W3C home > Mailing lists > Public > public-css-commits@w3.org > January 2012

csswg/css3-transforms Overview.html,1.6,1.7 Transforms.src.html,1.9,1.10

From: Simon Fraser via cvs-syncmail <cvsmail@w3.org>
Date: Thu, 26 Jan 2012 17:14:07 +0000
To: public-css-commits@w3.org
Message-Id: <E1RqStj-0007tI-VA@lionel-hutz.w3.org>
Update of /sources/public/csswg/css3-transforms
In directory hutz:/tmp/cvs-serv30164

Modified Files:
	Overview.html Transforms.src.html 
Log Message:
- Added definitions for the 3D transform-related properties
- Started writing some prose about the 3D rendering model
- Moved some content into normative sections, since we need some normative text describing how 3D is rendered


Index: Overview.html
===================================================================
RCS file: /sources/public/csswg/css3-transforms/Overview.html,v
retrieving revision 1.6
retrieving revision 1.7
diff -u -d -r1.6 -r1.7
--- Overview.html	24 Jan 2012 23:08:05 -0000	1.6
+++ Overview.html	26 Jan 2012 17:14:04 -0000	1.7
@@ -47,8 +47,18 @@
         color: #46A4E9;
     }
     
+    .prop-name {
+      font-weight: bold;
+    }
     
-
+    .prop-value {
+      font-family: monospace;
+    }
[...1183 lines suppressed...]
+   <li>backface-visibility, <a href="#backface-visibility"
+    title=backface-visibility><strong>11.</strong></a>
+
+   <li>perspective, <a href="#perspective"
+    title=perspective><strong>9.</strong></a>
+
+   <li>perspective-origin, <a href="#perspective-origin"
+    title=perspective-origin><strong>10.</strong></a>
+
+   <li>transform, <a href="#effects" title=transform><strong>6.</strong></a>
 
    <li>transform-origin, <a href="#transform-origin"
-    title=transform-origin><strong>4.</strong></a>
+    title=transform-origin><strong>7.</strong></a>
+
+   <li>transform-style, <a href="#transform-style"
+    title=transform-style><strong>8.</strong></a>
   </ul>
   <!--end-index-->
 </html>

Index: Transforms.src.html
===================================================================
RCS file: /sources/public/csswg/css3-transforms/Transforms.src.html,v
retrieving revision 1.9
retrieving revision 1.10
diff -u -d -r1.9 -r1.10
--- Transforms.src.html	25 Jan 2012 18:54:12 -0000	1.9
+++ Transforms.src.html	26 Jan 2012 17:14:05 -0000	1.10
@@ -45,8 +45,18 @@
         color: #46A4E9;
     }
     
+    .prop-name {
+      font-weight: bold;
+    }
     
-
+    .prop-value {
+      font-family: monospace;
+    }
+    
+    .term {
+      font-style: italic;
+    }
+     
      div.issue-marker {
          position: absolute;
          width: 20ex;
@@ -152,34 +162,123 @@
 
         <h2>Introduction</h2>
 
-        <p><em>This section is not normative.</em>
+          <p><em>This section is not normative.</em></p>
+          <p>
+            The CSS <a href="http://www.w3.org/TR/REC-CSS2/visuren.html">visual
+            formatting model</a> describes a coordinate system within which each
+            element is positioned. Positions and sizes in this coordinate space can
+            be thought of as being expressed in pixels, starting in the upper left
+            corner of the parent with positive values proceeding to the right and
+            down.
+          </p>
+          <p>
+            This coordinate space can be modified with the <span
+            class="prop-name">'transform'</span> property. Using transform, elements
+            can be translated, rotated and scaled in two or three dimensional space.
+          </p>
+          <p>
+            Additional properties make working with transforms easier, and allow the
+            author to control how nested three-dimensional transforms interact.
+          </p>
+          <ul>
+            <li>
+              The <span class="prop-name">'transform-origin'</span> property
+              provides a convenient way to control the origin about which transforms on
+              an element are applied.
+            </li>
+            <li>
+              The <span class="prop-name">'perspective'</span> property allows the author
+              to make child elements with three-dimensional transforms appear as if they live in a common
+              three-dimensional space.
+              The <span class="prop-name">'perspective-origin'</span> property provides control
+              over the origin at which perspective is applied, effectively changing the location of
+              the "vanishing point".
+            </li>
+            <li>
+              The <span class="prop-name">'transform-style'</span> 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 <span class="prop-name">'backface-visibility'</span> 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>
+          <p>
+            Note that while some values of the <span class="prop-name">'transform'</span> 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.
+          </p>
 
