I thought that the points in the layer would all have a z of 0 since
it's 2d, but that then the perspective transform would give them
"depth" based on m34 and the rotation of the sublayer, which would
then have to be mapped to an x,y point in 2d space.
yes, that's correct. We compute a single matrix to map 2D points from
the child layer into the parent. The points in the parent are 3D, but
then have their Z component set to zero to project them into the
parent's plane.
And then for each of the 4 original points of l3 (centered around
the middle of the rect) applying the transform.
You can't concatenate the matrices like that. Since each layer is
effectively a plane in 3D space, you need to project the point into
that plane every time. All that means is that you have to apply the
mapping from each child to its parent layer separately, setting the Z
component of the result to zero before mapping that 2D point into the
next superlayer.
John
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