Copyright © 2011 W3C® (MIT, ERCIM, Keio), All Rights Reserved. W3C liability, trademark and document use rules apply.
This CSS Image Values and Replaced Content module has two parts: First, it defines the syntax for <image> values in CSS. <image> values can be a single URI to an image, a list of URIs denoting a series of fallbacks, a reference to an element in the document, or gradients. Second, it defines properties used to control the interaction of replaced content and the CSS layout algorithms. These properties can affect the used image resolution for bitmaps, the replaced object's orientation, and whether and how to preserve the object's aspect ratio.
This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.
Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.
The (archived) public mailing list www-style@w3.org (see instructions) is preferred for discussion of this specification. When sending e-mail, please put the text “css3-images” in the subject, preferably like this: “[css3-images] …summary of comment…”
This document was produced by the CSS Working Group (part of the Style Activity).
This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.
The following features are at risk:
object-fit
’
image-orientation
’ property
image-resolution
’
This section is non-normative.
In CSS Levels 1 and 2, image values, such as those used in the
‘background-image
’ property, could
only be given by a single URI value. This module introduces additional
notations that allow a 2D image to be given as a list of URIs denoting
fallbacks, as a reference to an element in the document, and as a
gradient.
A document or implementation cannot conform to CSS Image Values & Replaced Content Level 3 alone, but can claim conformance to CSS Image Values & Replaced Content Level 3 if it satisfies the conformance requirements in this specification when implementing CSS or another host language that normatively references this specification.
Conformance to CSS Image Values & Replaced Content Level 3 is defined for three classes:
The conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification. All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]
Examples in this specification are introduced with the words "for
example" or are set apart from the normative text with
class="example"
, like this:
This is an example of an informative example.
Informative notes begin with the word "Note" and are set apart from the
normative text with class="note"
, like this:
Note, this is an informative note.
This specification defines the following units as part of the <resolution> value type:
px
’ unit
The <resolution> unit represents the size of a single "dot" of an
image. For raster images, a dot is an image pixel. A <resolution>
defines how many of these dots fit in a CSS ‘in
’, ‘cm
’, or ‘px
’ so that images can be sized accordingly.
The default resolution of raster images in CSS is ‘1dppx
’, which is equivalent to ‘96dpi
’. The concept of "resolution" does not apply
to vector image formats like SVG; generally, this means that any attempt
to change the resolution of a vector image is simply meaningless.
Individual uses of the <resolution> value define precisely what effects
they have on raster and vector images.
Printers tend to have substantially higher resolution than computer
monitors; due to this, an image that looks fine on the screen may look
pixellated when printed out. A <resolution> may be used in the
‘image-resolution
’ property to embed a
high-resolution image into the document that maintains an appropriate
size, ensuring attractive display both on screen and on paper:
img.high-res {
image-resolution: 300dpi;
}
With this set, an image meant to be 5 inches wide that was saved at 300dpi will actually display as 5 inches wide; without this set, the image would display as approximately 15.6 inches wide since the image is 15000 image pixels across, and by default there are 96 image pixels per inch.
The <image> value type denotes a 2D image. It is defined as
<image> = <url> | <image-list> | <element-reference> | <image-combination> | <gradient>
Image values can be used in many CSS properties, including the
‘background-image
’, ‘list-style-image
’, ‘cursor
’ properties [CSS21].
url()
’ functionThe simplest way to indicate an image is to reference an image file by
URL. This is done with the ‘url()
’ notation, defined in [CSS21].
In the example below, a background image is specified with ‘url()
’ syntax:
background-image: url(wavy.png);
A portion of an image may be referenced (clipped out and used as a standalone image) by use of media fragment identifiers. [MEDIA-FRAGS]
For example, given the following image* and CSS:
background-image: url('sprites.svg#xywh=40,0,20,20')
...the background of the element will be the portion of the image that starts at (40px,0px) and is 20px wide and tall, which is just the circle with a quarter filled in.
* SVG-in-<img> support required. Click the picture to view the SVG directly.