+          <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>
+          <div class="issue">
+            What do fixed backgrounds do in transforms? They should probably ignore
+            the transform completely, since - even transformed - the object should
+            be acting as "porthole" through which the fixed background can be viewed
+            in its original form.
+          </div>
+          
+        <!-- ======================================================================================================= -->
 
+        <h2 id="module-interactions">Module Interactions</h2>
+          <p>Write me</p>
+
+        <h2 id="module-interactions">CSS Values</h2>
+          <p>Write me</p>
+
+
+        <h2 id="definitions">Definitions</h2>
+          <p> When used in this specification, terms have the meanings assigned in
+            this section.
+          </p>
+          <dl>
+            <dt id="TermTransformableElement">
+              <span class="termDefine">transformable element</span>
+            </dt>
+            <dd>
               <p>
-                The CSS <a href="http://www.w3.org/TR/REC-CSS2/visuren.html">visual
-                formatting model</a> describes a coordinate system within which each
-                element is positioned. Positions and sizes in this coordinate space can
-                be thought of as being expressed in pixels, starting in the upper left
-                corner of the parent with positive values proceeding to the right and
-                down.
-              </p>
-              <p class="todo">
-                Need to mention the perspective transform to give a sense of depth.
-              </p>
-              <p>
-                This coordinate space can be modified with the <span
-                class="prop-name">'transform'</span> property. Using transform, elements
-                can be translated, rotated and scaled in two or three dimensional space.
-                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.
+                A transformable element in the HTML namespace which is either be a block-level or atomic inline-level
+                element, or an element the SVG namespace (see [[SVG11]]) which has the attributes 'transform',
+                'patternTransform' or 'gradientTransform'.
               </p>
+            </dd>
+
+            <dt id="Term3DRenderingContext">
+              <span class="termDefine">3D rendering context</span>
+            </dt>
+            <dd>
               <p>
-                Transforms apply to <a href="#TermTransformableElement">transformable elements</a>.
+                All elements in the same <span class="term">3D rendering context</span> share a common three-dimensional
+                coordinate system. Elements in the same 3D rendering context which intersect
+                may be rendered as intersecting by the user agent.
               </p>
+            </dd>
+            <!-- Define "three-dimensional transform" ? -->
+          </dl>
+
+          <!-- ======================================================================================================= -->
+              
+          <h2 id="transform-rendering">The Transform Rendering Model</h2>
+            <!-- This section is normative -->
               <p>
                 Specifying a value other than 'none' for the <span class="prop-name">'transform'</span>
                 property establishes a new <em>local coordinate system</em> at the element that it is
@@ -190,7 +289,20 @@
                 of these transforms defines a <em>current transformation matrix (CTM)</em> for the element.
               </p>
               <p>
-                The transform property does not affect the flow of the content
+                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>
+              <p>
+                Transforms apply to <a href="#TermTransformableElement">transformable elements</a>.
+              </p>
+              <!-- 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 value of the overflow
                 area takes into account transformed elements. This behavior is similar
                 to what happens when elements are translated via relative positioning.
@@ -200,64 +312,211 @@
                 see content that is transformed outside the visible area.
               </p>
               <p>
-                Any value other than 'none' for the transform results in the creation of
+                In the HTML namespace, 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>
-              <p class="todo">
-                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 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>
-              <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.
+              
+              <h3 id="transform-3d-rendering">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 the containing block.
+              </p>
+              <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 <span class="term">3D rendering context</span> or not, as described below.
+              </p>
+              <div class="issue issue-marker">
+                <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="issue">
-                What do fixed backgrounds do in transforms? They should probably ignore
-                the transform completely, since - even transformed - the object should
-                be acting as "porthole" through which the fixed background can be viewed
-                in its original form.
-              </div><!-- ======================================================================================================= -->
-              <h2 id="definitions">
-                Definitions
-              </h2>
-              <p> When used in this specification, terms have the meanings assigned in
-                this section.
+              <p>
+                An element with a three-dimensional transform that is not contained in a
+                <span class="term">3D rendering context</span> 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>
-              <dl>
-                <dt id="TermTransformableElement">
-                  <span class="termDefine">transformable element</span>
-                </dt>
-                <dd>
-                  <p>
-                    A transformable element can either be a block-level or atomic inline-level
-                    element, not splitted into multiple inline-level boxes,
-                    from the HTML namespace, or an element which has the attributes 'transform',
-                    'patternTransform' or 'gradientTransform' in the SVG namespace (see [[SVG11]]).
-                  </p>
-                </dd>
-              </dl>
+              
+              <div class="example">
+<pre>
+</pre>
+                <p>Insert example of simple 3d transform.
+                </p>
+                <div class="figure">
+                </div>
+              </div>
+              
+              <p>
+                An element with a three-dimensional transform that is contained in a
+                <span class="term">3D rendering context</span> can visibly interact with other elements
+                in that same 3D rendering context; the set of elements participating in the same 
+                <span class="term">3D rendering context</span> 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 <span class="term">3D rendering context</span>
+                through each element that is a containing block for the given element, as described below.
+                <!-- More detail required, probably with matrices -->
+              </p>
+
+              <p>
+                A <span class="term">3D rendering context</span> is established via the following:
+              </p>
+              <ul>
+                <li>
+                  A transformable element has a value for the <span type="prop-name">perspective</span> property
+                  which is not 'none' and greater than 0, and which is not itself part of a 3D rendering context.
+                </li>
+                <!-- Part of is wooly. Need to clarify. -->
+                <li>
+                  An transformable element whose computed value of <span type="prop-name">transform-style</span> is
+                  <span class="prop-value">'perserve-3d'</span>, and which itself is not part of a 3D rendering context.
+                </li>
+              </ul>
+
+              <div class="issue issue-marker">
+                <p class="desc">Should intersection behavior be normative?</p>
+              </div>
+              <p>
+                An element whose computed value for <span type="prop-name">transform-style</span> is
+                <span style="prop-value">'preserve-3d'</span>, and which is itself a member of a
+                <span class="term">3D rendering context</span>, extends the 3D rendering context of which it is a member
+                to the elements for which it acts as a containing block.
+              </p>
+
+              <p>
+                An element participates in a <span class="term">3D rendering context</span> if its containing block
+                establishes or extends a <span class="term">3D rendering context</span>.
+              </p>
+              
+              <!-- Need more exposition here on establishes vs. participates -->
+
+              <p>
+                The final value of the transform used to render an element in a <span class="term">3D rendering context</span>
+                is computed as follows:
+              </p>
+              <ol>
+                <!-- Clarify pre- vs post-multiply? -->
+                <li>Start with the identity matrix</li>
+                <li>If the root of the <span class="term">3D rendering context</span> has perspective, multiply that into
+                  the matrix, taking perspective-origin into account.</li>
+                <li>For each ancestor of the element in question, compute a transformation matrix by:
+                  <ol>
+                    <li>applying perpsective and perspective-origin, if any</li>
+                    <li>applying a translation equivalent to the horizontal and vertical offset of the element relative to
+                      its containing block as specified by CSS layout rules <!-- (tighten this!) --></li>
+                    <li>applying transform and transform-origin, if any</li>
+                  </ol>
+                  The target matrix is then multiplied with the matrix for this ancestor.
+                </li>
+              </ol>
+
+              <!-- Discuss whether an element that establishes a 3D rendering context also participates in it; notably,
+              does the foreground/background of the element intersect with transformed members of the 3D rendering context? -->
+              <!-- Discuss whether non-transformed memebers of the <span class="term">3D rendering context</span> participate in intersection -->
+
+              <div class="issue issue-marker">
+                <p class="desc">Does transform-style follow containing block, or parent element ancestry?</p>
+              </div>
+              <div class="issue issue-marker">
+                <p class="desc">Does perspective establish a 3D rendering context?</p>
+              </div>
+
+
+              <div class="example">
+<pre>
+</pre>
+                <p>Example of simple 3D rendering context
+                </p>
+                <div class="figure">
+                </div>
+              </div>
+  
+              <div class="example">
+<pre>
+</pre>
+                <p>Example of deeper 3D rendering context
+                </p>
+                <div class="figure">
+                </div>
+              </div>
+
+            <div class="example">
+<pre>
+</pre>
+              <p>Example of flattening inside 3D rendering context
+              </p>
+              <div class="figure">
+              </div>
+            </div>
+
+
+<!--
+                <p>
+  	              The <span class="prop-name">'transform-style'</span> property defines
+                  how nested elements are rendered in three-dimensional space. If the <span
+                  class="prop-name">'transform-style'</span> is <span
+                  class="prop-value">'flat'</span>, all children of this element are
+                  rendered flattened into the 2D plane of the element. Therefore, rotating
+                  the element about the X or Y axes will cause children positioned at
+                  positive or negative Z positions to appear on the element's plane, rather
+                  than in front of or behind it. If the <span
+                  class="prop-name">'transform-style'</span> is <span
+                  class="prop-value">'preserve-3d'</span>, this flattening is not performed,
+                  so children maintain their position in three-dimensional space.
+                </p>
+                <p>
+                  This flattening takes place at each element, so preserving a hierarchy
+                  of elements in three-dimensional space requires that each element in the hierarchy
+                  have the value <span class="prop-value">'preserve-3d'</span> for <span
+                  class="prop-name">'transform-style'</span>. But since <span
+                  class="prop-name">'transform-style'</span> affects only an element's
+                  children, the leaf nodes in a hierarchy do not require the 
+                  <span class="prop-name">'transform-style: preserve-3d'</span> to be specified.
+                </p>
+                
+                <p>
+                  For some effects, the author will have to ensure that an ancestor
+                  element to a subtree using <span class="prop-value">'preserve-3d'</span>
+                  has a <span class="prop-name">'transform-style'</span> of <span
+                  class="prop-value">'flat'</span> (or the default). Otherwise, the
+                  elements in the 3D tree may be located behind ancestor elements and,
+                  thus, invisible (hidden behind an ancestor's background).
+                </p>
+
+                <p>
+                  Note that while 'preserve-3d' can be specified on an element, the effect may not
+                  be possible. Elements that have <span class="prop-name">'overflow'</span> set to
+                  <span class="prop-value">'hidden'</span> are unable to keep their children in 3D.
+                  In this case the element will behave as if the property was set to 
+                  <span class="prop-value">'flat'</span>.
+                </p>
+                -->
+              
+              <!-- ======================================================================================================= -->
 
