OpenGL: glGetTexImage

no-reply@allegro.cc (Erin Maus) — Fri, 11 Sep 2009 01:53:02 +0000

I'm wondering that, when using glGetTexImage, what format is the texture in? For example, say the texture is 1920 x 1080 and the pixel format is R8G8B8A8. Now, I figure I can't simple have a loop like this:

for (int x = 0; x < TextureWidth; x++)
      {
        for (int y = 0; y < TextureHeight; y++)
        {
          bla.SetPixel(x, y, Color.FromArgb(glImage[x, y, 0], glImage[x, y, 1], glImage[x, y, 2]));
        }
      }

I figure this because if I try that, then the image is...strange:

{"name":"599389","src":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/e\/e\/eef27875ec139019b9b9af4aee1199f8.png","w":480,"h":270,"tn":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/e\/e\/eef27875ec139019b9b9af4aee1199f8"}

When it should be more like this:

{"name":"599390","src":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/c\/b\/cb8379e5a153e21575901f031d752776.png","w":283,"h":162,"tn":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/c\/b\/cb8379e5a153e21575901f031d752776"}

Basically, how do I use the data from glGetTexImage into the standard (0, 0) is top left and (W, H) is bottom right?

no-reply@allegro.cc (Thomas Harte) — Fri, 11 Sep 2009 02:26:52 +0000

Not sure exactly what language you're using there for "glImage[x, y, 0]" to reference anything other than the very first byte, but with R8G8B8A8 (assuming you mean that you're specifying a format of GL_RGBA and a type of GL_UNSIGNED_BYTE) each pixel will be a byte of red, then a byte of green, then a byte of blue, then a byte of alpha.

So, example code:

#SelectExpand
  1void printPixelsOfMipMap(int mipMapLevel)
  2{
  3   GLint textureWidth, textureHeight;
  4
  5   glGetTexLevelParameter(GL_TEXTURE_2D, mipMapLevel, GL_TEXTURE_WIDTH, &textureWidth);
  6   glGetTexLevelParameter(GL_TEXTURE_2D, mipMapLevel, GL_TEXTURE_HEIGHT, &textureHeight);
  7
  8   GLubyte *buffer = (GLubyte *)malloc(textureWidth*textureHeight*4);
  9
 10   glGetTexImage(GL_TEXTURE_2D, mipMapLevel, GL_RGBA, GL_UNSIGNED_BYTE, buffer);
 11
 12   for(int y = 0; y < textureHeight; y++)
 13      for(int x = 0; x < textureWidth; x++)
 14      {
 15          int startAddressOfPixel = ((y*textureWidth) + x)*4;
 16
 17          printf("pixel at %d %d has colour r=%d g=%d b=%d a=%d\n", x, y, buffer[startAddressOfPixel], buffer[startAddressOfPixel+1], buffer[startAddressOfPixel+2], startAddressOfPixel[startAddressOfPixel+3]);
 18      }
 19
 20   free(buffer);
 21}

no-reply@allegro.cc (Erin Maus) — Fri, 11 Sep 2009 02:34:30 +0000

Peculiar. I was using C#.

With a few changes based on your code it works now.

Thanks!