Note that a legacy UA that doesn't understand the media
fragments notation will ignore the fragment and simply display the
entirety of an image specified with ‘url
’. However, since URLs with media fragment
identifiers can also be used in the ‘image()
’ notation defined below, authors can take
advantage of CSS's forward-compatible parsing rules to provide a fallback
when using an image fragment URL:
In the example below, the ‘image()
’
notation is used together with the media fragment syntax, so that UAs
that don't support media fragments fail to parse the second declaration
and use the first.
background-image: url('swirl.png'); /* old UAs */ background-image: image('sprites.png#xywh=10,30,60,20'); /* new UAs */
image()
’ notationThe ‘image()
’ function allows an
author to specify an image with fallback images to be used if the original
image can't be decoded or is a type that the browser doesn't recognize.
Additionally, the author can specify a color as an ultimate fallback to be
used when none of the images can be.
Along with each URL, the author may specify a directionality, similar to
adding a dir
attribute to an element in HTML. The image
represented by the function takes on the directionality of the used URL.
If a directional image is used on or in an element with opposite
directionality, the image must be flipped in the inline direction (as if
it was transformed by, e.g., scaleX(-1)
, if the inline
direction is the X axis).
So that authors can take advantage of CSS's forwards-compatible parsing
rules to provide a fallback for image slices, implementations that support
the ‘image()
’ notation must
support the xywh=#,#,#,#
form of media fragment identifiers
for images. [MEDIA-FRAGS]
The ‘image()
’ notation is defined as:
<image-list> = image( [ <image-decl> , ]* [ <image-decl> | <color> ] ) <image-decl> = <string> [ ltr | rtl ]?
Each <string>
must represent a URL.
Multiple arguments can be given separated by commas, in which case the
function represents the first <string> representing an image that the
browser can successfully load and display. The final argument can specify
a <color> to serve as an ultimate fallback; this can be used, e.g. for
‘background-image
’, to ensure
adequate contrast if none of the preceding <image-decl>s can be used.
If the final argument is a <color>, it represents a solid-color image
of the given color with no intrinsic
dimensions.
The rule below would tell the UA to load ‘wavy.svg
’ if it can; failing that to load
‘wavy.png
’; failing that to display
‘wavy.gif
’; and finally, if none of
the images can be loaded and displayed, to use the color ‘rgba(0,0,255,0.5)
’ to create a dimensionless
background image. For example, the browser might not understand how to
render SVG images, the PNG may be malformed, and the GIF might not exist
on the server and return an HTML 404 error page instead of an image.
background-image: image("wavy.svg", 'wavy.png' , "wavy.gif", rgba(0,0,255,0.5));
The ‘background-size
’ property
specifies that dimensionless images by default stretch to cover the
entire background positioning area [CSS3BG], so if none of the
specified images can be displayed the background will be painted
semi-transparent blue. As with any image, this fallback will be painted
over the ‘background-color
’ (if
any).
At times, one may need a solid-color image for a property or function
that does not accept the <color> type directly. The ‘image()
’ function can be used for this: by
specifying only a color without any URLs, the function
immediately falls back to representing a solid-color image of the chosen
color.
background-image: image(rgba(0,0,255,.5)), url("bg-image.png");
In the above, the background is the image "bg-image.png", overlaid with partially-transparent blue.
element()
’ functionThe ‘element()
’ function allows an
author to use an element in the document as an image. As the referenced
element changes appearance, the image changes as well. This can be used,
for example, to create live previews of the next/previous slide in a
slideshow, or to reference a canvas element for a fancy generated gradient
or even an animated background. The syntax for ‘element()
’ is:
<element-reference> = element( [<id-selector> | <identifier> ] )
where <id-selector> is an ID selector [SELECT], and <identifier> is an identifer [CSS3VAL].
If the argument to the ‘element()
’
function is an ID selector, the function references the element matched by
the selector. If it's an identifier, the function references the element
whose CSS element reference
identifier is the given identifier. (CSS does not define how an
element acquires a CSS
element reference identifier; that is determined by the host
language.)