               <h2 id="transform-property">
                 The <span class="prop-name">'transform'</span> Property
               </h2>
               <p>
-                A two-dimensional transformation is applied to the coordinate system an element
-                renders in through the <span
-                class="prop-name">'transform'</span> property. This property contains a
+                A transformation is applied to the coordinate system an element
+                renders in through the <span class="prop-name">'transform'</span> 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 (either defined in
@@ -333,10 +592,12 @@
                 </tbody>
               </table>
               
-              <div class="issue issue-marker"><p class="desc">We need to resolve whether the computed value is the same as the specified value, or matrix().</p></div>
+              <div class="issue issue-marker">
+                <p class="desc">We need to resolve whether the computed value is the same as the specified value, or matrix().</p>
+              </div>
               <p>The computed value of the transform property is a matrix() or matrix3d() value that describes the matrix that results from concatenating the individual transform functions. If the resulting matrix can be represented as a two-dimensional matrix with no loss of information, then a matrix() value is returned, otherwise a matrix3d() value. For elements with no transform applied, the computed value is 'none'.</p>
+
               <!-- ======================================================================================================= -->
-   
               
               <h3 id="svg-transform">The SVG transform attribute</h3>
               
@@ -373,9 +634,15 @@
                   </ul>
               </div>
 
+              <!-- ======================================================================================================= -->
+
               <h2 id="transform-origin-property">
                 The <span class="prop-name">'transform-origin'</span> Property
               </h2>
+              <div class="issue issue-marker">
+                <p class="desc">Need to add 3D transform-origin variant in a way that is not ambiguous
+                  with the background-origin syntax (<a href="https://www.w3.org/Bugs/Public/show_bug.cgi?id=15432">bug 15432</a>).</p>
+              </div>
               <p>
                 The <span class="prop-name">'transform-origin'</span> property
                 establishes the origin of transformation for a coordinate system. This property
@@ -495,6 +762,375 @@
     transform-origin: top left;
 }                  
               </pre>
+              <div class="issue">
+                  <p class="desc">Need to determine how to extend transform-origin to allow it ot affect the z-origin
+                    with three-dimensional transforms (<a href="https://www.w3.org/Bugs/Public/show_bug.cgi?id=14647">Bug 14647</a>).
+              </div
+
+              <!-- ======================================================================================================= -->
+
+              <h2 id="transform-style-property">
+                The <span class="prop-name">'transform-style'</span> Property
+              </h2>
+              <p>
+                Property summary here.
+              </p>
+              <table class="propdef">
+                <tbody>
+                  <tr>
+                    <td>
+                      <em>Name:</em>
+                    </td>
+                    <td>
+                      <dfn id="transform-style">transform-style</dfn>
+                    </td>
+                  </tr>
+                  <tr>
+                    <td>
+                      <em>Value:</em>
+                    </td>
+                    <td>
+                      flat | preserve-3d
+                    </td>
+                  </tr>
+                  <tr>
+                    <td>
+                      <em>Initial:</em>
+                    </td>
+                    <td>
+                      flat
+                    </td>
+                  </tr>
+                  <tr>
+                    <td>
+                      <em>Applies&nbsp;to:</em>
+                    </td>
+                    <td>
+                      <a href="#TermTransformableElement">transformable elements</a>
+                    </td>
+                  </tr>
+                  <tr>
+                    <td>
+                      <em>Inherited:</em>
+                    </td>
+                    <td>
+                      no
+                    </td>
+                  </tr>
+                  <tr>
+                    <td>
+                      <em>Percentages:</em>
+                    </td>
+                    <td>
+                      N/A
+                    </td>
+                  </tr>
+                  <tr>
+                    <td>
+                      <em>Media:</em>
+                    </td>
+                    <td>
+                      visual
+                    </td>
+                  </tr>
+                  <tr>
+                    <td>
+                      <em>Computed value:</em>
+                    </td>
+                    <td>
+                      Same as specified value.
+                    </td>
+                  </tr>
+                </tbody>
+              </table>
+
+              <p>
+                A value of <span class="prop-value">preserve-3d</span> for <span class="prop-name">transform-style</span>
+                establishes a stacking context.
+              </p>
+
+              <div class="issue issue-marker">
+                <p class="desc">Should this affect the computed value of transform-style?</p>
+              </div>
+              <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 
+                <span class="prop-name">transform-style</span>: <span class="prop-value">preserve-3d</span>:
+                <ul>
+                  <li><span class="prop-name">overflow</span>: any value other than 'visible'.</li>
+                  <li><span class="prop-name">opacity</span>: any value other than 1.</li>
+                  <li><span class="prop-name">filter</span>: any value other than 'none'.</li>
+                  <!-- Others? -->
+                </ul>
+              </p>
+
+              <!-- ======================================================================================================= -->
+
+              <h2 id="perspective-property">
+                The <span class="prop-name">'perspective'</span> Property
+              </h2>
+              <p>
+                
+              <p>
+                The <span class="prop-name">'perspective'</span> property applies the
+                same transform as the <span
+                class="prop-value">perspective(&lt;length&gt;)</span> transform
+                function, except that it applies only to the positioned or transformed
+                children of the element, not to the transform on the element itself.
+              </p>
+
+              <table class="propdef">
+                <tbody>
+                  <tr>
+                    <td>
+                      <em>Name:</em>
+                    </td>
+                    <td>
+                      <dfn id="perspective">perspective</dfn>
+                    </td>
+                  </tr>
+                  <tr>
+                    <td>
+                      <em>Value:</em>
+                    </td>
+                    <td>
+                      none | &lt;length&gt;
+                    </td>
+                  </tr>
+                  <tr>
+                    <td>
+                      <em>Initial:</em>
+                    </td>
+                    <td>
+                      none
+                    </td>
+                  </tr>
+                  <tr>
+                    <td>
+                      <em>Applies&nbsp;to:</em>
+                    </td>
+                    <td>
+                      <a href="#TermTransformableElement">transformable elements</a>
+                    </td>
+                  </tr>
+                  <tr>
+                    <td>
+                      <em>Inherited:</em>
+                    </td>
+                    <td>
+                      no
+                    </td>
+                  </tr>
+                  <tr>
+                    <td>
+                      <em>Percentages:</em>
+                    </td>
+                    <td>
+                      N/A
+                    </td>
+                  </tr>
+                  <tr>
+                    <td>
+                      <em>Media:</em>
+                    </td>
+                    <td>
+                      visual
+                    </td>
+                  </tr>
+                  <tr>
+                    <td>
+                      <em>Computed value:</em>
+                    </td>
+                    <td>
+                      Same as specified value.
+                    </td>
+                  </tr>
+                </tbody>
+              </table>
+
+              <p>
+                If the value is <span
+                class="prop-value">'none'</span>, less than or equal to 0 no perspective transform is applied.
+              </p>
+              <p>
+                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.
+              </p>
+              
+              <!-- ======================================================================================================= -->
+              
+              <h2 id="perspective-origin-property">
+                The <span class="prop-name">'perspective-origin'</span> Property
+              </h2>
+              <p>
+                The <span class="prop-name">'perspective-origin'</span> property
+                establishes the origin for the <em>perspective</em> property. It
+                effectively sets the X and Y position at which the viewer appears to be
+                looking at the children of the element.
+              </p>
+              <table class="propdef">
+                <tbody>
+                  <tr>
+                    <td>
+                      <em>Name:</em>
+                    </td>
+                    <td>
+                      <dfn id="perspective-origin">perspective-origin</dfn>
+                    </td>
+                  </tr>
+                  <tr>
+                    <td>
+                      <em>Value:</em>
+                    </td>
+                    <td>
+                      [ [ &lt;percentage&gt; | &lt;length&gt; | left | center | right ] [
+                      &lt;percentage&gt; | &lt;length&gt; | top | center | bottom ]? ] | [ [ left |
