Is there any way to read the rectangle from the texture?

OpenGL OpenGL: Advanced Coding
1

I have large (2048x2048) compressed RGBA texture. i need to read small (24x24) uncompressed rectangle from it. glGetTexImage() function reads the whole texture, uncompresses it and returns me lots of values i don’t really need. does anyone know the way to read only the part of the texture?

2

i don’t think so. even if you use only a simple compression (like run-length-encoding), you’ll loose the information about which compressed byte belongs to which x-y-coordinate. you cannot tell which byte belongs to the 24x24 rectangle unless you uncompress the whole image.

3

but i think if we are talking about DXT compression it is possible. so as i understood there is no function which can read me the part of the texture

4

You could render a section of texture with the proper scaling and filter settings and read it back with glReadPixels.

5

but i think if we are talking about DXT compression it is possible.
What makes you say that?

so as i understood there is no function which can read me the part of the texture
Are you sure about that?

6

Originally posted by Leghorn:
Are you sure about that?
if you know the function could you please name it?

7

Honestly, if you need to have access to the texture data, you shouldn’t have deleted your copy of it from memory. That way, there’s 0 performance cost associated with it.

8

There is no function to download a section of a texture. If you need to decompress your DXT texture, there is code hanging around. Look at DevIL
http://openil.sourceforge.net/

9

thanks everyone for your tips :slight_smile: i created the static buffer in memory which changes its data only when i read other texture. so i don’t call glGetTexImage() every time i need to access my compressed texture. i’ve got speedup up to 30 times :slight_smile: