- From: Dr. Olaf Hoffmann <Dr.O.Hoffmann@gmx.de>
- Date: Sat, 21 Apr 2012 14:40:34 +0100
- To: www-style@w3.org, public-fx@w3.org

Hello, this is about: http://www.w3.org/TR/2012/WD-css3-transforms-20120403/#mathematical-description The draft shows some examples, what can be assumed as the effect of several types of transformations and in this chapter 17. it provides a 'Mathematical Description of Transform Functions' - well, better it provides only matrices, no direct relation to the effect of such a matrix on the presentation. For 2D-Transforms it is sufficiently described in SVG already what the effect for a point r = (x, y, 1) for a matrix M is, r_p representation in the previous coordinate system, r_c in the current coordinate system (respectively r=(x, y, z, ?) for three dimensions?): r_p = M r_c I think this should be noted in this draft as well - and this is even more important for the matrices related to 3D-transformations, because it is not obvious, what the relation is. The old SVG transform draft http://www.w3.org/TR/SVG-Transforms/ has slightly more advanced descriptions. Well, even with these formulas I do not get something similar to for example 5 of the current draft. Due to my experience with perspective transforms, for a central projection, what seems to be intended here in examples like 4,5, one needs an additional transformation like (index _p here for projected) (x_p, y_p) = (x_c, y_c) * l/z_c with l a length. Obviously the fourth dimension of the matrices is intended for this, but the relation to such a transformation is not decribed. The parallel projection as intended in example 3 is simpler, one just has to use a simple 3x2 (respectively 4x2) matrix, to extract only the x and y components. I suggest to decribe/define the effect of such matrices in detail as a functional relation between the representation of an arbitrary point r_c in the current coordinate system to the representation of this projected point r_p. Other solutions for the problem are possible as well of course, but without a precise description at least the effect of the 3D transforms are undefined and those of the 2D transforms are applicable only for SVG, that has already a precise description for 2D. Best wishes Olaf PS: Is it really useful to change the preferred mailing list for this draft to the www-style list instead of the public-fx, as for the previous draft? Because the draft applies still to SVG as well, I added the public-fx...

Received on Saturday, 21 April 2012 13:41:05 UTC