The image represented by the ‘element()
’ function can vary based on a number of
factors. The function must represent the image described by the first set
of conditions, following, that are true:
The function represents an image with width and height equal to the bounding box of the referenced element. The image must be constructed by rendering the referenced element and its descendants at the same size that the element would be in its document, over an infinite transparent-black canvas, positioned so that the edges of the referenced element's bounding box is flush with the edges of the image. Note: Because images clip anything outside their bounds by default, this means that decorations that extend outside the bounding box, like box-shadows or some border-images, may be clipped.
If the referenced element has a transform applied to it or an ancestor, the transform must be ignored when rendering the element as an image. If the referenced element is broken across pages, the element must be displayed as if the page content areas were joined flush in the pagination direction (elements broken across lines or columns just render with their bounding box, as normal, which may have unintended visual effects).
Implementations may either re-use existing bitmap data generated for the referenced element or regenerate the display of the element to maximize quality at the image's size (for example, if the implementation detects that the referenced element is an SVG fragment); in the latter case, the layout of the referenced element in the image must not be changed by the regeneration process. That is, the image must look identical to the referenced element, modulo rasterization quality.
TODO: Create examples of all of the above cases.
Host languages may define that some elements provide a paint source. Paint sources have an intrinsic width, height, and appearance, separate from the process of rendering, and so may be used as images even when they're not being rendered. Examples of elements that provide paint sources are the <linearGradient>, <radialGradient>, and <pattern> elements in SVG, or the <img>, <video>, and <canvas> elements in HTML.
element()
’The ‘element()
’ function can produce
nonsensical circular relationships, such as an element using itself as its
own background. These relationships can be easily and reliably detected
and resolved, however, by keeping track of a dependency graph and using
common cycle-detection algorithms.
Populate the dependency graph initially by having every element depend
on each of its children. Then, whenever a property on an element A uses
the ‘element()
’ function to refer to an
element B, add an edge to the graph by having A depend on B. If a
dependency cycle is detected, any ‘element()
’ functions that produced a dependency in
the cycle must represent a fully transparent image with no intrinsic
dimensions.
Someone else needs to review this and make sure that I'm not missing any cycles.
A gradient is an image that smoothly fades from one color to another. These are commonly used for subtle shading in background images, buttons, and many other things. The two functions described in this section allow an author to specify such an image in a terse syntax, so that the UA can generate the image automatically when rendering the page. The syntax of a <gradient> is:
<gradient> = [ <linear-gradient> | <radial-gradient> | <repeating-linear-gradient> | <repeating-radial-gradient> ]
where <linear-gradient>, <radial-gradient>, <repeating-linear-gradient>, and <repeating-radial-gradient> are defined in their applicable sections below.
Gradients are a type of image, and can be used anywhere an image can,
such as in the ‘background-image
’
or ‘list-style-image
’ properties.
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. Elsewhere in this section this rectangle is simply called the "box".
A gradient has no intrinsic
dimensions. This means that, for example, if you use a gradient in
the ‘background-image
’ property
(with ‘background-size
’ at the
default value and ‘background-repeat
’ not equal to ‘round
’), the box will simply be the size of
the background sizing area. Similarly, for a gradient used as a
list-style-image, the box would be a 1em square.
A linear gradient is created by specifying a gradient-line and then several colors placed along that line. The image is constructed by creating an infinite canvas and painting it with lines 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.
<linear-gradient> = linear-gradient(
[ [ <angle> | [ to <side-or-corner> ] ] ,]?
<color-stop>[, <color-stop>]+
)
<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 may be specified in two different ways. The first is by specifying the angle the gradient-line should assume; for the purposes of this argument, 0deg points upwards, 90deg points toward the right, and positive angles go clockwise. The starting-point and ending-point of the gradient-line are determined by extending a line in both directions from the center of the box at the angle specified. In the direction of the angle, 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 box in that direction. The starting-point is determined identically, except in the opposite direction of the angle.
