View Full Version : Image Manipulation

01-16-2007, 04:23 PM
I want to do the following in some code I am working on. I posted this to see if anybody could show me why it would be a bad idea. I have a 3D geometry rendered to the screen. I will be using both perspective and orthographic projection. I need the following functionality:

1. Zoom to the center of the image - glScalef(z, z, 1)
2. Pan the image - glTranslatef(x, y, 0)
3. Rotate the image in plane - glRotatef(r, 0, 0, 1)
4. Flip the image horizontally (fx) and vertically (fy) - glScalef(1 - 2 * fx, 1 - 2 * fy, 1)

I would like to perform all of these calculations in the projection matrix. I believe this should be fine, since none of the operations above affect the direction or size of Z. I also think this would be a good idea since lighting normals will be unaffected by the scaling. So, what, if anything, is wrong with this approach?

01-16-2007, 04:34 PM
Hehe.. Is this a quiz/riddle of some kind, jtipton? If so, what do we win if we get the right answer?

P.S. I could use a new set of lawn chairs.

01-16-2007, 05:01 PM
Does it have to be a "new" set of lawn chairs? By the way, thanks again for the Delphi link. That site is great.

Bruce Merry
01-16-2007, 09:53 PM
Sounds like it should work. The only thing that might go slightly wrong is that the frustum will change relative to the geometry, which can affect non-linear vertex operations (like per-vertex lighting). You can avoid that in the non-rotated case by fiddling with glViewport to keep the frustum the same, although performance under a strong zoom (very large viewport) may depend on the driver.

Also remember that if you flip the image on one axis, you need to change glFrontFace.

01-17-2007, 05:46 AM
I am not following the first 2 scenarios, but the glFrontFace catch is a good one.

01-17-2007, 01:13 PM
I do the zoom in projection matrix, use glFrustum or gluPickMatrix
With glFrustum, you just input smaller values than previous.
glFrustum(-1.0, 1.0, -1.0, 1.0, 1.0, 1000.0)
glFrustum(-0.9, 0.9, -0.9, 0.9, 0.9, 1000.0)

All the others, I do on the modelview else it messes up glTexGen(GL_SPHERE_MAP) and perhaps there was another reason.

01-17-2007, 03:30 PM
How does it mess up texture generation? Wouldn't it just compute the coords without factoring in the transforms that I put into the projection matrix. The effect I want is to rotate, zoom, pan, and flip the result of the rendering. The "image" should be the same, just transformed in 2D.