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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html lang=en>
<head profile="http://www.w3.org/2006/03/hcard"><meta
content="text/html; charset=utf-8" http-equiv=Content-Type>
<title>CSS Transforms</title>
<link href="../default.css" rel=stylesheet type="text/css">
<link href="http://www.w3.org/StyleSheets/TR/W3C-ED.css" rel=stylesheet
type="text/css">
<style type="text/css">
.term {
font-style: italic;
}
.todo {
font-weight: bold;
border-left: 0.5em solid #f44;
padding-left: 1em;
margin-top: 0.5em;
color: #a0a0a0;
}
.todo:before {
content: "TO DO : ";
color: #f44;
}
</style>
<body>
<div class=head id=div-head> <!--begin-logo-->
<p><a href="http://www.w3.org/"><img alt=W3C height=48
src="http://www.w3.org/Icons/w3c_home" width=72></a> <!--end-logo-->
<h1>CSS Transforms</h1>
<h2 class="no-num no-toc" id=longstatus-date>Editor's Draft 14 March 2012</h2>
<dl>
<dt>This version:
<dd> <a
href="http://www.w3.org/TR/2012/ED-css3-transforms-20120314/">http://dev.w3.org/csswg/css3-transforms/</a>
<!--http://www.w3.org/TR/2012/WD-css3-transforms-20120314-->
<dt>Latest version:
<dd><a
href="http://www.w3.org/TR/css3-transforms">http://www.w3.org/TR/css3-transforms/</a>
<dt>Editor's draft:
<dd><a
href="http://dev.w3.org/csswg/css3-transforms/">http://dev.w3.org/csswg/css3-transforms/</a>
<dt>Previous version:
<dd>None
<dt id=editors-list>Editors:
<dd>Simon Fraser (<a href="http://www.apple.com/">Apple Inc</a>)
<simon.fraser @apple.com>
<dd>Dean Jackson (<a href="http://www.apple.com/">Apple Inc</a>) <dino
@apple.com>
<dd>David Hyatt (<a href="http://www.apple.com/">Apple Inc</a>) <hyatt
@apple.com>
<dd>Chris Marrin (<a href="http://www.apple.com/">Apple Inc</a>)
<cmarrin @apple.com>
<dd>Edward O'Connor (<a href="http://www.apple.com/">Apple Inc</a>)
<eoconnor @apple.com>
<dd>Dirk Schulze (<a href="http://www.adobe.com/">Adobe Systems, Inc</a>)
<dschulze @adobe.com>
<dd>Aryeh Gregor (<a href="http://www.mozilla.org/">Mozilla</a>) <ayg
@aryeh.name>
<dt>Issues list:
<dd><a
href="https://www.w3.org/Bugs/Public/buglist.cgi?query_format=advanced&product=CSS&component=Transforms&resolution=---&cmdtype=doit">in
Bugzilla</a>
<dt>Test suite:
<dd>none yet
</dl>
<!--begin-copyright-->
<p class=copyright><a
href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright"
rel=license>Copyright</a> © 2012 <a
href="http://www.w3.org/"><acronym
title="World Wide Web Consortium">W3C</acronym></a><sup>®</sup> (<a
href="http://www.csail.mit.edu/"><acronym
title="Massachusetts Institute of Technology">MIT</acronym></a>, <a
href="http://www.ercim.eu/"><acronym
title="European Research Consortium for Informatics and Mathematics">ERCIM</acronym></a>,
<a href="http://www.keio.ac.jp/">Keio</a>), All Rights Reserved. W3C <a
href="http://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">liability</a>,
<a
href="http://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">trademark</a>
and <a
href="http://www.w3.org/Consortium/Legal/copyright-documents">document
use</a> rules apply.</p>
<!--end-copyright-->
<hr title="Separator for header">
</div>
<h2 class="no-num no-toc" id=abstract>Abstract</h2>
<p>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.
<h2 class="no-num no-toc" id=status>Status of this document</h2>
<p class=note> This specification merges the former CSS 2D Transforms and
CSS 3D Transforms specifications and will also merge CSS Transforms and
SVG Transforms. The merge is in progress and the specification is not yet
ready for review.</p>
<!--begin-status-->
<p>This is a public copy of the editors' draft. It is provided for
discussion only and may change at any moment. Its publication here does
not imply endorsement of its contents by W3C. Don't cite this document
other than as work in progress.