Alternately, the direction may be specified with keywords that denote the direction. If the argument is "to top", "to right", "to bottom", or "to left", the gradient must be rendered identically to "0deg", "90deg", "180deg", or "270deg", respectively. If the argument specifies a corner to angle towards, the gradient must be rendered identically to an angle-based gradient with an angle chosen such that the endpoint of the gradient is in the same quadrant as the indicated corner, and a line drawn perpendicular to the gradient-line through the center of the box intersects the two neighboring corners.
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.
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 distance markers - the starting-point defines where 0%, 0px, etc are located when specifying color-stops, and the ending-point defines where 100% is located. Color-stops are allowed to have positions before 0% or after 100%.
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)
* SVG-in-HTML support required to view the image.
In a radial gradient, rather than colors smoothly fading from one side of the box to the other as with linear gradients, they instead emerge from a single point and smoothly spread outward in a circular or elliptical shape.
A radial gradient is specified by first pinpointing the center of the gradient, where the 0% ellipse will be, then specifying the size and shape of the 100% ellipse, ending with a list of color-stops just like a linear-gradient. Between the center and the ending-ellipse, and past the ending-ellipse, concentric ellipses are drawn and colored according to the specified color-stops.
<radial-gradient> = radial-gradient( [<position>,]? [[ [<shape> || <size>] | [<length> | <percentage>]{2} ],]? <color-stop>[, <color-stop>]+ )
The first argument to the function specifies the center of the ellipse.
The <position>
value type is
defined as the positioning syntax of ‘background-position
’
and is resolved in the same way, using the center-point as the subject and
the content box as the positioning area. [CSS21] [CSS3BG]
If this argument is omitted, it defaults to ‘center
’.
The second argument to the function specifies the size and shape of the ending-ellipse. This can be specified in two ways, with different characteristics:
The size and shape of the ending-ellipse can be defined implicitly with a size and shape keyword. The <shape> keyword is defined as:
<shape> = circle | ellipse
‘circle
’ indicates that the
ending-ellipse will be a circle with a constant radius. ‘ellipse
’ indicates that the gradient-shape will
be an axis-aligned ellipse (that is, its major and minor radiuses will
be horizontal and vertical, not necessarily in that order).
The <size> keyword is defined as:
<size> = closest-side | closest-corner | farthest-side | farthest-corner | contain | cover
Its values are defined in the list below. For the purpose of these definitions, consider the box edges as extending infinitely in both directions, rather than being finite line segments.
If the <shape> is ‘circle
’, the ending-shape must be a circle
sized so that it exactly meets the closest side of the box.
If the <shape> is ‘ellipse
’, the ending-shape must be an ellipse
sized so that it exactly meets the closest horizontal and vertical
sides of the box.
closest-side
’, except the ending-shape must
be sized based on the farthest side(s).
If the <shape> is ‘circle
’, the ending-shape must be a circle
sized so that it exactly meets the closest corner of the box.
If the <shape> is ‘ellipse
’, the ending-shape must be an ellipse
with the same aspect-ratio it would have if ‘closest-side
’ were specified, but sized so
that it exactly meets the closest corner of the box.
closest-corner
’, except the ending-shape
must be sized based on the farthest corner. If <shape> is ‘ellipse
’, the ending-shape must have the same
aspect ratio it would have if ‘farthest-side
’ were specified.
closest-side
’, and has the same meaning.
farthest-corner
’, and has the same meaning.
Alternately, the ending-shape's size and shape can be defined explicitly, by providing two lengths or percentages. These measure the length of the horizontal and vertical axises of the ellipse, respectively. (The axis length is the length from the center of the ellipse to the edge, similar to the radius of a circle, not the diameter.)
Percentages used in the first value are relative to the width of the box, while percentages used in the second value are relative to the height of the box.
Negative lengths or percentages are a syntax error.
Some combinations of position, size, and shape will produce a degenerate shape - a circle or ellipse with a
radius of 0. This will occur if the center is on a box edge and the second
argument is ‘circle closest-side
’,
‘ellipse closest-side
’, or ‘ellipse closest-corner
’, or if the center is on a
box corner and the shape is ‘closest-corner
’, or if the size and shape are
given explicitly and either of the radiuses are ‘0
’. In these instances, the gradient must be be
rendered as follows:
0px
’.