+                      center | right ] || [ top | center | bottom ] ]
+                    </td>
+                  </tr>
+                  <tr>
+                    <td>
+                      <em>Initial:</em>
+                    </td>
+                    <td>
+                      50% 50%
+                    </td>
+                  </tr>
+                  <tr>
+                    <td>
+                      <em>Applies&nbsp;to:</em>
+                    </td>
+                    <td>
+                      <a href="#TermTransformableElement">transformable elements</a>
+                    </td>
+                  </tr>
+                  <tr>
+                    <td>
+                      <em>Inherited:</em>
+                    </td>
+                    <td>
+                      no
+                    </td>
+                  </tr>
+                  <tr>
+                    <td>
+                      <em>Percentages:</em>
+                    </td>
+                    <td>
+                      refer to the size of the element's border box
+                    </td>
+                  </tr>
+                  <tr>
+                    <td>
+                      <em>Media:</em>
+                    </td>
+                    <td>
+                      visual
+                    </td>
+                  </tr>
+                  <tr>
+                    <td>
+                      <em>Computed value:</em>
+                    </td>
+                    <td>
+                      Same as specified value.
+                    </td>
+                  </tr>
+                </tbody>
+              </table>
+              
+              <!-- ======================================================================================================= -->
+
+              <h2 id="backface-visibility-property">
+                The <span class="prop-name">'backface-visibility'</span> Property
+              </h2>
+              <p>
+                The <span class="prop-name">'backface-visibility'</span> 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
+                degrees (for instance) would cause the back side of the element to face
+                the viewer.
+              </p>
+              <!-- This should not be in a normative section. -->
+              <p>
+                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.
+              </p>
+              <table class="propdef">
+                <tbody>
+                  <tr>
+                    <td>
+                      <em>Name:</em>
+                    </td>
+                    <td>
+                      <dfn id="backface-visibility">backface-visibility</dfn>
+                    </td>
+                  </tr>
+                  <tr>
+                    <td>
+                      <em>Value:</em>
+                    </td>
+                    <td>
+                      visible | hidden
+                    </td>
+                  </tr>
+                  <tr>
+                    <td>
+                      <em>Initial:</em>
+                    </td>
+                    <td>
+                      visible
+                    </td>
+                  </tr>
+                  <tr>
+                    <td>
+                      <em>Applies&nbsp;to:</em>
+                    </td>
+                    <td>
+                      block-level and inline-level elements
+                    </td>
+                  </tr>
+                  <tr>
+                    <td>
+                      <em>Inherited:</em>
+                    </td>
+                    <td>
+                      no
+                    </td>
+                  </tr>
+                  <tr>
+                    <td>
+                      <em>Percentages:</em>
+                    </td>
+                    <td>
+                      N/A
+                    </td>
+                  </tr>
+                  <tr>
+                    <td>
+                      <em>Media:</em>
+                    </td>
+                    <td>
+                      visual
+                    </td>
+                  </tr>
+                  <tr>
+                    <td>
+                      <em>Computed value:</em>
+                    </td>
+                    <td>
+                      Same as specified value.
+                    </td>
+                  </tr>
+                </tbody>
+              </table>
+              
 
               <!-- ======================================================================================================= -->
 
@@ -1239,6 +1875,15 @@
 
         <h2 id="changes">Changes</h2>
 
+        <h3 id="changes_on_January_26_2012">
+          Changes by Simon Fraser on January 26 2012
+          <ul>
+            <li>Added definitions for the 3D transform-related properties</li>
+            <li>Started writing some prose about the 3D rendering model</li>
+            <li>Moved some content into normative sections, since we need some normative text describing how 3D is rendered</li>
+          </ul>
+        </h3>
+
         <h3 id="changes_on_January_25_2012">
           Changes by Simon Fraser on January 25 2012
         </h3>
Received on Thursday, 26 January 2012 17:14:10 UTC

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