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[css-transforms] Zero value in perspective #4279

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[css-transforms] Zero value in perspective #4279

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10 changes: 5 additions & 5 deletions css-transforms-2/Overview.bs
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Original file line number Diff line number Diff line change
Expand Up @@ -769,11 +769,9 @@ 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.
:: Distance to the center of projection. Negative values are invalid.

Issue: Verify that projection is the distance to the center of projection.

Expand Down Expand Up @@ -902,7 +900,7 @@ In the following <dfn export>3d transform functions</dfn>, a <<zero>> behaves th
: <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.
:: 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. Negative values are invalid.


Transform function primitives and derivatives {#transform-primitives}
Expand Down Expand Up @@ -1265,10 +1263,12 @@ One translation unit on a matrix is equivalent to 1 pixel in the local coordinat
<img src="images/rotate3dvariables.png" alt="\newline sc = \sin (\alpha/2) \cdot \cos (\alpha/2) \newline sq = \sin^2 (\alpha/2)" width="221" height="50">

<li id="PerspectiveDefined">
A perspective projection matrix with the parameter <em>d</em> is equivalent to the matrix:
A perspective projection matrix with the parameter <em>d</em> is equivalent to the matrix

<img src="images/perspective.png" alt="\begin{bmatrix} 1 & 0 & 0 & 0 \\ 0 & 1 & 0 & 0 \\ 0 & 0 & 1 & 0 \\ 0 & 0 & -1/d & 1 \end{bmatrix}" width="143" height="106">

when <em>d</em> is non-zero, and a 4x4 identity matrix when <em>d</em> is zero.

</ul>

The SVG 'transform' Attribute {#svg-transform}
Expand Down