If this argument is omitted, it defaults to ‘ellipse cover
’.
If only one argument is provided before the color-stops, and it could be
interpreted as either a position or an explicit size (for example, in
‘radial-gradient(10% 10%, red, blue)
’),
it must be interpreted as a position.
Color-stops are placed on an imaginary line extending from the center of
the gradient toward the right, with the 0% point at the center of the
gradient, and 100% at the point where the line intersects the
ending-ellipse. The color of each ellipse is equal to the color of the
line where the ellipse intersects it. Distances past 100% can be
specified, and simply indicate a color-stop placed on the line a
corresponding distance from the center. Negative distances are allowed in
a radial gradient and work the same as in linear gradients with respect to
setting the color of the gradient-line, but colors before the
starting-point of the gradient-line
are not displayed. For example, ‘radial-gradient(red
-50px, yellow 100px)
’ would produce an elliptical gradient
which starts with a reddish-orange color in the center (the color 1/3
between red and yellow) and transitions to yellow at 100px wide.
Brad suggests that we could drop the position/sizing arguments and just use background-position and background-size. This would force all non-background uses of radial gradient to be centered and box-filling. Is this acceptable or not?
All of the following examples are applied to a box that is 200px wide and 100px tall.
These examples demonstrate the basic syntax for radial gradients:
radial-gradient(yellow, green);
radial-gradient(center, ellipse cover, yellow 0%, green 100%);
radial-gradient(50% 50%, farthest-corner, 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(bottom left, farthest-side, red, yellow 50px, green);
Here we illustrate a ‘contain
’ gradient.
radial-gradient(20px 30px, contain, red, yellow, green);
radial-gradient(20px 30px, 20px 30px, red, yellow, green);
radial-gradient(20px 30px, circle contain, red, yellow, green);
radial-gradient(20px 30px, 20px 20px, red, yellow, green);
In addition to the ‘linear-gradient()
’ and ‘radial-gradient()
’ functions, this specification
defines ‘repeating-linear-gradient()
’
and ‘repeating-radial-gradient()
’
functions. These two functions take the same values and are interpreted
the same as their respective non-repeating siblings defined previously:
<repeating-linear-gradient> = repeating-linear-gradient(
[ [ <angle> | [ to <side-or-corner> ] ] ,]?
<color-stop>[, <color-stop>]+
)
<side-or-corner> =
[ [left | right] || [top | bottom] ] |
[ [start | end] || [before |after] ]
<repeating-radial-gradient> = repeating-radial-gradient(
[<'background-position'>,]?
[[
[<shape> || <size>]
|
[<length> | <percentage>]{2}
],]?
<color-stop>[, <color-stop>]+
)
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 basically 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(20px 30px, circle contain, 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 height of a repeating radial gradient 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.
To find the average color of a gradient, run these steps:
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);
<color-stop> = <color> [ <percentage> | <length> ]?
Color-stops are points placed along the line defined by the gradient-line at the beginning of the rule. Color-stops must be specified in order. Percentages refer to the length of the gradient-line, with 0% being at the starting point and 100% being at the ending point. Lengths are measured from the starting-point in the direction of the ending-point. Color-stops are usually placed between the starting-point and ending-point, but that's not required; the gradient-line extends infinitely in both directions, and a color-stop can be placed at any position on the line.
At each color-stop, the line is the color of the color-stop. Between two color-stops, the line's color is linearly interpolated between the colors of the two color-stops, with the interpolation taking place in premultiplied RGBA space. Before the first color-stop, the line is the color of the first color-stop. After the last color-stop, the line is the color of the last color-stop.
The following steps must be applied in order to process the list of color-stops. After applying these rules, all color-stops will have a definite position and they will be in ascending order:
If multiple color-stops have the same position, they produce an infinitesimal transition from the one specified first in the rule to the one specified last. In effect, the color suddenly changes at that position rather than smoothly transitioning.
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.
