Thank you for the guidelines for skewing forms.

I've followed Matthias' instructions (and the associated math doc) and skewing works well this way through WarpBlt. (The code linked by Yoshiki was too slow and didn't work fully).

I wrote the math conversions in a C DLL, and it would probably be useful for people. Is there any place where I could put the code so the community can use it ?

Cheers,
Iulian

---------- Forwarded message ----------
From: Matthias Berth <matthias.berth@googlemail.com>
Date: Tue, Jul 22, 2008 at 7:10 PM
Subject: Re: [Newbies] Skewing Forms in Squeak V2
To: "A friendly place to get answers to even the most basic questions about Squeak." <beginners@lists.squeakfoundation.org>


So it comes down to this:

given a projective transformation T from a rectangle (your form) to a
quad Q (what you want to draw on screen), how can you express the same
transformation in terms of mapping a quad to a rectangle (what WarpBlt
needs)?

One possible plan is:

1) determine the transformation's parameters

2) take the bounding box of Q (a rectangle R) and transform it by the
inverse of T, giving you a quad Q'

3) use WarpBlt to transform pixels in Q' (should be a superset of your
form) to R

4) mask the result with your original quad Q, using pixel-by-pixel
operations on Forms

I think all you need in terms of mathematics for this is in
http://www.cs.cmu.edu/~ph/869/www/notes/proj/proj.pdf

Hope this helps, feel free to ask about the details,

Matthias