<p>The (<a
href="http://lists.w3.org/Archives/Public/www-style/">archived</a>) public
mailing list <a
href="mailto:www-style@w3.org?Subject=%5Bcss3-transforms%5D%20PUT%20SUBJECT%20HERE">
www-style@w3.org</a> (see <a
href="http://www.w3.org/Mail/Request">instructions</a>) is preferred for
discussion of this specification. When sending e-mail, please put the text
“css3-transforms” in the subject, preferably like this:
“[<!---->css3-transforms<!---->] <em>…summary of
comment…</em>”
<p>This document was produced by the <a href="/Style/CSS/members">CSS
Working Group</a> (part of the <a href="/Style/">Style Activity</a>).
<p>This document was produced by a group operating under the <a
href="/Consortium/Patent-Policy-20040205/">5 February 2004 W3C Patent
Policy</a>. W3C maintains a <a href="/2004/01/pp-impl/32061/status"
rel=disclosure>public list of any patent disclosures</a> 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 <a
href="/Consortium/Patent-Policy-20040205/#def-essential">Essential
Claim(s)</a> must disclose the information in accordance with <a
href="/Consortium/Patent-Policy-20040205/#sec-Disclosure">section 6 of the
W3C Patent Policy</a>.</p>
<!--end-status-->
<p> The <a href=ChangeLog>list of changes made to this specification</a> is
available.
<h2 class="no-num no-toc" id=contents>Table of contents</h2>
<!--begin-toc-->
<ul class=toc>
<li><a href="#introduction"><span class=secno>1. </span>Introduction</a>
<li><a href="#module-interactions"><span class=secno>2. </span>Module
Interactions</a>
<li><a href="#css-values"><span class=secno>3. </span>CSS Values</a>
<li><a href="#definitions"><span class=secno>4. </span>Definitions</a>
<li><a href="#transform-rendering"><span class=secno>5. </span>The
Transform Rendering Model</a>
<ul class=toc>
<li><a href="#transform-3d-rendering"><span class=secno>5.1. </span>3D
Transform Rendering</a>
</ul>
<li><a href="#transform-property"><span class=secno>6. </span> The
‘<code class=css><code
class=property>transform</code></code>’ Property </a>
<li><a href="#svg-transform"><span class=secno>7. </span> The SVG
‘<code class=css><code
class=property>transform</code></code>’ Attribute </a>
<ul class=toc>
<li><a href="#transform-attribute-specificity"><span class=secno>7.1.
</span>SVG ‘<code class=css><code
class=property>transform</code></code>’ attribute specificity</a>
<li><a href="#svg-syntax"><span class=secno>7.2. </span>Syntax of the
SVG ‘<code class=css><code
class=property>transform</code></code>’ attribute</a>
<ul class=toc>
<li><a href="#svg-transform-list"><span class=secno>7.2.1.
</span>Transform List</a>
<li><a href="#svg-functional-notation"><span class=secno>7.2.2.
</span>Functional Notation</a>
<li><a href="#svg-data-types"><span class=secno>7.2.3. </span>SVG Data
Types</a>
<ul class=toc>
<li><a href="#svg-transform-value"><span class=secno>7.2.3.1.
</span>The <var><translation-value></var> and
<var><length></var> type</a>
<li><a href="#svg-angle"><span class=secno>7.2.3.2. </span>The
<var><angle></var> type</a>
<li><a href="#svg-number"><span class=secno>7.2.3.3. </span>The
<var><number></var> type</a>
</ul>
</ul>
<li><a href="#svg-gradient-transform-pattern-transform"><span
class=secno>7.3. </span>The SVG ‘<code
class=property>gradientTransform</code>’ and ‘<code
class=property>patternTransform</code>’ attributes</a>
<li><a href="#svg-transformation-functions"><span class=secno>7.4.
</span>SVG transformation functions</a>
<li><a href="#svg-three-dimensional-functions"><span class=secno>7.5.
</span>SVG and 3D transformation functions</a>
<li><a href="#svg-object-bounding-box"><span class=secno>7.6.
