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<h1>CSS Transforms Module Level 1</h1>
<pre class='metadata'>
Status: ED
ED: http://dev.w3.org/csswg/css-transforms/
TR: http://www.w3.org/TR/css3-transforms/
Previous Version: http://www.w3.org/TR/2012/WD-css3-transforms-20120911/
Previous Version: http://www.w3.org/TR/2012/WD-css3-transforms-20120403/
Shortname: css-transforms
Link Defaults: svg (property) to/stroke/fill, css-flexbox-1 (property) display, css-masking-1 (property) clip/clip-path, filters-1 (property) filter
Level: 1
Group: fxtf
Editor: Simon Fraser, <a href="http://www.apple.com/">Apple Inc</a>, simon.fraser@apple.com
Editor: Dean Jackson, <a href="http://www.apple.com/">Apple Inc</a>, dino@apple.com
Editor: Edward O'Connor, <a href="http://www.apple.com/">Apple Inc</a>, eoconnor@apple.com
Editor: Dirk Schulze, <a href="http://www.adobe.com/">Adobe Systems Inc</a>, dschulze@adobe.com
Former Editor: David Hyatt, <a href="http://www.apple.com/">Apple Inc</a>, hyatt@apple.com
Former Editor: Chris Marrin, <a href="http://www.apple.com/">Apple Inc</a>, cmarrin@apple.com
Former Editor: Aryeh Gregor, <a href="http://www.mozilla.org/">Mozilla</a>, ayg@aryeh.name
Test Suite: http://test.csswg.org/suites/css3-transforms/nightly-unstable/
Abstract: CSS transforms allows elements styled with CSS to be transformed in two-dimensional or three-dimensional space. This specification is the convergence of the <a href="http://www.w3.org/TR/css3-2d-transforms/">CSS 2D transforms</a>, <a href="http://www.w3.org/TR/css3-3d-transforms/">CSS 3D transforms</a> and <a href="http://www.w3.org/TR/2009/WD-SVG-Transforms-20090320/">SVG transforms</a> specifications.
!Issues List: <a href="https://www.w3.org/Bugs/Public/buglist.cgi?query_format=advanced&product=CSS&component=Transforms&resolution=---&cmdtype=doit">in Bugzilla</a>
</pre>
<script src='http://test.csswg.org/harness/annotate.js#css-transforms-1_dev' type='text/javascript' defer></script>
<style type="text/css">
.example {
clear:both
}
th {
text-align:left
}
.pseudo-code {
font-family:monospace
}
.pseudo-code > ol {
list-style-type:decimal
}
.pseudo-code > ol > li > ol {
list-style-type:lower-latin
}
.pseudo-code > ol > li > ol > li > ol {
list-style-type:lower-roman
}
.pseudo-code ul {
list-style-type:disc
}
dd > p:nth-child(1) {
margin-top:0
}
</style>
<h2 id="intro">Introduction</h2>
<em>This section is not normative.</em>
The CSS <a href="http://www.w3.org/TR/REC-CSS2/visuren.html">visual formatting model</a> describes a coordinate system within each element is positioned. Positions and sizes in this coordinate space can be thought of as being expressed in pixels, starting in the origin of point with positive values proceeding to the right and down.
This coordinate space can be modified with the 'transform' property. Using transform, elements can be translated, rotated and scaled in two or three dimensional space.
Additional properties make working with transforms easier, and allow the author to control how nested three-dimensional transforms interact.
<ul>
<li>
The 'transform-origin' property provides a convenient way to control the origin about which transforms on an element are applied.
</li>
<li>
The 'perspective' property allows the author to make child elements with three-dimensional transforms appear as if they live in a common three-dimensional space. The 'perspective-origin' property provides control over the origin at which perspective is applied, effectively changing the location of the "vanishing point".
</li>
<li>
The 'transform-style' property allows 3D-transformed elements and their 3D-transformed descendants to share a common three-dimensional space, allowing the construction of hierarchies of three-dimensional objects.
</li>
<li>
The 'backface-visibility' property comes into play when an element is flipped around via three-dimensional transforms such that its reverse side is visible to the viewer. In some situations it is desirable to hide the element in this situation, which is possible using the value of ''hidden'' for this property.
</li>
</ul>
Note: While some values of the 'transform' property allow an element to be transformed in a three-dimensional coordinate system, the elements themselves are not three-dimensional objects. Instead, they exist on a two-dimensional plane (a flat surface) and have no depth.
<h2 id="module-interactions">Module Interactions</h2>
This module defines a set of CSS properties that affect the visual rendering of elements to which those properties are applied; these effects are applied after elements have been sized and positioned according to the <a href="http://www.w3.org/TR/CSS2/visuren.html" title="Visual formatting model">Visual formatting model</a> from [[!CSS21]]. Some values of these properties result in the creation of a <a href="http://www.w3.org/TR/CSS2/visuren.html#containing-block" title="Visual formatting model">containing block</a>, and/or the creation of a <a spec="css21">stacking context</a>.
Three-dimensional transforms can also affect the visual layering of elements, and thus override the back-to-front painting order described in <a href="http://www.w3.org/TR/CSS2/zindex.html" title="Elaborate description of Stacking Contexts">Appendix E</a> of [[!CSS21]].
Transforms affect the results of the Element Interface extensions <a href="http://www.w3.org/TR/cssom-view/#dom-element-getclientrects">getClientRects()</a> and <a href="http://www.w3.org/TR/cssom-view/#dom-element-getboundingclientrect">getBoundingClientRect()</a>, which are specified in [[CSSOM-VIEW]].
Transforms affect the rendering of backgrounds on elements with a value of ''fixed'' for the 'background-attachment' property, which is specified in [[!CSS3BG]].
<h2 id="css-values">CSS Values</h2>
This specification follows the <a href="http://www.w3.org/TR/CSS21/about.html#property-defs">CSS property definition conventions</a> from [[!CSS21]]. Value types not defined in these specifications are defined in CSS Values and Units Module Level 3 [[!CSS3VAL]].
In addition to the property-specific values listed in their definitions, all properties defined in this specification also accept the <a href="http://www.w3.org/TR/CSS21/cascade.html#value-def-inherit">inherit</a> keyword as their property value. For readability it has not been repeated explicitly.
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|__ $$__/
| $$ /$$$$$$ /$$$$$$ /$$$$$$/$$$$ /$$$$$$$
| $$ /$$__ $$ /$$__ $$| $$_ $$_ $$ /$$_____/
| $$| $$$$$$$$| $$ \__/| $$ \ $$ \ $$| $$$$$$
| $$| $$_____/| $$ | $$ | $$ | $$ \____ $$
| $$| $$$$$$$| $$ | $$ | $$ | $$ /$$$$$$$/
|__/ \_______/|__/ |__/ |__/ |__/|_______/
-->
<h2 id="terminology">Terminology</h2>
When used in this specification, terms have the meanings assigned in this section.
