied RGBA colors, and total-length as the distance between first and last color stops.
For each adjacent pair of color-stops, define weight as half the distance between the two color-stops, divided by total-length. Add two entries to list, the first obtained by representing the color of the first color-stop in premultiplied sRGBA and scaling all of the components by weight, and the second obtained in the same way with the second color-stop.
Sum the entries of list 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.

For example, the following gradient is rendered as a solid light-purple image (equal to rgb(75%,50%,75%)):

repeating-linear-gradient(red 0px, white 0px, blue 0px);

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:

repeating-linear-gradient(red 0px, white .1px, blue .2px);

3.4. Defining Gradient Color

The colors in gradients are specified using color stops (a <color> and a corresponding position on the gradient line) and color transition hints (a position between two