glReadPixels*

The first two arguments of glReadPixels specify the lower-left corner of the region you want to grab, so try using something like:

glReadPixels( 0, SCREEN_HEIGHT, IMAGE_SIZE, IMAGE_SIZE, GL_RED, GL_UNSIGNED_BYTE, &heightmap);

instead.

I am not saving it as a bitmap, just in raw form. I just create an array of values[IMAGE_SIZE][IMAGE_SIZE] and assign grayscale color to each point via a Perlion noise function I got off of the internet.

I then draw the image to the screen.

I then read the pixels when save is pressed as follows (this includes the snipped originally posted above:

void SaveImageAsRaw( char* filename )
{
glReadPixels( 0, 0, IMAGE_SIZE, IMAGE_SIZE, GL_RED, GL_UNSIGNED_BYTE, &heightmap);


//write them out
FILE* pFile;

//open a file to write to
pFile= fopen( filename, "wb" );


//write the data to the file
fwrite( heightmap, 1, IMAGE_SIZE*IMAGE_SIZE, pFile );

//Close the file
fclose( pFile );



}

Monder: When I tried what you suggested I get a completely white image written out.

Width and Heigth can't be negative. You basically have three options:

1 - Flip the image with a bunch of memory copying CPU side
2 - Use glPixelZoom which is very slow
3 - Copy the framebuffer to a texture and redraw the image back to the framebuffer on a quad then use glReadPixels

GL_INVALID_VALUE is set if either width or height is negative.

Quote:

Original post by GKW 2 - Use glPixelZoom which is very slow

That's not always true. With certain implementations ("drivers") X and Y zoom factors of -1.0 are special-cased so that they're relatively quick. I think the specific example I'm recalling is NVidia's implementation, but other vendors may do this as well. Still, if it's a performance critical section of code I wouldn't necessarily trust glPixelZoom to always be quick :).

I have a nvidia card. It is over 20 times slower to use glPixelZoom than to use render-to-texture to flip a 32bit image.

In OpenGL pixel 0,0 is in the bottom left corner (math standard) but most bitmap formats have (0,0) at the *top* left corner (tv standard). Since you usually write out one scanline at a time anyway to get the pitch right, just write the scnalines in reversed order and you'll be fine.