<dl>
<dt><dfn>bounding box</dfn></dt>
<dd>
A bounding box is the object bounding box for all SVG elements without an associated CSS layout box and the border box for all other elements. The bounding box of a table is the border box of its <a href="http://www.w3.org/TR/CSS21/tables.html#model">table wrapper box</a>, not its table box.
</dd>
<dt><dfn>transformable element</dfn></dt>
<dd>
A transformable element is an element in one of these categories:
<ul>
<li>
an element whose layout is governed by the CSS box model which is either a <a href="http://www.w3.org/TR/CSS2/visuren.html#block-level">block-level</a> or <a href="http://www.w3.org/TR/CSS2/visuren.html#x13">atomic inline-level element</a>, or whose 'display' property computes to ''table-row'', ''table-row-group'', ''table-header-group'', ''table-footer-group'', ''table-cell'', or ''table-caption'' [[!CSS21]]
</li>
<li>
an element in the SVG namespace and not governed by the CSS box model which has the attributes 'transform', <a>'patternTransform'</a> or <a>'linearGradient/gradientTransform'</a> [[!SVG11]].
</li>
</ul>
</dd>
<dt><dfn>user coordinate system</dfn></dt>
<dt><dfn>local coordinate system</dfn></dt>
<dd>
In general, a coordinate system defines locations and distances on the current canvas. The current local coordinate system (also user coordinate system) is the coordinate system that is currently active and which is used to define how coordinates and lengths are located and computed, respectively, on the current canvas.
</dd>
<dt><dfn>perspective matrix</dfn></dt>
<dd>
A matrix computed from the values of the 'perspective' and 'perspective-origin' properties as described <a href="#perspective-matrix-computation">below</a>.
</dd>
<dt><dfn>transformation matrix</dfn></dt>
<dd>
A matrix that defines the mathematical mapping from one coordinate system into another. It is computed from the values of the 'transform' and 'transform-origin' properties as described <a href="#transformation-matrix-computation">below</a>.
</dd>
<dt><dfn>current transformation matrix (CTM)</dfn></dt>
<dd>
A matrix that defines the mapping from the local coordinate system into the viewport coordinate system.
</dd>
<dt><dfn>accumulated 3D transformation matrix</dfn></dt>
<dd>
A matrix computed for elements in a <i>3D rendering context</i>, as described <a href="#accumulated-3d-transformation-matrix-computation">below</a>.
</dd>
<dt><dfn>identity transform function</dfn></dt>
<dd>
A <a href="#transform-functions">transform function</a> that is equivalent to a identity 4x4 matrix (see <a href="#mathematical-description">Mathematical Description of Transform Functions</a>). Examples for identity transform functions are ''translate(0)'', ''translate3d(0, 0, 0)'', ''translateX(0)'', ''translateY(0)'', ''translateZ(0)'', ''scale(1)'', ''scaleX(1)'', ''scaleY(1)'', ''scaleZ(1)'', ''rotate(0)'', ''rotate3d(1, 1, 1, 0)'', ''rotateX(0)'', ''rotateY(0)'', ''rotateZ(0)'', ''skew(0, 0)'', ''skewX(0)'', ''skewY(0)'', ''matrix(1, 0, 0, 1, 0, 0)'' and ''matrix3d(1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1)''. A special case is perspective: ''perspective(infinity)''. The value of m<sub>34</sub> becomes infinitesimal small and the transform function is therefore assumed to be equal to the identity matrix.
</dd>
<dt><dfn>3D rendering context</dfn></dt>
<dd>
A containing block hierarchy of one or more levels, instantiated by elements with a computed value for the 'transform-style' property of ''preserve-3d'', whose elements share a common three-dimensional coordinate system.
</dd>
</dl>
<!--
/$$$$$$ /$$$$$$$ /$$$$$$ /$$
/$$__ $$| $$__ $$ /$$__ $$ | $$
|__/ \ $$| $$ \ $$ | $$ \__/ /$$ /$$| $$$$$$$
/$$$$$$/| $$ | $$ /$$$$$$| $$$$$$ | $$ | $$| $$__ $$
/$$____/ | $$ | $$|______/ \____ $$| $$ | $$| $$ \ $$
| $$ | $$ | $$ /$$ \ $$| $$ | $$| $$ | $$
| $$$$$$$$| $$$$$$$/ | $$$$$$/| $$$$$$/| $$$$$$$/
|________/|_______/ \______/ \______/ |_______/
-->
<h2 id="two-dimensional-subset">Two Dimensional Subset</h2>
UAs may not always be able to render three-dimensional transforms and then just support a two-dimensional subset of this specification. In this case <a href="#three-d-transform-functions">three-dimensional transforms</a> and the properties 'transform-style', 'perspective', 'perspective-origin' and 'backface-visibility' must not be supported. Section <a href="#transform-3d-rendering">3D Transform Rendering</a> does not apply. Matrix decomposing uses the technique taken from the "unmatrix" method in "Graphics Gems II, edited by Jim Arvo", simplified for the 2D case. Section <a href="#mathematical-description">Mathematical Description of Transform Functions</a> is still effective but can be reduced by using a 3x3 transformation matrix where <em>a</em> equals <em>m<sub>11</sub></em>, <em>b</em> equals <em>m<sub>12</sub></em>, <em>c</em> equals <em>m<sub>21</sub></em>, <em>d</em> equals <em>m<sub>22</sub></em>, <em>e</em> equals <em>m<sub>41</sub></em> and <em>f</em> equals <em>m<sub>42</sub></em> (see <a href="#MatrixDefined">A 2D 3x2 matrix with six parameter</a>).
<div class="figure">
<img src="3x3matrix.png" alt="3x3 matrix" title="\begin{bmatrix} a & c & e \\ b
& d & f \\ 0 & 0 & 1 \end{bmatrix}" width="82" height="79">
<p class="caption">
3x3 matrix for two-dimensional transformations.
</p>
</div>
<div class="example">
<p>
Authors can easily provide a fallback if UAs do not provide support for three-dimensional transforms. The following example has two property definitions for 'transform'. The first one consists of two two-dimensional transform functions. The second one has a two-dimensional and a three-dimensional transform function.
</p>
<pre>div {
transform: scale(2) rotate(45deg);
transform: scale(2) rotate3d(0, 0, 1, 45deg);
}</pre>
<p>
With 3D support, the second definition will override the first one. Without 3D support, the second definition is invalid and a UA falls back to the first definition.
</p>
</div>
<!--
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| $$__ $$ | $$
| $$ \ $$ /$$$$$$ /$$$$$$$ /$$$$$$$ /$$$$$$ /$$$$$$
| $$$$$$$/ /$$__ $$| $$__ $$ /$$__ $$ /$$__ $$ /$$__ $$
| $$__ $$| $$$$$$$$| $$ \ $$| $$ | $$| $$$$$$$$| $$ \__/
| $$ \ $$| $$_____/| $$ | $$| $$ | $$| $$_____/| $$
| $$ | $$| $$$$$$$| $$ | $$| $$$$$$$| $$$$$$$| $$
|__/ |__/ \_______/|__/ |__/ \_______/ \_______/|__/
-->
<h2 id="transform-rendering">The Transform Rendering Model</h2>
<em>This section is normative.</em>
Specifying a value other than ''none'' for the 'transform' property establishes a new <i>local coordinate system</i> at the element that it is applied to. The mapping from where the element would have rendered into that local coordinate system is given by the element's <i>transformation matrix</i>. Transformations are cumulative. That is, elements establish their local coordinate system within the coordinate system of their parent. From the perspective of the user, an element effectively accumulates all the 'transform' properties of its ancestors as well as any local transform applied to it. The accumulation of these transforms defines a <i>current transformation matrix (CTM)</i> for the element.
The coordinate space is a coordinate system with two axes: the X axis increases horizontally to the right; the Y axis increases vertically downwards. Three-dimensional transform functions extend this coordinate space into three dimensions, adding a Z axis perpendicular to the plane of the screen, that increases towards the viewer.
<div class="figure">
<img src="coordinates.svg" width="270" height="240" alt="Demonstration of the initial coordinate space">
<p class="caption">
Demonstration of the initial coordinate space.
</p>
</div>
<p id="transformation-matrix-computation">
The <i>transformation matrix</i> is computed from the 'transform' and 'transform-origin' properties as follows:
<ol>
<li>Start with the identity matrix.</li>
<li>Translate by the computed X, Y and Z values of 'transform-origin'</li>
<li>Multiply by each of the transform functions in 'transform' property from left to right</li>
<li>Translate by the negated computed X, Y and Z values of 'transform-origin'</li>
</ol>
</p>
Transforms apply to <i>transformable elements</i>.
<div class="example">
<pre>
div {
transform: translate(100px, 100px);
}</pre>
<p>
This transform moves the element by 100 pixels in both the X and Y directions.
</p>
<div class="figure">
<img src="examples/translate1.svg" alt="The 100px translation in X and Y"
width="470" height="250">
</div>
</div>
<div class="example">
<pre>div {
height: 100px; width: 100px;
transform-origin: 50px 50px;
transform: rotate(45deg);
}</pre>
<p>
The 'transform-origin' property moves the point of origin by 50 pixels in both the X and Y directions. The transform rotates the element clockwise by 45° about the point of origin. After all transform functions were applied, the translation of the origin gets translated back by -50 pixels in both the X and Y directions.
</p>
<div class="figure">
<img alt="The point of origin gets translated temporary" src="examples/origin1.svg" width="735" height="250">
</div>
</div>
<div class="example">
<pre>
div {
height: 100px; width: 100px;
transform: translate(80px, 80px) scale(1.5, 1.5) rotate(45deg);
}</pre>
<p>
This transform moves the element by 80 pixels in both the X and Y directions, then scales the element by 150%, then rotates it 45° clockwise about the Z axis. Note that the scale and rotation operate about the center of the element, since the element has the default transform-origin of ''50% 50%''.
</p>
<div class="figure">
<img src="examples/compound_transform.svg" alt="The transform specified above"
width="270" height="270">
</div>
<p class="note">
Note that an identical rendering can be obtained by nesting elements with the equivalent transforms:
</p>
<pre>
<div style="transform: translate(80px, 80px)">
<div style="transform: scale(1.5, 1.5)">
<div style="transform: rotate(45deg)"></div>
</div>
</div></pre>
</div>
For elements whose layout is governed by the CSS box model, the transform property does not affect the flow of the content surrounding the transformed element. However, the extent of the overflow area takes into account transformed elements. This behavior is similar to what happens when elements are offset via relative positioning. Therefore, if the value of the 'overflow' property is ''scroll'' or ''auto'', scrollbars will appear as needed to see content that is transformed outside the visible area.
For elements whose layout is governed by the CSS box model, any value other than ''none'' for the transform results in the creation of both a stacking context and a containing block. The object acts as a containing block for fixed positioned descendants.
<p class="issue">
Is this effect on ''position: fixed'' necessary? If so, need to go into more detail here about why fixed positioned objects should do this, i.e., that it's much harder to implement otherwise. See <a href="https://www.w3.org/Bugs/Public/show_bug.cgi?id=16328">Bug 16328</a>.
</p>
<a href="http://www.w3.org/TR/css3-background/#fixed0">Fixed backgrounds</a> on the root element are affected by any transform specified for that element. For all other elements that are effected by a transform (i.e. have a transform applied to them, or to any of their ancestor elements), a value of ''fixed'' for the 'background-attachment' property is treated as if it had a value of ''scroll''. The computed value of 'background-attachment' is not affected.
Note: If the root element is transformed, the transformation applies to the entire canvas, including any background specified for the root element. Since <a href="http://www.w3.org/TR/css3-background/#special-backgrounds">the background painting area for the root element</a> is the entire canvas, which is infinite, the transformation might cause parts of the background that were originally off-screen to appear. For example, if the root element's background were repeating dots, and a transformation of ''scale(0.5)'' were specified on the root element, the dots would shrink to half their size, but there will be twice as many, so they still cover the whole viewport.
<h3 id="3d-transform-rendering">3D Transform Rendering</h3>
Normally, elements render as flat planes, and are rendered into the same plane as their containing block. Often this is the plane shared by the rest of the page. Two-dimensional transform functions can alter the appearance of an element, but that element is still rendered into the same plane as its containing block.
Three-dimensional transforms can result in transformation matrices with a non-zero Z component (where the Z axis projects out of the plane of the screen). This can result in an element rendering on a different plane than that of its containing block. This may affect the front-to-back rendering order of that element relative to other elements, as well as causing it to intersect with other elements. This behavior depends on whether the element is a member of a <i>3D rendering context</i>, as described below.
<div class="issue">
<p class="desc">
This description does not exactly match what WebKit implements. Perhaps it should be changed to match current implementations? See <a href="https://www.w3.org/Bugs/Public/show_bug.cgi?id=19637">Bug 19637</a>.
</p>
</div>
<div class="example">
<p>
This example shows the effect of three-dimensional transform applied to an element.
</p>
<pre>
<style>
div {
height: 150px;
width: 150px;
}
.container {
border: 1px solid black;
}
.transformed {
transform: rotateY(50deg);
}
</style>
<div class="container">
<div class="transformed"></div>
</div></pre>
<div class="figure">
<img src="examples/simple-3d-example.png" width="210" height="190" alt="Div with a rotateY transform.">
</div>
<p>
The transform is a 50° rotation about the vertical, Y axis. Note how this makes the blue box appear narrower, but not three-dimensional.
</p>
</div>
The 'perspective' and 'perspective-origin' properties can be used to add a feeling of depth to a scene by making elements higher on the Z axis (closer to the viewer) appear larger, and those further away to appear smaller. The scaling is proportional to <var>d</var>/(<var>d</var> − <var>Z</var>) where <var>d</var>, the value of 'perspective', is the distance from the drawing plane to the the assumed position of the viewer's eye.
<div class="figure">
<img alt="Diagram of scale vs. Z position" src="perspective_distance.png">
<p class="caption">
Diagrams showing how scaling depends on the 'perspective' property and Z position. In the top diagram, <var>Z</var> is half of <var>d</var>. In order to make it appear that the original circle (solid outline) appears at <var>Z</var> (dashed circle), the circle is scaled up by a factor of two, resulting in the light blue circle. In the bottom diagram, the circle is scaled down by a factor of one-third to make it appear behind the original position.
</p>
</div>
Normally the assumed position of the viewer's eye is centered on a drawing. This position can be moved if desired – for example, if a web page contains multiple drawings that should share a common perspective – by setting 'perspective-origin'.
<div class="figure">
<img alt="Diagram of different perspective-origin" src="perspective_origin.png">
<p class="caption">
Diagram showing the effect of moving the perspective origin upward.
</p>
</div>
<p id="perspective-matrix-computation">
The <i>perspective matrix</i> is computed as follows:
<ol>
<li>Start with the identity matrix.</li>
<li>Translate by the computed X and Y values of 'perspective-origin'</li>
<li>Multiply by the matrix that would be obtained from the ''perspective()'' transform function, where the length is provided by the value of the 'perspective' property</li>
<li>Translate by the negated computed X and Y values of 'perspective-origin'</li>
</ol>
</p>
<div class="example">
<p>
This example shows how perspective can be used to cause three-dimensional transforms to appear more realistic.
</p>
<pre>
<style>
div {
height: 150px;
width: 150px;
}
.container {
perspective: 500px;
border: 1px solid black;
}
.transformed {
transform: rotateY(50deg);
}
</style>
<div class="container">
<div class="transformed"></div>
</div></pre>
<div class="figure">
<img src="examples/simple-perspective-example.png" width="210" height="190" alt="Div with a rotateY transform, and perspective on its container">
</div>
<p>
The inner element has the same transform as in the previous example, but its rendering is now influenced by the perspective property on its parent element. Perspective causes vertices that have positive Z coordinates (closer to the viewer) to be scaled up in X and Y, and those further away (negative Z coordinates) to be scaled down, giving an appearance of depth.
</p>
</div>
An element with a three-dim
34B2
ensional transform that is not contained in a <i>3D rendering context</i> renders with the appropriate transform applied, but does not intersect with any other elements. The three-dimensional transform in this case can be considered just as a painting effect, like two-dimensional transforms. Similarly, the transform does not affect painting order. For example, a transform with a positive Z translation may make an element look larger, but does not cause that element to render in front of elements with no translation in Z.
An element with a three-dimensional transform that is contained in a <i>3D rendering context</i> can visibly interact with other elements in that same 3D rendering context; the set of elements participating in the same <i>3D rendering context</i> may obscure each other or intersect, based on their computed transforms. They are rendered as if they are all siblings, positioned in a common 3D coordinate space. The position of each element in that three-dimensional space is determined by accumulating the transformation matrices up from the element that establishes the <i>3D rendering context</i> through each element that is a containing block for the given element, as described below.
<div class="example">
<pre>
<style>
div {
height: 150px;
width: 150px;
}
.container {
perspective: 500px;
border: 1px solid black;
}
.transformed {
transform: rotateY(50deg);
background-color: blue;
}
.child {
transform-origin: top left;
transform: rotateX(40deg);
background-color: lime;
}
</style>
<div class="container">
<div class="transformed">
<div class="child"></div>
</div>
</div>
</pre>
<p>
This example shows how nested 3D transforms are rendered in the absence of ''transform-style: preserve-3d''. The blue div is transformed as in the previous example, with its rendering influenced by the perspective on its parent element. The lime element also has a 3D transform, which is a rotation about the X axis (anchored at the top, by virtue of the transform-origin). However, the lime element is being rendered into the plane of its parent because it is not a member of a 3D rendering context; the parent is "flattening".
</p>
<div class="figure">
<img src="examples/3d-rendering-context-flat.png" width="240" height="200" alt="Nested 3D transforms, with flattening">
</div>
</div>
Elements establish and participate in 3D rendering contexts as follows:
<ul>
<li>
A <i>3D rendering context</i> is established by a a <i>transformable element</i> whose computed value for 'transform-style' is ''preserve-3d'', and which itself is not part of a 3D rendering context. Note that such an element is always a containing block. An element that establishes a 3D rendering context also participates in that context.
</li>
<li>
An element whose computed value for 'transform-style' is ''preserve-3d'', and which itself participates in a <i>3D rendering context</i>, extends that 3D rendering context rather than establishing a new one.
</li>
<li>
An element participates in a <i>3D rendering context</i> if its containing block establishes or extends a <i>3D rendering context</i>.
</li>
</ul>
<p id="accumulated-3d-transformation-matrix-computation">
The final value of the transform used to render an element in a <i>3D rendering context</i> is computed by accumulating an <i>accumulated 3D transformation matrix</i> as follows:
</p>
<ol>
<li>Start with the identity matrix.</li>
<li>For each containing block between the root of the <i>3D rendering context</i> and the element in question:
<ol>
<li>multiply the accumulated matrix with the <i>perspective matrix</i> on the element's containing block (if any). That containing block is not necessarily a member of the 3D rendering context.</li>
<li>apply to the accumulated matrix a translation equivalent to the horizontal and vertical offset of the element relative to its containing block as specified by the CSS visual formatting model.</li>
<li>multiply the accumulated matrix with the <i>transformation matrix</i>.</li>
</ol>
</li>
</ol>
<div class="example">
<pre>
<style>
div {
height: 150px;
width: 150px;
}
.container {
perspective: 500px;
border: 1px solid black;
}
.transformed {
<b>transform-style: preserve-3d</b>;
transform: rotateY(50deg);
background-color: blue;
}
.child {
transform-origin: top left;
transform: rotateX(40deg);
background-color: lime;
}
</style></pre>
<p>
This example is identical to the previous example, with the addition of ''transform-style: preserve-3d'' on the blue element. The blue element now establishes a 3D rendering context, of which the lime element is a member. Now both blue and lime elements share a common three-dimensional space, so the lime element renders as tilting out from its parent, influenced by the perspective on the container.
</p>
<div class="figure">
<img src="examples/3d-rendering-context-3d.png" width="240" height="200" alt="Nested 3D transforms, with preserve-3d.">
</div>
</div>
Elements in the same <i>3D rendering context</i> may intersect with each other. User agents must render intersection by subdividing the planes of intersecting elements as described by <a href="http://en.wikipedia.org/wiki/Newell's_algorithm">Newell's algorithm</a>.
Untransformed elements in a <i>3D rendering context</i> render on the Z=0 plane, yet may still intersect with transformed elements.
Within a <i>3D rendering context</i>, the rendering order of non-intersecting elements is based on their position on the Z axis after the application of the accumulated transform. Elements at the same Z position render in <a href="http://www.w3.org/TR/CSS2/zindex.html#painting-order">stacking context order</a>.
<div class="example">
<pre>
<style>
.container {
background-color: rgba(0, 0, 0, 0.3);
transform-style: preserve-3d;
perspective: 500px;
}
.container > div {
position: absolute;
left: 0;
}
.container > :first-child {
transform: rotateY(45deg);
background-color: orange;
top: 10px;
height: 135px;
}
.container > :last-child {
transform: translateZ(40px);
background-color: rgba(0, 0, 255, 0.75);
top: 50px;
height: 100px;
}
</style>
<div class="container">
<div></div>
<div></div>
</div></pre>
<p>
This example shows show elements in a 3D rendering context can intersect. The container element establishes a 3D rendering context for itself and its two children. The children intersect with each other, and the orange element also intersects with the container.
</p>
<div class="figure">
<img src="examples/3d-intersection.png" width="200" height="200" alt="Intersecting sibling elements.">
</div>
</div>
Using three-dimensional transforms, it's possible to transform an element such that its reverse side is towards the viewer. 3D-transformed elements show the same content on both sides, so the reverse side looks like a mirror-image of the front side (as if the element were projected onto a sheet of glass). Normally, elements whose reverse side is towards the viewer remain visible. However, the 'backface-visibility' property allows the author to make an element invisible when its reverse side is towards the viewer. This behavior is "live"; if an element with ''backface-visibility: hidden'' were animating, such that its front and reverse sides were alternately visible, then it would only be visible when the front side were towards the viewer.
<div class="issue">
<p class="desc">
This wording needs clarification; 'backface-visibility' works per-3D rendering context, not relative to the root.
</p>
</div>
<h3 id="processing-of-perspective-transformed-boxes">
Processing of Perspective-Transformed Boxes
</h3>
<div class="issue">
<p class="desc">
This is a first pass at an attempt to precisely specify how exactly to transform elements using the provided matrices. It might not be ideal, and implementer feedback is encouraged. See <a href="https://www.w3.org/Bugs/Public/show_bug.cgi?id=15605">bug 15605</a>.
</p>
</div>
The <i>accumulated 3D transformation matrix</i> is a 4×4 matrix, while the objects to be transformed are two-dimensional boxes. To transform each corner (<var>a</var>, <var>b</var>) of a box, the matrix must first be applied to (<var>a</var>, <var>b</var>, 0, 1), which will result in a four-dimensional point (<var>x</var>, <var>y</var>, <var>z</var>, <var>w</var>). This is transformed back to a three-dimensional point (<var>x</var>′, <var>y</var>′, <var>z</var>′) as follows:
If <var>w</var> > 0, (<var>x</var>′, <var>y</var>′, <var>z</var>′) = (<var>x</var>/<var>w</var>, <var>y</var>/<var>w</var>, <var>z</var>/<var>w</var>).
If <var>w</var> = 0, (<var>x</var>′, <var>y</var>′, <var>z</var>′) = (<var>x</var> ⋅ <var>n</var>, <var>y</var> ⋅ <var>n</var>, <var>z</var> ⋅ <var>n</var>). <var>n</var> is an implementation-dependent value that should be chosen so that <var>x</var>′ or <var>y</var>′ is much larger than the viewport size, if possible. For example, (5px, 22px, 0px, 0) might become (5000px, 22000px, 0px), with <var>n</var> = 1000, but this value of <var>n</var> would be too small for (0.1px, 0.05px, 0px, 0). This specification does not define the value of <var>n</var> exactly. Conceptually, (<var>x</var>′, <var>y</var>′, <var>z</var>′) is <a href="http://en.wikipedia.org/wiki/Plane_at_infinity">infinitely far</a> in the direction (<var>x</var>, <var>y</var>, <var>z</var>).
If <var>w</var> < 0 for all four corners of the transformed box, the box is not rendered.
If <var>w</var> < 0 for one to three corners of the transformed box, the box must be replaced by a polygon that has any parts with <var>w</var> < 0 cut out. This will in general be a polygon with three to five vertices, of which exactly two will have <var>w</var> = 0 and the rest <var>w</var> > 0. These vertices are then transformed to three-dimensional points using the rules just stated. Conceptually, a point with <var>w</var> < 0 is "behind" the viewer, so should not be visible.
<div class="example">
<pre><style>
.transformed {
height: 100px;
width: 100px;
background: lime;
transform: perspective(50px) translateZ(100px);
}
</style></pre>
<p>
All of the box's corners have <var>z</var>-coordinates greater than the perspective. This means that the box is behind the viewer and will not display. Mathematically, the point (<var>x</var>, <var>y</var>) first becomes (<var>x</var>, <var>y</var>, 0, 1), then is translated to (<var>x</var>, <var>y</var>, 100, 1), and then applying the perspective results in (<var>x</var>, <var>y</var>, 100, −1). The <var>w</var>-coordinate is negative, so it does not display. An implementation that doesn't handle the <var>w</var> < 0 case separately might incorrectly display this point as (−<var>x</var>, −<var>y</var>, −100), dividing by −1 and mirroring the box.
</p>
</div>
<div class="example">
<pre><style>
.transformed {
height: 100px;
width: 100px;
background: radial-gradient(yellow, blue);
transform: perspective(50px) translateZ(50px);
}
</style></pre>
<p>
Here, the box is translated upward so that it sits at the same place the viewer is looking from. This is like bringing the box closer and closer to one's eye until it fills the entire field of vision. Since the default transform-origin is at the center of the box, which is yellow, the screen will be filled with yellow.
</p>
<p>
Mathematically, the point (<var>x</var>, <var>y</var>) first becomes (<var>x</var>, <var>y</var>, 0, 1), then is translated to (<var>x</var>, <var>y</var>, 50, 1), then becomes (<var>x</var>, <var>y</var>, 50, 0) after applying perspective. Relative to the transform-origin at the center, the upper-left corner was (−50, −50), so it becomes (−50,
−50, 50, 0). This is transformed to something very far to the upper left, such as (−5000, −5000, 5000). Likewise the other corners are sent very far away. The radial gradient is stretched over the whole box, now enormous, so the part that's visible without scrolling should be the color of the middle pixel: yellow. However, since the box is not actually infinite, the user can still scroll to the edges to see the blue parts.
</p>
</div>
<div class="issue">
<p class="desc">
WebKit doesn't render this box unless the translateZ() is < 50px.
</p>
</div>
<div class="example">
<pre><style>
.transformed {
height: 50px;
width: 50px;
background: lime;
border: 25px solid blue;
transform-origin: left;
transform: perspective(50px) rotateY(-45deg);
}
</style></pre>
<p>
The box will be rotated toward the viewer, with the left edge staying fixed while the right edge swings closer. The right edge will be at about <var>z</var> = ''70.7px'', which is closer than the perspective of ''50px''. Therefore, the rightmost edge will vanish ("behind" the viewer), and the visible part will stretch out infinitely far to the right.
