OpenGL Programming Guide

Chapter 13Now That You Know

Alternative Uses for glDrawPixels() and glCopyPixels()

You might think of glDrawPixels() as a way to draw a rectangular region of pixels to the screen. Although this is often what it's used for, some other interesting uses are outlined here.

• Video - Even if your machine doesn't have special video hardware, you can display short movie clips by repeatedly drawing frames with glDrawPixels() in the same region of the back buffer and then swapping the buffers. The size of the frames you can display with reasonable performance using this method depends on your hardware's drawing speed, so you might be limited to 100 × 100 pixel movies (or smaller) if you want smooth fake video.
  
  

• Airbrush - In a paint program, your airbrush (or paintbrush) shape can be simulated using alpha values. The color of the paint is represented as the color values. To paint with a circular brush in blue, repeatedly draw a blue square with glDrawPixels() where the alpha values are largest in the center and taper to zero at the edges of a circle centered in the square. Draw using a blending function that uses alpha of the incoming color and (1-alpha) of the color already at the pixel. If the alpha values in the brush are all much less than 1, you have to paint over an area repeatedly to get a solid color. If the alpha values are near 1, each brush stroke pretty much obliterates the colors underneath.
  
  

• Filtered Zooms - If you zoom a pixel image by a nonintegral amount, OpenGL effectively uses a box filter, which can lead to rather severe aliasing effects. To improve the filtering, jitter the resulting image by amounts less than a pixel and redraw it multiple times, using alpha blending to average the resulting pixels. The result is a filtered zoom.
  
  

• Transposing Images - You can swap same-size images in place with glCopyPixels() using the XOR operation. With this method, you can avoid having to read the images back into processor memory. If A and B represent the two images, the operation looks like this:

  
  

  1. A = A XOR B
     
     

  2. B = A XOR B
     
     

  3. A = A XOR B

OpenGL Programming Guide