[css-transforms-2] Per WG resolution, clarify that perspective()/'perspective' takes values down to zero, and clamps to 1px to avoid infinity and problems near infinity. Fixes w3c#413, closes w3c#4279.

Author: tabatkins

css-transforms-2/Overview.bs

@@ -82,7 +82,7 @@ Terminology {#terminology}
 :: A 4x4 matrix which does not fulfill the requirements of an [=2D matrix=].
 
 : <dfn>identity transform function</dfn>
-:: In addition to the identity transform function in CSS Transforms, examples for identity transform functions include ''translate3d(0, 0, 0)'', ''translateZ(0)'', ''scaleZ(1)'', ''rotate3d(1, 1, 1, 0)'', ''rotateX(0)'', ''rotateY(0)'', ''rotateZ(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.
+:: In addition to the identity transform function in CSS Transforms, examples for identity transform functions include ''translate3d(0, 0, 0)'', ''translateZ(0)'', ''scaleZ(1)'', ''rotate3d(1, 1, 1, 0)'', ''rotateX(0)'', ''rotateY(0)'', ''rotateZ(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. 1111lllll
 
 : <dfn>perspective matrix</dfn>
 :: A matrix computed from the values of the 'perspective' and 'perspective-origin' properties as described <a href="#perspective-matrix-computation">below</a>.
@@ -760,7 +760,7 @@ The 'perspective' Property {#perspective-property}
 
 <pre class='propdef'>
 Name: perspective
-Value: none | <<length>>
+Value: none | <<length [0, ∞]>>
 Initial: none
 Applies to: <a>transformable elements</a>
 Inherited: no
@@ -769,14 +769,20 @@ Computed value: the keyword ''perspective/none'' or an absolute length
 Animation type: by computed value
 </pre>
 
-Where <<length>> values must be positive.
-
 <dl dfn-type=value dfn-for="perspective">
 	: <dfn><<length>></dfn>
 	:: Distance to the center of projection.
 
 		Issue: Verify that projection is the distance to the center of projection.
 
+		As very small <<length>> values can produce bizarre rendering results
+		and stress the numerical accuracy of transform calculations,
+		values less than ''1px'' must be treated as ''1px''
+		for rendering purposes.
+		(This clamping does not affect the underlying value,
+		so ''perspective: 0;'' in a stylesheet
+		will still serialize back as ''0''.)
+
 	: <dfn>none</dfn>
 	:: No perspective transform is applied. The effect is mathematically similar to an infinite <<length>> value. All objects appear to be flat on the canvas.
 
@@ -901,8 +907,11 @@ In the following <dfn export>3d transform functions</dfn>, a <<zero>> behaves th
 :: same as ''rotate3d(0, 1, 0, &lt;angle>)''.
 : <span class='prod'><dfn>rotateZ()</dfn> = rotateZ( [ <<angle>> | <<zero>> ] )</span>
 :: same as ''rotate3d(0, 0, 1, &lt;angle>)'', which is a 3d transform equivalent to the 2d transform ''rotate(&lt;angle>)''.
-: <span class='prod'><dfn>perspective()</dfn> = perspective( <<length>> )</span>
-:: specifies a <a href="#PerspectiveDefined">perspective projection matrix</a>. This matrix scales points in X and Y based on their Z value, scaling points with positive Z values away from the origin, and those with negative Z values towards the origin. Points on the z=0 plane are unchanged. The parameter represents the distance of the z=0 plane from the viewer. Lower values give a more flattened pyramid and therefore a more pronounced perspective effect. For example, a value of 1000px gives a moderate amount of foreshortening and a value of 200px gives an extreme amount. The value for depth must be greater than zero, otherwise the function is invalid.
+: <span class='prod'><dfn>perspective()</dfn> = perspective( <<length [0,∞]>> )</span>
+:: specifies a <a href="#PerspectiveDefined">perspective projection matrix</a>. This matrix scales points in X and Y based on their Z value, scaling points with positive Z values away from the origin, and those with negative Z values towards the origin. Points on the z=0 plane are unchanged. The parameter represents the distance of the z=0 plane from the viewer. Lower values give a more flattened pyramid and therefore a more pronounced perspective effect. For example, a value of 1000px gives a moderate amount of foreshortening and a value of 200px gives an extreme amount.
+
+	If the depth value is less than ''1px'',
+	it must be treated as ''1px'' for the purpose of rendering.
 
 
 Transform function primitives and derivatives {#transform-primitives}