OpenGL: glGetTexImage

Erin Maus

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?

Thomas Harte

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}

Erin Maus

Peculiar. I was using C#.

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

Thanks!

Thread #601551. Printed from Allegro.cc