</span>Object bounding box units</a>
<li><a href="#transform-attribute-dom"><span class=secno>7.7. </span>SVG
DOM interface for the ‘<code class=css><code
class=property>transform</code></code>’ attribute</a>
<li><a href="#svg-animation"><span class=secno>7.8. </span>SVG
Animation</a>
<ul class=toc>
<li><a href="#svg-animate-element"><span class=secno>7.8.1. </span>The
SVG ‘<code class=property>animate</code>’ and
‘<code class=property>set</code>’ element</a>
<li><a href="#svg-attribute-name"><span class=secno>7.8.2. </span>The
SVG ‘<code class=property>attributeName</code>’
attribute</a>
<li><a href="#svg-attribute-type"><span class=secno>7.8.3. </span>The
SVG ‘<code class=property>attributeType</code>’
attribute</a>
<li><a href="#svg-animateTransform-extension"><span class=secno>7.8.4.
</span>The SVG ‘<code
class=property>animateTransform</code>’ element</a>
</ul>
</ul>
<li><a href="#transform-origin-property"><span class=secno>8. </span> The
‘<code class=css><code
class=property>transform-origin</code></code>’ Property </a>
<li><a href="#transform-style-property"><span class=secno>9. </span> The
‘<code class=css><code
class=property>transform-style</code></code>’ Property </a>
<li><a href="#perspective-property"><span class=secno>10. </span> The
‘<code class=css><code
class=property>perspective</code></code>’ Property </a>
<li><a href="#perspective-origin-property"><span class=secno>11. </span>
The ‘<code class=css><code
class=property>perspective-origin</code></code>’ Property </a>
<li><a href="#backface-visibility-property"><span class=secno>12. </span>
The ‘<code class=css><code
class=property>backface-visibility</code></code>’ Property </a>
<li><a href="#transform-functions"><span class=secno>13. </span> The
Transformation Functions </a>
<ul class=toc>
<li><a href="#two-d-transform-functions"><span class=secno>13.1.
</span>2D Transformation Functions</a>
<li><a href="#three-d-transform-functions"><span class=secno>13.2.
</span>3D Transformation Functions</a>
</ul>
<li><a href="#transform-values"><span class=secno>14. </span> Transform
Values and Lists </a>
<li><a href="#animation"><span class=secno>15. </span> Transitions and
Animations between Transform Values </a>
<li><a href="#matrix-decomposition"><span class=secno>16. </span> Matrix
Decomposition for Animation </a>
<ul class=toc>
<li><a href="#unmatrix"><span class=secno>16.1. </span>Unmatrix</a>
<li><a href="#animating-the-components"><span class=secno>16.2.
</span>Animating the components</a>
<li><a href="#recomposing-the-matrix"><span class=secno>16.3.
</span>Recomposing the matrix</a>
</ul>
<li><a href="#mathematical-description"><span class=secno>17. </span>
Mathematical Description of Transformation Functions </a>
<li><a href="#references"><span class=secno>18. </span>References</a>
<ul class=toc>
<li class=no-num><a href="#normative-references">Normative
references</a>
<li class=no-num><a href="#other-references">Other references</a>
</ul>
<li class=no-num><a href="#property-index">Property index</a>
<li class=no-num><a href="#index">Index</a>
</ul>
<!--end-toc-->
<h2 id=introduction><span class=secno>1. </span>Introduction</h2>
<p><em>This section is not normative.</em>
<p> 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.
<p> This coordinate space can be modified with the ‘<a
href="#effects"><code class=css><code
class=property>transform</code></code></a>’ property. Using
transform, elements can be translated, rotated and scaled in two or three
dimensional space.
<p> Additional properties make working with transforms easier, and allow
the author to control how nested three-dimensional transforms interact.
<ul>
<li> The ‘<a href="#transform-origin"><code class=css><code
class=property>transform-origin</code></code></a>’ property
provides a convenient way to control the origin about which transforms on
an element are applied.
<li> The ‘<a href="#perspective"><code class=css><code
class=property>perspective</code></code></a>’ property allows the
author to make child elements with three-dimensional transforms appear as
if they live in a common three-dimensional space. The ‘<a
href="#perspective-origin"><code class=css><code
class=property>perspective-origin</code></code></a>’ property
provides control over the origin at which perspective is applied,
effectively changing the location of the "vanishing point".
<li> The ‘<a href="#transform-style"><code class=css><code
class=property>transform-style</code></code></a>’ 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> The ‘<a href="#backface-visibility"><code class=css><code
class=property>backface-visibility</code></code></a>’ 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 ‘<code
class=property>hidden</code>’ for this property.
</ul>
<p> Note that while some values of the ‘<a href="#effects"><code
class=css><code class=property>transform</code></code></a>’ 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.
<div class=issue> There are two roles for transformations in layout: (1)
transformations that adjust the position of the affected content without
changing the normal layout of that content (much like relative
positioning) and (2) transformation of the content prior to layout that
affects the layout of that content. See <a
href="http://lists.w3.org/Archives/Public/www-style/2007Oct/0209">http://lists.w3.org/Archives/Public/www-style/2007Oct/0209</a>
for examples of both cases. The "transform" property (as defined in this
document) is equally useful for both roles. This document is focused on
satisfying the first role. There is, however, an architectural question
that arises because there needs to be a way to distinguish which role an
author of a stylesheet wants. The key question is which is the default
behavior/role for the "transform" property and how is the other
behavior/role indicated by a stylesheet author. If you have an opinion on
this topic, please send feedback.</div>
<!-- ======================================================================================================= -->
<h2 id=module-interactions><span class=secno>2. </span>Module Interactions</h2>
<p>Write me
<h2 id=css-values><span class=secno>3. </span>CSS Values</h2>
<p>Write me
<h2 id=definitions><span class=secno>4. </span>Definitions</h2>
<p> When used in this specification, terms have the meanings assigned in
this section.
<dl>
<dt id=TermBoundingBox><dfn id=bounding-box>bounding box</dfn>
<dd>
<p> 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.</p>
<dt id=TermTransformableElement><dfn
id=transformable-element>transformable element</dfn>
<dd>
<p> A transformable element in the HTML namespace 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 an element in the SVG namespace (see <a
href="#SVG11" rel=biblioentry>[SVG11]<!--{{SVG11}}--></a>) which has the
attributes ‘<a href="#effects"><code class=css><code
class=property>transform</code></code></a>’, ‘<code
class=property>patternTransform</code>’ or ‘<code
class=property>gradientTransform</code>’.</p>
<dt id=TermPerspectiveMatrix><dfn id=perspective-matrix>perspective
matrix</dfn>
<dd>
<p> A matrix computed from the values of the ‘<a
href="#perspective"><code class=css><code
class=property>perspective</code></code></a>’ and ‘<a
href="#perspective-origin"><code class=css><code
class=property>perspective-origin</code></code></a>’ properties as
described <a href="#perspective-matrix-computation">below</a>.</p>
<dt id=TermTransformationMatrix><dfn
id=transformation-matrix>transformation matrix</dfn>
<dd>
<p> A matrix computed from the values of the ‘<a
href="#effects"><code class=css><code
class=property>transform</code></code></a>’ and ‘<a
href="#transform-origin"><code class=css><code
class=property>transform-origin</code></code></a>’ properties as
described <a href="#transformation-matrix-computation">below</a>.</p>
<dt id=Term3DRenderingContext><dfn id=d-rendering-context>3D rendering
context</dfn>
<dd>
<p> A containing block hierarchy of one or more levels, instantiated by
elements with a computed value for the ‘<a
href="#transform-style"><code class=css><code
class=property>transform-style</code></code></a>’ property of
‘<code class=css><code class=css>preserve-3d</code></code>’,
whose elements share a common three-dimensional coordinate system.</p>
</dd>
<!-- Define "three-dimensional transform" ? -->
</dl>
<!-- ======================================================================================================= -->
<h2 id=transform-rendering><span class=secno>5. </span>The Transform
Rendering Model</h2>
<!-- This section is normative -->
<p> Specifying a value other than ‘<code class=css><code
class=css>none</code></code>’ for the ‘<a
href="#effects"><code class=css><code
class=property>transform</code></code></a>’ property establishes a
new <em>local coordinate system</em> 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 <a
href="#TermTransformationMatrix"><i>transformation matrix</i></a>.
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
‘<a href="#effects"><code class=css><code
class=property>transform</code></code></a>’ properties of its
ancestors as well as any local transform applied to it. The accumulation
of these transforms defines a <em>current transformation matrix (CTM)</em>
for the element.
<p> The coordinate space behaves as described in the <a
href="http://www.w3.org/TR/SVG/coords.html#EstablishingANewUserSpace">coordinate
system transformations</a> section of the SVG 1.1 specification. This 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 extent this coordinate space into three dimensions,
adding a Z axis perpendicular to the plane of the screen, that increases
towards the viewer.
<p id=transformation-matrix-computation> The <a
href="#TermTransformationMatrix"><i>transformation matrix</i></a> is
computed from the ‘<a href="#effects"><code class=css><code
class=property>transform</code></code></a>’ and ‘<a
href="#transform-origin"><code class=css><code
class=property>transform-origin</code></code></a>’ properties as
follows:
<ol>
<li>Start with the identity matrix.
<li>Translate by the computed X, Y and Z values of ‘<a
href="#transform-origin"><code class=css><code
class=property>transform-origin</code></code></a>’
<li>Multiply by each of the transform functions in ‘<a
href="#effects"><code class=css><code
class=property>transform</code></code></a>’ property in turn
<li>Translate by the negated computed X, Y and Z values of ‘<a
href="#transform-origin"><code class=css><code
class=property>transform-origin</code></code></a>’
</ol>
<p> Transforms apply to <a class=term
href="#transformable-element">transformable elements</a>.
<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 alt="The 100px translation in X and Y"
src=transform1.png></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 ‘<code class=css><code class=css>50%
50%</code></code>’.</p>
<div class=figure> <img alt="The transform specified above"
src="compound_transform.png"></div>
<p>Note that an identical rendering can be obtained by nesting elements
with the equivalent transforms:
<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>
<!-- This "in the HTML namespace" is awkward. Is there a better way? -->
<p> In the HTML namespace, 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 ‘<code class=css><code
class=property>overflow</code></code>’ property is ‘<code
class=css><code class=css>scroll</code></code>’ or ‘<code
class=css><code class=css>auto</code></code>’, scrollbars will
appear as needed to see content that is transformed outside the visible
area.
<p> In the HTML namespace, any value other than ‘<code
class=css><code class=css>none</code></code>’ 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 affect 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.
<p> <a href="http://dev.w3.org/csswg/css3-background/#fixed0">Fixed
backgrounds</a> are affected by any transform specified for the root
element, and not by any other transforms.
<p class=note> Thus an element with a fixed background still acts like a
"porthole" into an image that's fixed to the viewport, and transforms on
the element affect the porthole, not the background behind it. On the
other hand, transforming the root element will still transform everything
on the page, rather than everything except for fixed backgrounds.
<h3 id=transform-3d-rendering><span class=secno>5.1. </span>3D Transform
Rendering</h3>
<!-- Maybe define "tranform container" in the definitions, and use it everywhere
in place of "containing block"? I'm not sure if "containing block" is exactly right. -->
<p> 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.
<p> Three-dimensional transforms can result in transformation matrices with
a non-zero Z component<!-- clarify -->, potentially lifting them off the
plane of their containing block. Because of this, elements with
three-dimensional transformations could potentially render in an
front-to-back order that different from the normal CSS rendering order,
and intersect with each other. Whether they do so depends on whether the
element is a member of a <a class=term href="#d-rendering-context">3D
rendering context</a>, 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?</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 alt="Div with a rotateY transform." height=190
src="examples/simple-3d-example.png" width=210></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>
<p> The ‘<a href="#perspective"><code class=css><code
class=property>perspective</code></code></a>’ and ‘<a
href="#perspective-origin"><code class=css><code
class=property>perspective-origin</code></code></a>’ 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.
<p id=perspective-matrix-computation> The <a
href="#TermPerspectiveMatrix"><i>perspective matrix</i></a> is computed as
follows: <!-- Make this more mathy, with matrices? -->
<ol>
<li>Start with the identity matrix.
<li>Translate by the computed X and Y values of ‘<a
href="#perspective-origin"><code class=css><code
class=property>perspective-origin</code></code></a>’
<li>Multiply by the matrix that would be obtained from the ‘<code
class=css><a href="#perspective-function"><code
class=css>perspective(<length>)</code></a></code>’ transform
function, where the length is provided by the value of the ‘<a
href="#perspective"><code class=css><code
class=property>perspective</code></code></a>’ property
<li>Translate by the negated computed X and Y values of ‘<a
href="#perspective-origin"><code class=css><code
class=property>perspective-origin</code></code></a>’
</ol>
<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
alt="Div with a rotateY transform, and perspective on its container"
height=190 src="examples/simple-perspective-example.png" width=210></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>
<p> An element with a three-dimensional transform that is not contained in
a <a class=term href="#d-rendering-context">3D rendering context</a>
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.
