Question

I’m working with glReadPixels for some work, and it’s behaving strangely. In particular, I run the following test:

unsigned char pixels[ 512 * 512 * 4 ];

int mid = 0;
while (mid < 512) {
  printf("Test %d : ", mid);
  memset(pixels, 0, 512*512*4);
  pixels[1] = 0x42;
  glReadPixels( 0, 0, mid, 511, GL_RGBA, GL_UNSIGNED_BYTE, pixels);
  printf(" %x ", pixels[1]);
  if (pixels[0] == 0) {
    printf("0\n");
  } else {
    printf("1\n");
  }
  mid++;
}

With output:

Test 0 :  42 0
Test 1 :  67 1
Test 2 :  67 1
Test 3 :  67 1
Test 4 :  42 0
Test 5 :  67 1
Test 6 :  67 1
Test 7 :  67 1
Test 8 :  42 0
Test 9 :  67 1
Test 10 :  67 1
Test 11 :  67 1

I have already drawn data to the entire screen (the 0x67 is the correct byte for the color I’m drawing) so I’m not sure what’s going on here.

The main thing I actually want is to get a copy of every pixel currently on the screen inside of an array I can access not on the GPU. I’m not sure if this is the ‘correct’ way about getting it, but it’s the only way I know of. Is there a better way of doing this?

Answer 1

You could use this code to first create an empty texture (use it just once, before your update loop):

GLuint texID;

GLubyte *data = (GLubyte*)malloc(width*height*4*sizeof(GLubyte));
ZeroMemory(data,width*height*4*sizeof(GLubyte));

glGenTextures(1, (GLuint*)texID);
glBindTexture(GL_TEXTURE_2D, texID);
glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_NEAREST);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, data);

Then use the following to copy the current buffer to your texture (use this at the end of every update loop, just before you swap buffers):

glBindTexture(GL_TEXTURE_2D,texID);
glCopyTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, 0, 0, width, height, 0);

And finally use this to retrieve an array with RGBA pixel data (right after the previous bit):

GLubyte *pixels = (GLubyte*)malloc(width*height*4*sizeof(GLubyte));
glBindTexture(GL_TEXTURE_2D,texID);
glGetTexImage(GL_TEXTURE_2D, 0, GL_RGBA, GL_UNSIGNED_BYTE, pixels);

It all depends on what you want to do with this pixel data ofcourse. This may not help you at all, but I hope it will ;).