//----------------------- vCaptureScreen
void vCaptureScreen (char * filename, int iFrameWidth, int iFrameHeight)
{
  FILE *fp = fopen (filename, "wb");
  if (fp == NULL) {
    printf ("Invalid output filename\n");
    return;
  }  

  int data_size = iFrameWidth*iFrameHeight*3;
  unsigned char *framebuffer = new unsigned char[data_size];
  if (framebuffer == NULL) {
    printf ("Unable to allocate enough memory to capture screen");
    return;
  }
    
  glReadPixels (0, 0, iFrameWidth, iFrameHeight, GL_RGB, GL_UNSIGNED_BYTE, framebuffer);

  // now flip vertical
  int x, y, i1, i2;
  unsigned char temp;

  for (x = 0; x < iFrameWidth; x++){
    for (y = 0; y < iFrameHeight/2; y++){
      i1 = (y*iFrameWidth + x)*3; // this pixel
      i2 = ((iFrameHeight - y - 1)*iFrameWidth + x) * 3; // its opposite (across x-axis)

      // swap pixels 
      temp = framebuffer[i1];
      framebuffer[i1] = framebuffer[i2];
      framebuffer[i2] = temp;

      i1++; i2++;
      temp = framebuffer[i1];
      framebuffer[i1] = framebuffer[i2];
      framebuffer[i2] = temp;

      i1++; i2++;
      temp = framebuffer[i1];
      framebuffer[i1] = framebuffer[i2];
      framebuffer[i2] = temp;

    }
  }
  
  fprintf (fp, "P6\n%d %d\n%d\n", iFrameWidth, iFrameHeight, 255);
  fwrite (framebuffer, data_size, 1, fp);
  delete framebuffer;
  fclose(fp);
}