<p> An element with a three-dimensional transform that is contained in a <a
class=term href="#d-rendering-context">3D rendering context</a> can
visibly interact with other elements in that same 3D rendering context;
the set of elements participating in the same <a class=term
href="#d-rendering-context">3D rendering context</a> 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 <a class=term href="#d-rendering-context">3D rendering
context</a> through each element that is a containing block for the given
element, as described below.
<!-- More detail required, probably with matrices -->
<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 ‘<code class=css><code class=css>transform-style:
preserve-3d</code></code>’. 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 alt="Nested 3D transforms, with flattening"
height=200 src="examples/3d-rendering-context-flat.png" width=240></div>
</div>
<p>Elements establish and participate in 3D rendering contexts as follows:
<ul>
<li> A <a class=term href="#d-rendering-context">3D rendering context</a>
is established by a a <a class=term
href="#transformable-element">transformable element</a> whose computed
value for ‘<a href="#transform-style"><code class=css><code
class=property>transform-style</code></code></a>’ is ‘<code
class=css><code class=css>preserve-3d</code></code>’, 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> An element whose computed value for ‘<a
href="#transform-style"><code class=css><code
class=property>transform-style</code></code></a>’ is ‘<code
class=css><code class=css>preserve-3d</code></code>’, and which
itself participates in a <a class=term href="#d-rendering-context">3D
rendering context</a>, extends that 3D rendering context rather than
establishing a new one.
<li> An element participates in a <a class=term
href="#d-rendering-context">3D rendering context</a> if its containing
block establishes or extends a <a class=term
href="#d-rendering-context">3D rendering context</a>.
</ul>
<p> The final value of the transform used to render an element in a <a
class=term href="#d-rendering-context">3D rendering context</a> is
computed by accumulating a matrix as follows:
<ol>
<li>Start with the identity matrix
<li>For each containing block between the root of the <a class=term
href="#d-rendering-context">3D rendering context</a> and the element in
question:
<ol>
<li>multiply the accumulated matrix with the <a
href="#TermPerspectiveMatrix"><i>perspective matrix</i></a> on the
element's containing block (if any). That containing block is not
necessarily a member of the 3D rendering context.
<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.
<!-- (tighten this!) -->
<li>multiply the accumulated matrix with the <a
href="#TermTransformationMatrix"><i>transformation matrix</i></a>.
</ol>
</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
‘<code class=css><code class=css>transform-style:
preserve-3d</code></code>’ 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 alt="Nested 3D transforms, with preserve-3d."
height=200 src="examples/3d-rendering-context-3d.png" width=240></div>
</div>
<p> Elements in the same <a class=term href="#d-rendering-context">3D
rendering context</a> 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>.
<p> Untransformed elements in a <a class=term
href="#d-rendering-context">3D rendering context</a> render on the Z=0
plane, yet may still intersect with transformed elements.
<p> Within a <a class=term href="#d-rendering-context">3D rendering
context</a>, 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 eachother, and
the orange element also intersects with the container.</p>
<div class=figure> <img alt="Intersecting sibling elements." height=200
src="examples/3d-intersection.png" width=200></div>
</div>
<p> Using three-dimensional transforms, it's possible to transform an
element such that its reverse side is towards the viewer. 3D-tranformed
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 ‘<a
href="#backface-visibility"><code class=css><code
class=property>backface-visibility</code></code></a>’ 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
‘<code class=css><code class=css>backface-visibility:
hidden</code></code>’ 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.</p>
<!-- ======================================================================================================= -->
<h2 id=transform-property><span class=secno>6. </span> The ‘<a
href="#effects"><code class=css><code
class=property>transform</code></code></a>’ Property</h2>
<p> A transformation is applied to the coordinate system an element renders
in through the ‘<a href="#effects"><code class=css><code
class=property>transform</code></code></a>’ 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
Transformation Functions</a>, then multiplying the matrices.
<table class=propdef>
<tbody>
<tr>
<td> <em>Name:</em>
<td> <dfn id=effects>transform</dfn>
<tr>
<td> <em>Value:</em>
<td> none | <transform-function> [ <transform-function> ]*
<tr>
<td> <em>Initial:</em>
<td> none
<tr>
<td> <em>Applies to:</em>
<td> <a href="#TermTransformableElement">transformable elements</a>
<tr>