1. linear-gradient(red, white 20%, blue)
==
linear-gradient(red 0%, white 20%, blue 100%)
2. linear-gradient(red 40%, white, black, blue)
==
linear-gradient(red 40%, white 60%, black 80%, blue 100%)
3. linear-gradient(red -50%, white, blue)
===
linear-gradient(red -50%, white 25%, blue 100%)
4. linear-gradient(red -50px, white, blue)
===
linear-gradient(red -50px, white calc(-25px + 50%), blue 100%)
5. linear-gradient(red 20px, white 0px, blue 40px)
==
linear-gradient(red 20px, white 20px, blue 40px)
6. linear-gradient(red, white -50%, black 150%, blue)
==
linear-gradient(red 0%, white 0%, black 150%, blue 150%)
7. linear-gradient(red 80px, white 0px, black, blue 100px)
==
linear-gradient(red 80px, white 80px, black 90px, blue 100px)
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:
linear-gradient(90deg, red, transparent, blue)
In premultiplied space, transitions to or from "transparent" always look nice:
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:
* SVG-in-HTML support required to view the images.
Note: It is recommended that authors not mix different types
of units, such as px, em, or %, in a single rule, as this can cause a
color-stop to unintentionally try to move before an earlier one. For
example, the rule ‘background-image:
linear-gradient(red, yellow 100px, blue 50%)
’ would work as
expected as long as the background area is at least 200px tall. If it was
150px tall, however, the blue color-stop's position would be equivalent to
"75px", which precedes the yellow color-stop, and would be corrected to a
position of 100px.
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.
Images used in CSS may come from a number of sources, from defined image formats (such as gif, jpeg, etc), dedicated markup formats (such as SVG), and CSS-specific formats (such as the linear-gradient() value type defined in this specification). As well, a document may contain many other types of objects, such as video, plugins, or nested documents. These images and objects (just objects 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.
In order to define this handling, we define a few terms, to make it easier to refer to various concepts:
An object's intrinsic dimensions are its preferred, natural width, height, and aspect ratio, if they exist. There can be an intrinsic height and intrinsic width, defining a definite rectangle. (Most bitmap images fall into this category.) There can be an intrinsic aspect ratio defining the relation of the width to the height, but no definite size. (SVG images designed to scale may fall into this category.) There can be just an intrinsic height or width. Or there can be no intrinsic dimensions at all, implying that the object has no preferred size or aspect ratio. (Embedded documents are often assumed to have no intrinsic size, as are CSS gradients, defined in this specification.)
If an object (such as an icon) has multiple sizes, then the largest size is used. If it has multiple aspect ratios of that size (or of no size), then the aspect ratio closest to the aspect ratio of the default object size is used.
object-fit
’ or ‘background-size
’ properties. The specified
size can be a definite width and height, a set of constraints, or a
combination thereof.
The default object size is a rectangle with a definite height and width used to determine the concrete object size when both the intrinsic dimensions and specified size are missing dimensions. It varies based on the context in which that the image is being laid out.
Below are some examples of default object sizing areas:
background-image
’
list-style-image
’
border-image
’
cursor
’
The only reason these are examples is because the proper place for the normative definitions of default object sizes is in the definitions for the relevant properties. These are the correct values, though.
Objects in CSS are sized and rendered as follows:
background-image
’ property or a @src
attribute on an <img> element, CSS queries the object for its intrinsic dimensions.
In the absence of more specific rules, an object's intrinsic dimensions are resolved to a concrete object size as follows:
If the specified size has
additional constraints, the concrete
object size must be sized to satisfy those constraints. For
example, the ‘min-width
’,
‘min-height
’, ‘max-width
’, and ‘max-height
’ properties specify slightly more
complex handling for sizing replaced elements, and ‘background-repeat: round
’ can further adjust the
size calculated by ‘background-size
’ so that the image fits a
whole number of times into the background positioning area.
object-fit
’ propertyName: | object-fit |
---|---|
Value: | fill | contain | cover | none | scale-down |
Initial: | fill |
Applies to: | replaced elements |
Inherited: | no |
Percentages: | N/A |
Media: | visual |
Computed value: | specified value |
The ‘object-fit
’ property specifies how the
contents of a replaced element should be scaled relative to the box
established by its used height and width. It also enables scaling a
replaced element's contents up to a specified maximum size or down to a
specified minimum size while preserving its aspect ratio. This property
never affects the size of the replaced element; it only affects the size
of the contents of the replaced element.
