OpenGL Programming Guide

Chapter 13Now That You Know

An Easy Fade Effect

Suppose you have an image that you want to fade gradually to some background color. Define a series of polygon stipple patterns, each of which has more bits turned on so that they represent denser and denser patterns. Then use these patterns repeatedly with a polygon large enough to cover the region over which you want to fade. For example, suppose you want to fade to black in sixteen steps. First define sixteen different pattern arrays:

GLubyte stips[16][4*32];

Then load them in such a way that each has one-sixteenth of the pixels in a 32x32 stipple pattern turned on. After that, the following code does the trick:

draw_the_picture(); 
glColor3f(0.0, 0.0, 0.0);    /* set color to black */ 
for (i = 0; i < 16; i++) { 
    glPolygonStipple(&stips[i][0]); 
    draw_a_polygon_large_enough_to_cover_the_whole_region(); 
}

In some OpenGL implementations, you might get better performance by first compiling the stipple patterns into display lists. During your initialization, do something like this:

#define STIP_OFFSET 100 
for (i = 0; i < 16; i++) { 
    glNewList(i+STIP_OFFSET, GL_COMPILE); 
    glPolygonStipple(&stips[i][0]); 
    glEndList(); 
} 

Then, replace this line in the first code fragment

glPolygonStipple(&stips[i][0]); 

with

glCallList(i); 

By compiling the command to set the stipple into a display list, OpenGL might be able to rearrange the data in the stips[ ][ ] array into the hardware-specific form required for maximum stipple-setting speed.

Another application for this technique is if you're drawing a changing picture, and you want to leave some blur behind that gradually fades out to give some indication of past motion. For example, suppose you're simulating a planetary system, and you want to leave trails on the planets to show a recent portion of their path. Again, assuming you want to fade in sixteen steps, set up the stipple patterns as before (using the display-list version, say), and have the main simulation loop look something like this:

current_stipple = 0; 
while (1) {                         /* loop forever */ 
    draw_the_next_frame(); 
    glCallList(current_stipple++); 
    if (current_stipple == 16) current_stipple = 0; 
    glColor3f(0.0, 0.0, 0.0);       /* set color to black */ 
    draw_a_polygon_large_enough_to_cover_the_whole_region(); 
} 

Each time through the loop, you clear one-sixteenth of the pixels. Any pixel that hasn't had a planet on it for sixteen frames is certain to be cleared to black. Of course, if your system supports blending in hardware, it's easier to blend in a certain amount of background color with each frame.

See "Displaying Points, Lines, and Polygons" for polygon stippling details, Chapter 4, "Display Lists," for more information about display lists, and "Blending" for information about blending.

OpenGL Programming Guide