From: Ian Mallett <geometrian@gm...>  20110609 20:48:42

On Thu, Jun 9, 2011 at 1:24 PM, Derakon <derakon@...> 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 reversetransform 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. > Switch lines 11 and 12. 