</p>
<p>
Mathematically, the top right vertex of the box was originally (100, −50), relative to the transform-origin. It is first expanded to (100, −50, 0, 1). After applying the transform specified, this will get mapped to about (70.71, −50, 70.71, −0.4142). This has <var>w</var> = −0.4142 < 0, so we need to slice away the part of the box with <var>w</var> < 0. This results in the new top-right vertex being (50, −50, 50, 0). This is then mapped to some faraway point in the same direction, such as (5000, −5000, 5000), which is up and to the right from the transform-origin. Something similar is done to the lower right corner, which gets mapped far down and to the right. The resulting box stretches far past the edge of the screen.
</p>
<p>
Again, the rendered box is still finite, so the user can scroll to see the whole thing if he or she chooses. However, the right part has been chopped off. No matter how far the user scrolls, the rightmost ''30px'' or so of the original box will not be visible. The blue border was only ''25px'' wide, so it will be visible on the left, top, and bottom, but not the right.
</p>
<p>
The same basic procedure would apply if one or three vertices had <var>w</var> < 0. However, in that case the result of truncating the <var>w</var> < 0 part would be a triangle or pentagon instead of a quadrilateral.
</p>
</div>
<!--
/$$ /$$$$$$
| $$ /$$__ $$
/$$$$$$ /$$$$$$ /$$$$$$ /$$$$$$$ /$$$$$$$| $$ \__//$$$$$$ /$$$$$$
|_ $$_/ /$$__ $$|____ $$| $$__ $$ /$$_____/| $$$$ /$$__ $$ /$$__ $$
| $$ | $$ \__/ /$$$$$$$| $$ \ $$| $$$$$$ | $$_/ | $$ \ $$| $$ \__/
| $$ /$$| $$ /$$__ $$| $$ | $$ \____ $$| $$ | $$ | $$| $$
| $$$$/| $$ | $$$$$$$| $$ | $$ /$$$$$$$/| $$ | $$$$$$/| $$
\___/ |__/ \_______/|__/ |__/|_______/ |__/ \______/ |__/
/$$$$$$/$$$$
| $$_ $$_ $$
| $$ \ $$ \ $$
| $$ | $$ | $$
| $$ | $$ | $$
|__/ |__/ |__/
-->
<h2 id="transform-property">The 'transform' Property</h2>
A transformation is applied to the coordinate system an element renders in through the 'transform' property. This property contains a list of <a href="#transform-functions">transform functions</a>. The final transformation value for a coordinate system is obtained by converting each function in the list to its corresponding matrix like defined in <a href="#mathematical-description">Mathematical Description of Transform Functions</a>, then multiplying the matrices.
<pre class='propdef'>
Name: transform
Value: none | <<transform-list>>
Initial: none
Applies to: <i>transformable elements</i>
Inherited: no
Percentages: refer to the size of <i>bounding box</i>
Computed value: As specified, but with relative lengths converted into absolute lengths.
Media: visual
Animatable: as <a href="#interpolation-of-transforms">transform</a>
</pre>
Any computed value other than ''none'' for the transform results in the creation of both a stacking context and a containing block. The object acts as a containing block for fixed positioned descendants.
<pre class=prod><dfn><transform-list></dfn> = <<transform-function>>+</pre>
<!--
/$$ /$$$$$$
| $$ /$$__ $$
/$$$$$$ /$$$$$$ /$$$$$$ /$$$$$$$ /$$$$$$$| $$ \__//$$$$$$ /$$$$$$
|_ $$_/ /$$__ $$|____ $$| $$__ $$ /$$_____/| $$$$ /$$__ $$ /$$__ $$
| $$ | $$ \__/ /$$$$$$$| $$ \ $$| $$$$$$ | $$_/ | $$ \ $$| $$ \__/
| $$ /$$| $$ /$$__ $$| $$ | $$ \____ $$| $$ | $$ | $$| $$
| $$$$/| $$ | $$$$$$$| $$ | $$ /$$$$$$$/| $$ | $$$$$$/| $$
\___/ |__/ \_______/|__/ |__/|_______/ |__/ \______/ |__/
/$$ /$$
|__/ |__/
/$$$$$$/$$$$ /$$$$$$ /$$$$$$ /$$ /$$$$$$ /$$ /$$$$$$$
| $$_ $$_ $$ /$$$$$$ /$$__ $$ /$$__ $$| $$ /$$__ $$| $$| $$__ $$
| $$ \ $$ \ $$|______/| $$ \ $$| $$ \__/| $$| $$ \ $$| $$| $$ \ $$
| $$ | $$ | $$ | $$ | $$| $$ | $$| $$ | $$| $$| $$ | $$
| $$ | $$ | $$ | $$$$$$/| $$ | $$| $$$$$$$| $$| $$ | $$
|__/ |__/ |__/ \______/ |__/ |__/ \____ $$|__/|__/ |__/
/$$ \ $$
| $$$$$$/
\______/
-->
<h2 id="transform-origin-property">The 'transform-origin' Property</h2>
<pre class='propdef'>
Name: transform-origin
Value: [ left | center | right | top | bottom | <<percentage>> | <<length>> ]<br> | <br> [ left | center | right | <<percentage>> | <<length>> ]<br> [ top | center | bottom | <<percentage>> | <<length>> ] <<length>>?<br> |<br> [ center | [ left | right ] ] && [ center | [ top | bottom ] ] <<length>>?
Initial: 50% 50%
Applies to: <i>transformable elements</i>
Inherited: no
Percentages: refer to the size of <i>bounding box</i>
Computed value: For <<length>> the absolute value, otherwise a percentage
Media: visual
Animatable: as <a href="http://dev.w3.org/csswg/css3-transitions/#animtype-simple-list">simple list</a> of <a href="http://dev.w3.org/csswg/css3-transitions/#animtype-lpcalc">length, percentage, or calc</a>
</pre>
The default value for SVG elements without associated CSS layout box is ''0 0''.
The values of the 'transform' and 'transform-origin' properties are used to compute the <i>transformation matrix</i>, as described above.
If only one value is specified, the second value is assumed to be <a value for=transform-origin>center</a>. If one or two values are specified, the third value is assumed to be ''0px''.
If two or more values are defined and either no value is a keyword, or the only used keyword is <a value for=transform-origin>center</a>, then the first value represents the horizontal position (or offset) and the second represents the vertical position (or offset). A third value always represents the Z position (or offset) and must be of type <<length>>.
<dl dfn-for="transform-origin" dfn-type="value">
<dt><<percentage>></dt>
<dd>
A percentage for the horizontal offset is relative to the width of the <i>bounding box</i>. A percentage for the vertical offset is relative to height of the <i>bounding box</i>. The value for the horizontal and vertical offset represent an offset from the top left corner of the <i>bounding box</i>.
</dd>
<dt><<length>></dt>
<dd>
<p>
A length value gives a fixed length as the offset. The value for the horizontal and vertical offset represent an offset from the top left corner of the <i>bounding box</i>.