Not all replaced elements can be scaled, but images typically can.
If the replaced element's content do not have an intrinsic aspect ratio
(which may be derived from an intrinsic width and height), all of the
values for ‘object-fit
’ are treated as ‘fill
’. Otherwise, the contents are scaled as
follows:
Set the content's size to the concrete object size obtained by running the object sizing algorithm with a specified size and a default object size equal to the replaced element's used width and height.
This will make the contents exactly fill the replaced element.
Determine the used ‘height’ and ‘width’ of the element as usual, except if both ‘height’ and ‘width’ are ‘auto’, and the used value of at least one of ‘max-width’ and ‘max-height’ is not ‘none’, then compute the element's used width and used height as though the intrinsic dimensions of the contents were infinitely large numbers whose ratio is the actual intrinsic ratio of the contents.
Set the content's size to the largest width and height that has the same aspect ratio as the content's intrinsic aspect ratio, and additionally has neither width nor height larger than the replaced element's used width and height, respectively.
Determine the used ‘height’ and ‘width’ of the element as usual, except if both ‘height’ and ‘width’ are ‘auto’, and the used value of at least one of ‘min-width’ and ‘min-height’ is not ‘none’, then compute the element's used width and used height as though the intrinsic dimensions of the contents were infinitely small numbers whose ratio is the actual intrinsic ratio of the contents.
Set the content's size to the smallest width and height that has the same aspect ratio as the content's intrinsic aspect ratio, and additionally has neither width nor height smaller than the replaced element's used width and height, respectively.
Set the content's size to the concrete object size obtained by running the object sizing algorithm with no specified size, and a default object size equal to the replaced element's used width and height.
Size the content as if ‘none
’
or ‘contain
’ were specified, whichever would
result in a smaller size.
Note that both ‘none
’ and ‘contain
’
respect the content's intrinsic aspect ratio, so the concept of
"smaller" is well-defined.
If the content does not completely fill the replaced element, the
unfilled space shows the replaced element's background. Replaced elements
always clip their contents, similar to the effects of ‘overflow:hidden
’ on non-replaced elements, so any
content that would extend beyond the edges of the box are simply not
displayed. See the ‘object-position
’ property for positioning
the object with respect to the element's box.
An example showing how four of the values of ‘object-fit
’
cause the replaced element (blue figure) to be scaled to fit its
height/width box (shownwith a green background), using the initial value
for ‘object-position
’. The fifth value,
‘scale-down, in this case
looks identical to
’contain.
Note: the ‘object-fit
’ property has similar semantics
to the fit attribute in [SMIL10].
Note: Per the CSS⇋Object Negotiation algorithm, the concrete object size (or, in this case, the size of the content) does not directly scale the object itself - it is merely passed to the object as information about the size of the visible canvas. How to then draw into that size is up to the image format. In particular, raster images always scale to the given size, while SVG uses the given size as the size of the "SVG Viewport" (a term defined by SVG) and then uses the values of several attributes on the root <svg> element to determine how to draw itself.
User agents MAY accept ‘image-fit
’ as an alias for ‘object-fit
’, as
a previous version of this specification used that name. Authors must not
use ‘image-fit
’ in their
stylesheets.
object-position
’ propertyName: | object-position |
---|---|
Value: | <position> |
Initial: | 50% 50% |
Applies to: | replaced elements |
Inherited: | no |
Percentages: | refer to width and height of box itself |
Media: | visual |
Computed value: | specified value |
The ‘object-position
’ property determines the
alignment of the replaced element inside its box. The <position>
value type is defined as
the positioning syntax of ‘background-position
’
and is resolved in the same way, using the image as the subject and the
content box as the positioning area. [CSS21] [CSS3BG]
Note that areas of the box not covered by the replaced element will show the element's background.
