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vrdhananjay

Accessing a part of a texture

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Hey everyone, Hope you can help me out with this one. I load an image to a texture. I want to do this CPU to GPU transfer only once! I want to select process parts of this texture many times by copying it into a smaller textures .For eg if my orginal image was 256 X 256, I want to access 4 X 4 subtextures. The only thing I found was glTexSubImage2D. This is useless to me, since it needs pixels from the CPU. As i said I want to do only one transfer, and then access only parts of this large texture. I checked out this thread also. http://www.gamedev.net/community/forums/topic.asp?topic_id=325623 But then I dont think it solves my problem. Thanks in advance, Dhananjay

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You generally do this with texture coordinates. Look into glTexCoord2f. It takes values in the range [0.0, 1.0]. This is a value you set per vertex, so if you wanted to show just a quarter of an image on a quad, you might do something like this:


glBegin(GL_QUADS);
glTexCoord2f(0.5f, 0.0f); glVertex3f( 1.0f, -1.0f, 0.0f );
glTexCoord2f(0.5f, 0.5f); glVertex3f( 1.0f, 1.0f, 0.0f );
glTexCoord2f(0.0f, 0.5f); glVertex3f( -1.0f, 1.0f, 0.0f );
glTexCoord2f(0.0f, 0.0f); glVertex3f( -1.0f, -1.0f, 0.0f );
glEnd();


This will draw a quad on the screen and map the lower left 1/4 of the texture onto the quad.

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