</p>
<p>
For SVG elements without an associated CSS layout box the horizontal and vertical offset represent an offset from the point of origin of the element's local coordinate space.
</p>
</dd>
<dt><dfn>top</dfn>
<dd>Computes to ''0%'' for the vertical position.
<dt><dfn>right</dfn>
<dd>Computes to ''100%'' for the horizontal position.
<dt><dfn>bottom</dfn>
<dd>Computes to ''100%'' for the vertical position.
<dt><dfn>left</dfn>
<dd>Computes to ''0%'' for the horizontal position.
<dt><dfn>center</dfn>
<dd>
Computes to ''50%'' (''left 50%'') for the horizontal position if the horizontal position is not otherwise specified, or ''50%'' (''top 50%'') for the vertical position if it is.
</dl>
The <a href="http://www.w3.org/TR/cssom/#resolved-value">resolved value</a> of 'transform-origin' is the <a href="http://www.w3.org/TR/CSS21/cascade.html#used-value">used value</a> (i.e., percentages are resolved to absolute lengths).
<!--
/$$ /$$$$$$
| $$ /$$__ $$
/$$$$$$ /$$$$$$ /$$$$$$ /$$$$$$$ /$$$$$$$| $$ \__//$$$$$$ /$$$$$$
|_ $$_/ /$$__ $$|____ $$| $$__ $$ /$$_____/| $$$$ /$$__ $$ /$$__ $$
| $$ | $$ \__/ /$$$$$$$| $$ \ $$| $$$$$$ | $$_/ | $$ \ $$| $$ \__/
| $$ /$$| $$ /$$__ $$| $$ | $$ \____ $$| $$ | $$ | $$| $$
| $$$$/| $$ | $$$$$$$| $$ | $$ /$$$$$$$/| $$ | $$$$$$/| $$
\___/ |__/ \_______/|__/ |__/|_______/ |__/ \______/ |__/
/$$ /$$
| $$ | $$
/$$$$$$/$$$$ /$$$$$$$ /$$$$$$ /$$ /$$| $$ /$$$$$$
| $$_ $$_ $$ /$$$$$$ /$$_____/|_ $$_/ | $$ | $$| $$ /$$__ $$
| $$ \ $$ \ $$|______/| $$$$$$ | $$ | $$ | $$| $$| $$$$$$$$
| $$ | $$ | $$ \____ $$ | $$ /$$| $$ | $$| $$| $$_____/
| $$ | $$ | $$ /$$$$$$$/ | $$$$/| $$$$$$$| $$| $$$$$$$
|__/ |__/ |__/ |_______/ \___/ \____ $$|__/ \_______/
/$$ | $$
| $$$$$$/
\______/
-->
<h2 id="transform-style-property">The 'transform-style' Property</h2>
<pre class='propdef'>
Name: transform-style
Value: flat | preserve-3d
Initial: flat
Applies to: <i>transformable elements</i>
Inherited: no
Percentages: N/A
Computed value: Same as specified value.
Media: visual
Animatable: no
</pre>
A value of ''preserve-3d'' for 'transform-style' establishes a stacking context.
<p class="issue">Keyword description missing. Description of flattening behavior is missing. Does rendering operate in 'z-index' order? Do we still create a 3D context and let planes intersect? See <a href="https://www.w3.org/Bugs/Public/show_bug.cgi?id=22427">Bug 22427</a> <a href="https://www.w3.org/Bugs/Public/show_bug.cgi?id=23015">Bug 23015</a>.</p>
The following CSS property values require the user agent to create a flattened representation of the descendant elements before they can be applied, and therefore override the behavior of ''transform-style: preserve-3d'':
<ul>
<li>'overflow': any value other than ''visible''.</li>
<li>'filter': any value other than ''none''.</li>
<li>'clip': any value other than ''auto''.</li>
<li>'clip-path': any value other than ''none''.</li>
<li>'isolation': used value of ''isolate''.</li>
<li>'mask-image': any value other than ''none''.</li>
<li>'mask-box-image-source': any value other than ''none''.</li>
<li>'mix-blend-mode': any value other than ''normal''.</li>
</ul>
The computed value of 'transform-style' is not affected.
The values of the 'transform' and 'transform-origin' properties are used to compute the <i>transformation matrix</i>, as described above.
<!--
/$$$$$$ /$$$$$$ /$$$$$$ /$$$$$$$ /$$$$$$ /$$$$$$ /$$$$$$$
/$$__ $$ /$$__ $$ /$$__ $$ /$$_____/ /$$__ $$ /$$__ $$ /$$_____/
| $$ \ $$| $$$$$$$$| $$ \__/| $$$$$$ | $$ \ $$| $$$$$$$$| $$
| $$ | $$| $$_____/| $$ \____ $$| $$ | $$| $$_____/| $$
| $$$$$$$/| $$$$$$$| $$ /$$$$$$$/| $$$$$$$/| $$$$$$$| $$$$$$$
| $$____/ \_______/|__/ |_______/ | $$____/ \_______/ \_______/
| $$ | $$
| $$ | $$
|__/ |__/
/$$ /$$
| $$ |__/
/$$$$$$ /$$ /$$ /$$ /$$$$$$
|_ $$_/ | $$| $$ /$$//$$__ $$
| $$ | $$ \ $$/$$/| $$$$$$$$
| $$ /$$| $$ \ $$$/ | $$_____/
| $$$$/| $$ \ $/ | $$$$$$$
\___/ |__/ \_/ \_______/
-->
<h2 id="perspective-property">The 'perspective' Property</h2>
<pre class='propdef'>
Name: perspective
Value: none | <<length>>
Initi
16E8
al: none
Applies to: <i>transformable elements</i>
Inherited: no
Percentages: N/A
Computed value: Absolute length or "none".
Media: visual
Animatable: as <a href="http://dev.w3.org/csswg/css3-transitions/#animtype-length">length</a>
</pre>
If the value is ''none'', no perspective transform is applied. Lengths must be positive.
The use of this property with any value other than ''none'' establishes a stacking context. It also establishes a containing block (somewhat similar to ''position: relative''), just like the 'transform' property does.
The values of the 'perspective' and 'perspective-origin' properties are used to compute the <i>perspective matrix</i>, as described above.
<!--
/$$$$$$ /$$$$$$ /$$$$$$ /$$$$$$$ /$$$$$$ /$$$$$$ /$$$$$$$
/$$__ $$ /$$__ $$ /$$__ $$ /$$_____/ /$$__ $$ /$$__ $$ /$$_____/
| $$ \ $$| $$$$$$$$| $$ \__/| $$$$$$ | $$ \ $$| $$$$$$$$| $$
| $$ | $$| $$_____/| $$ \____ $$| $$ | $$| $$_____/| $$
| $$$$$$$/| $$$$$$$| $$ /$$$$$$$/| $$$$$$$/| $$$$$$$| $$$$$$$
| $$____/ \_______/|__/ |_______/ | $$____/ \_______/ \_______/
| $$ | $$
| $$ | $$
|__/ |__/
/$$ /$$ /$$
| $$ |__/ |__/
/$$$$$$ /$$ /$$ /$$ /$$$$$$ /$$$$$$ /$$$$$$ /$$ /$$$$$$
|_ $$_/ | $$| $$ /$$//$$__ $$ /$$$$$$ /$$__ $$ /$$__ $$| $$ /$$__ $$
| $$ | $$ \ $$/$$/| $$$$$$$$|______/| $$ \ $$| $$ \__/| $$| $$ \ $$
| $$ /$$| $$ \ $$$/ | $$_____/ | $$ | $$| $$ | $$| $$ | $$
| $$$$/| $$ \ $/ | $$$$$$$ | $$$$$$/| $$ | $$| $$$$$$$
\___/ |__/ \_/ \_______/ \______/ |__/ |__/ \____ $$
/$$ \ $$
| $$$$$$/
\______/
/$$
|__/
/$$ /$$$$$$$
| $$| $$__ $$
| $$| $$ \ $$
| $$| $$ | $$
| $$| $$ | $$
|__/|__/ |__/
-->
<h2 id="perspective-origin-property">The 'perspective-origin' Property</h2>
The 'perspective-origin' property establishes the origin for the 'perspective' property. It effectively sets the X and Y position at which the viewer appears to be looking at the children of the element.
<pre class='propdef'>
Name: perspective-origin
Value: [ left | center | right | top | bottom | <<percentage>> | <<length>> ]<br> |<br> [ left | center | right | <<percentage>> | <<length>> ]<br> [ top | center | bottom | <<percentage>> | <<length>> ]<br> |<br> [ center | [ left | right ] ] && [ center | [ top | bottom ] ]
Initial: 50% 50%
Applies to: <i>transformable elements</i>
Inherited: no
Percentages: refer to the size of the <i>bounding box</i>
Computed value: For <<length>> the absolute value, otherwise a percentage.
Media: visual
Animatable: as <a href="http://dev.w3.org/csswg/css3-transitions/#animtype-simple-list">simple list</a> of <a href="http://dev.w3.org/csswg/css3-transitions/#animtype-lpcalc">length, percentage, or calc</a>
</pre>
The values of the 'perspective' and 'perspective-origin' properties are used to compute the <i>perspective matrix</i>, as described above.
If only one value is specified, the second value is assumed to be <a value for=perspective-origin>center</a>.
If at least one of the two values is not a keyword, then the first value represents the horizontal position (or offset) and the second represents the vertical position (or offset).
The values for 'perspective-origin' represent an offset of the perspective origin from the top left corner of the <i>bounding box</i>.
<dl dfn-for="perspective-origin" dfn-type="value">
<dt><<percentage>></dt>
<dd><p>A percentage for the horizontal perspctive offset is relative to the width of the
<i>bounding box</i>.
A percentage for the vertical offset is relative to height of the
<i>bounding box</i>. The value for the horizontal
and vertical offset represent an offset from the top left corner of the
<i>bounding box</i>.
</p>
</dd>
<dt><<length>></dt>
<dd>
<p>A length value gives a fixed length as the offset. The value for the horizontal
and vertical offset represent an offset from the top left corner of the
<i>bounding box</i>.
</dd>
<dt><dfn>top</dfn></dt>
<dd>Computes to ''0%'' for the vertical position.</dd>
<dt><dfn>right</dfn></dt>
<dd>Computes to ''100%'' for the horizontal position.</dd>
<dt><dfn>bottom</dfn></dt>
<dd>Computes to ''100%'' for the vertical position.</dd>
<dt><dfn>left</dfn></dt>
<dd>Computes to ''0%'' for the horizontal position.</dd>
<dt><dfn>center</dfn></dt>
<dd>
Computes to ''50%'' (''left 50%'') for the horizontal position if the horizontal position is not otherwise specified, or ''50%'' (''top 50%'') for the vertical position if it is.
</dd>
</dl>
The <a href="http://www.w3.org/TR/cssom/#resolved-value">resolved value</a> of 'perspective-origin' is the <a href="http://www.w3.org/TR/CSS21/cascade.html#used-value">used value</a> (i.e., percentages are resolved to absolute lengths).
<!--
/$$ /$$ /$$$$$$
| $$ | $$ /$$__ $$
| $$$$$$$ /$$$$$$ /$$$$$$$| $$ /$$| $$ \__//$$$$$$ /$$$$$$$ /$$$$$$
| $$__ $$ |____ $$ /$$_____/| $$ /$$/| $$$$ |____ $$ /$$_____/ /$$__ $$
| $$ \ $$ /$$$$$$$| $$ | $$$$$$/ | $$_/ /$$$$$$$| $$ | $$$$$$$$
| $$ | $$ /$$__ $$| $$ | $$_ $$ | $$ /$$__ $$| $$ | $$_____/
| $$$$$$$/| $$$$$$$| $$$$$$$| $$ \ $$| $$ | $$$$$$$| $$$$$$$| $$$$$$$
|_______/ \_______/ \_______/|__/ \__/|__/ \_______/ \_______/ \_______/
/$$ /$$ /$$ /$$ /$$ /$$ /$$
|__/ |__/| $$ |__/| $$|__/ | $$
/$$ /$$ /$$ /$$$$$$$ /$$| $$$$$$$ /$$| $$ /$$ /$$$$$$ /$$ /$$
/$$$$$$| $$ /$$/| $$ /$$_____/| $$| $$__ $$| $$| $$| $$|_ $$_/ | $$ | $$
|______/ \ $$/$$/ | $$| $$$$$$ | $$| $$ \ $$| $$| $$| $$ | $$ | $$ | $$
\ $$$/ | $$ \____ $$| $$| $$ | $$| $$| $$| $$ | $$ /$$| $$ | $$
\ $/ | $$ /$$$$$$$/| $$| $$$$$$$/| $$| $$| $$ | $$$$/| $$$$$$$
\_/ |__/|_______/ |__/|_______/ |__/|__/|__/ \___/ \____ $$
/$$ | $$
| $$$$$$/
\______/
-->
<h2 id="backface-visibility-property">The 'backface-visibility' Property</h2>
The 'backface-visibility' property determines whether or not the "back" side of a transformed element is visible when facing the viewer. With an identity transform, the front side of an element faces the viewer. Applying a rotation about Y of 180°ree; (for instance) would cause the back side of the element to face the viewer.
Note: This property is useful when you place two elements back-to-back, as you would to create a playing card. Without this property, the front and back elements could switch places at times during an animation to flip the card. Another example is creating a box out of 6 elements, but where you want to see the inside faces of the box. This is useful when creating the backdrop for a 3 dimensional stage.
<pre class='propdef'>
Name: backface-visibility
Value: visible | hidden
Initial: visible
Applies to: <i>transformable elements</i>
Inherited: no
Percentages: N/A