OpenGL Programming Guide

Chapter 4Display Lists

Encapsulating Mode Changes

You can use display lists to organize and store groups of commands to change various modes or set various parameters. When you want to switch from one group of settings to another, using display lists might be more efficient than making the calls directly, since the settings might be cached in a format that matches the requirements of your graphics system.

Display lists are likely to be more efficient when you're switching between multiple texture maps, for example. (Texture mapping is described in Chapter 9 .) Suppose you have two different textures that are fairly large, as textures tend to be, but that both of them fit into texture memory. Without display lists, you would have to load the data for the first texture, use it to draw some objects, wait for the second texture to be loaded into memory, and then use it for drawing. When you want to switch back to the first texture, it would have to be loaded into memory again rather than being plucked out of texture memory. There's no way for OpenGL to know that it's already been stored without the display-list mechanism to provide a "handle" to identify it. With display lists, both textures can be loaded into texture memory once and then used as often as necessary without having to be reloaded.

Another case where display lists are likely to be more efficient than immediate mode is for switching among various lighting, lighting-model, and material-parameter settings. (These topics are discussed in Chapter 6 .) You might also use display lists for stipple patterns, fog parameters, and clipping-plane equations. In general, you're guaranteed that executing display lists is at least as fast as making the relevant calls directly, but remember that some overhead is involved in jumping to a display list.

Example 4-9 shows how to use display lists to switch among three different line stipples. First, you call glGenLists() to allocate a display list for each stipple pattern and create a display list for each pattern. Then, you use glCallList() to switch from one stipple pattern to another.

Example 4-9 : Using Display Lists for Mode Changes

GLuint offset;
offset = glGenLists (3);

glNewList (offset, GL_COMPILE);
    glDisable (GL_LINE_STIPPLE);
glEndList ();

glNewList (offset+1, GL_COMPILE);
    glEnable (GL_LINE_STIPPLE);
    glLineStipple (1, 0x0F0F);
glEndList ();

glNewList (offset+2, GL_COMPILE);
    glEnable (GL_LINE_STIPPLE);
    glLineStipple (1, 0x1111);
glEndList ();


#define drawOneLine(x1,y1,x2,y2) glBegin(GL_LINES); \ 
    glVertex2f ((x1),(y1)); glVertex2f ((x2),(y2)); glEnd();

glCallList (offset);
drawOneLine (50.0, 125.0, 350.0, 125.0);

glCallList (offset+1);
drawOneLine (50.0, 100.0, 350.0, 100.0);

glCallList (offset+2);
drawOneLine (50.0, 75.0, 350.0, 75.0);

OpenGL Programming Guide