[Newbies] Skewing Forms in Squeak V2

[Matthias Berth]

Hi Iulian,

I'm glad to hear that it worked for you. I wonder why you implemented
it in a C DLL... As far as I can see, the manipulations with the
parameters would be done only once for every quad you want to
transform. The computation time for that should be negligible compared
to the time it takes to actually transform all the pixels in the quad
/ rectangle. At least that's what I was thinking, maybe i am wrong?

Cheers

Matthias

On Tue, Sep 23, 2008 at 1:08 AM, Iulian Wande Radu
<infinite.flow at gmail.com> wrote:
> 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 at 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 at 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
>
>
>
>
> On Thu, Jul 24, 2008 at 1:39 AM, Yoshiki Ohshima <yoshiki at vpri.org> wrote:
>>
>> At Tue, 22 Jul 2008 13:53:21 -0400,
>> Iulian Wande Radu wrote:
>> >
>> > Hi, I'd like to skew the graphics of a Form, is there an easy way to do
>> > this in v2 ?
>> >
>> > What I want is to take the rectangle from a normal form, transform it
>> > into an arbitrarily-defined quadrilateral, and
>> > then paint on the screen. I saw that WarpBlt does something like this..
>> > but it maps a quadrilateral section of a form
>> > onto a rectangle.
>>
>>  There was similar a discussion on this a while ago:
>>
>>
>> http://lists.squeakfoundation.org/pipermail/squeak-dev/2001-February/013402.html
>>
>> -- Yoshiki
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>
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