GLSL changing textures, or rendering to texture?

OpenGL
1

Hello,

how would I write a shader which changes a texture?

I would like to write to a texture some how.

Does anyone know of an example of doing this?

Or can someone point me to a place on how to change textures with GLSL?

cheers,

2

GL_EXT_framebuffer_object

3

http://www.flashbang.se/postbreak.php?id=18
A somewhat simplistic tutorial, but it will work.
Just remember that you can’t write and read from/to the same texture, in that case you either have to use blending if you can, or two textures.

4

Thanks for the replies.

How can I use blending to read from the texture I just wrote to?

I want to do the following:

  • copy data to texture 0.
  • render to texture 1 with GLSL fragment program accessing texture 0.
  • read data out of texture 1 into memory.

I seem to have a program working with one texture which can do this:

  • copy data to texture 0.
  • render to texture 0 with GLSL fragment program.
  • read data out of texture 0 into memory.

cheers,

5

I seem to have to read from the texture 0, then read from the texture 1… and it kind of works:

glReadBuffer(GL_COLOR_ATTACHMENT0_EXT)
result = glReadPixels(0, 0, 0, 0,texFormat,GL_FLOAT)

glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT1_EXT, texTarget,tex2,0)
glReadBuffer(GL_COLOR_ATTACHMENT1_EXT)
result = glReadPixels(0, 0, texSize, texSize,texFormat,GL_FLOAT)

I say kind of works, because I get the expected result - however my program segfaults on exit. (might be related to python ref counts or something… maybe not related to this).

6

I’m surprised the readpixels calls work, they don’t even have the right amount of parameters.

N.

7

As the OP said, he is using Python.
http://pyopengl.sourceforge.net/documentation/manual/glReadPixels.3G.html

8

Oh, I see. I’ve been messing around a bit with PyOpenGL, only to tweak some small linux apps to suit my needs but until now I assumed the calls where always the same format. Thanks for pointing that out. That aside, Why would you call readpixels with (0,0,0,0,…)?

N.

9

Hi,

That first glReadPixels call seems to be required for the second one to work at all. Since I’m not interested in the data at all I pass 0,0 for height, and width. Changing it to say 16,16 or 1,1 makes no difference.

http://rene.f0o.com/~rene/stuff/gpudemo.py
It requires opengl-ctypes latest cvs, and crashes on exit. Python opengl is very similar to C opengl… but a few calls are different.

Can anyone see anything wrong with what I’m doing?

cu,

10

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