Re: [PyOpenGL-Users] Difference in transformation matrices

[Derakon <de...@gm...>]

Never mind, problem solved! The issue was that my second translation
was getting rotated, which was undesirable. In fact what I wanted was
to do this:

glTranslated(dx + cx, dy + cy)
glRotated(-angle, 0, 0, 1)
glTranslated(-cx, -cy, 0)

I'd tried the line swap that Ian suggested earlier, and it didn't work
(as noted in the last paragraph of my previous message), but this
appears to get me what I want.

-Chris

On Thu, Jun 9, 2011 at 1:24 PM, Derakon <de...@gm...> wrote:
> I have a program that displays 3D arrays of pixel data that have been
> transformed (by XYZ translation, rotation about the Z axis, and
> uniform scaling in X and Y -- so five parameters). When possible (i.e.
> Z offset is 0) I use OpenGL to show the transformation since this is
> fast. however, when there is a Z transform, I manually construct a
> transformation matrix with Numpy, invert it, and use it to map display
> coordinates into the data to determine what needs to be shown. That
> is, I have a bunch of XY coordinates, one for each pixel I want to
> display (e.g. from [0, 0] to [512, 512]), I reverse-transform them,
> this gives me a location in the dataset, and that gets me the value to
> display.
>
> There's a problem here: the two approaches generate different results.
> For example, here's my flat 512x512 test image (disregarding the Z
> dimension), untransformed (the grey lines are for reference and are
> not part of the image):
> http://derakon.dyndns.org/~chriswei/temp2/1.png
>
> Here's the image rotated 45° and translated 20 pixels in X, in OpenGL:
> http://derakon.dyndns.org/~chriswei/temp2/2.png
>
> (Ignore the grey triangles; they're just a display artifact)
>
> And here's that same image with the transformation applied via Numpy:
> http://derakon.dyndns.org/~chriswei/temp2/3.png
>
> The last example is the behavior I actually want -- rotation should be
> about the center of the image, and applied before translation occurs.
> However that's not what I'm getting from my OpenGL transformation
> code. I've put up a paste comparing the two approaches and what I get
> when I print their transformation matrices, here:
> http://pastebin.com/j31iLGbT
>
> Any ideas what I'm doing wrong here? I note that if I swap the order
> of the glRotated and glTranslated calls, then I get OpenGL matrices
> that look more like what I'm getting from Numpy, but of course the
> rotation is no longer about the center of the image, so the results
> are even more off.
>
> -Chris
>