[geometry] DOMPointReadonly matrixTransform() is poorly specified

[smfr]

https://drafts.fxtf.org/geometry/#dom-dompointreadonly-matrixtransform

Step 3 says "Post-multiply point with matrixObject." but what does that mean exactly? Specifically, it's unclear how to handle the w value.

Does it mean just initialize a matrix with the x, y, z, w values as:

[1 0 0 x]
[0 1 0 y]
[0 0 1 z]
[0 0 w 1]

and then post-multiply that with 'matrix'?

[dirkschulze]

@smfr Initially we just had x, y, z and it was a simple vector multiplied with a matrix (4th value or the vector beeing 1). With the perspective value I agree that this should be changed as you suggest. Then we would probably also need a way to get from the matrix back to the 4 value Point.

[zcorpan]

I believe it's implemented in chromium as a column vector

x
y
z
w

(except I think there's a bug in the "2d" code path)

What you suggest gives a different result.

[dirkschulze]

@zcorpan That means that w would correspond to m44. I need to check it but are you sure that this is correct?

[zcorpan]

It's how it's implemented in Chromium, and how it's tested in web-platform-tests, as far as I can tell.