User agents MAY accept ‘image-position
’ as an alias for ‘object-position
’, as a previous version of
this specification used that name. Authors must not use ‘image-position
’ in their stylesheets.
image-resolution
’ propertyThe image resolution is defined as the number of image pixels per
unit length, e.g., pixels per inch. Some image formats can record
information about the resolution of images. This information can be
helpful when determining the actual size of the image in the formatting
process. However, the information can also be wrong, in which case it
should be ignored. By default, CSS assumes a resolution of one image pixel
per CSS ‘px
’ unit; however, the
‘image-resolution
’ property allows using
some other resolution.
Name: | image-resolution |
---|---|
Value: | [from-image || <resolution>] snap? |
Initial: | 1dppx |
Applies to: | all elements |
Inherited: | yes |
Media: | visual |
Computed value: | specified value |
The ‘image-resolution
’ property specifies the
resolution of all images used in or on the element: images in content
(e.g. replaced elements and generated content), background images, list
markers, etc. Values have the following meanings:
1dppx
’ (the default
resolution).
This property must have no effect on vector images, as vector images do not have a concept of "resolution".
This rule specifies that the UA should use the image resolution found
in the image itself, falling back to 1 image pixel per CSS ‘px
’ unit.
img { image-resolution: from-image }
Using this rule, the image resolution is set to 300dpi and the resolution in the image, if any, is ignored.
img { image-resolution: 300dpi }
This rule, on the other hand, if used when the screen's resolution is 96dpi, would instead render the image at 288dpi (so that 3 image pixels map to 1 device pixel):
img { image-resolution: 300dpi snap; }
These rules both specify that the UA should use the image resolution found in the image itself. If the image has no resolution, the resolution is set to 300dpi.
img { image-resolution: from-image 300dpi } img { image-resolution: 300dpi from-image }
image-orientation
’ propertyImages from camera phones, digital cameras or scanners may be encoded sideways. For example, the first row of image data may represent the leftmost or rightmost column of image pixels. Furthermore, often such devices have limited resources, and do not have the capability to rotate the image into an upright orientation. However, this type of device may have internal knowledge or can accept input from its user as to the rotational correction to perform.
The image-orientation property provides a way to specify an "out-of-band" rotation to be applied to image source data. This facility is not intended to specify layout transformations such as arbitrary rotation or flipping the image in the horizontal or vertical direction. It is not needed to correctly orient an image when printing in landscape versus portrait orientation, as that rotation is done as part of layout. It should only be used to correct incorrectly-oriented images.
Name: | image-orientation |
---|---|
Value: | <angle> |
Initial: | 0deg |
Applies to: | images |
Inherited: | no |
Media: | visual |
Computed value: | specified value, rounded and normalized (see text) |
‘image-orientation
’ specifies an orthogonal
rotation to be applied to an image before it is laid out. CSS layout
processing applies to the image after rotation. This implies, for
example:
Positive values cause the image to be rotated to the right (in a clockwise direction), while negative values cause a rotation to the left. The computed value of the property is calculated by rounding the specified angle to the nearest quarter-turn (90deg, 100grad, .25turn, etc.), rounding away from 0 (that is, 45deg is rounded to 90deg, while -45deg is rounded to -90deg), then moduloing the value by 1 turn (360deg, 400grad, etc.).
The following example rotates the image 90 degrees clockwise:
img.ninety { image-orientation: 90deg } ... <img class="ninety" src=... />
The same effect could be achieved with, for example, an angle of -270deg or 450deg.
Thanks to Brad Kemper, Brian Manthos, and Alan Gresley for their contributions to the definition of gradients.
Property | Values | Initial | Applies to | Inh. | Percentages | Media |
---|---|---|---|---|---|---|
Name: | Value: | Initial: | Applies to: | Inherited: | Media: | Computed value: |
Name: | Value: | Initial: | Applies to: | Inherited: | Media: | Computed value: |
Name: | Value: | Initial: | Applies to: | Inherited: | Percentages: | Media: |
Name: | Value: | Initial: | Applies to: | Inherited: | Percentages: | Media: |
<position>
, ">6.5.