OpenGL Programming Guide

Chapter 4Display Lists

Creating and Executing a Display List

What's Stored in a Display List

When you're building a display list, only the values for expressions are stored in the list. Thus, if values in an array are subsequently changed, for example, the display-list values don't change. In the following code fragment, the display list contains a command to set the current color to black (0.0, 0.0, 0.0). The subsequent change of the value of the color_vector array to red (1.0, 0.0, 0.0) has no effect on the display list because the display list contains the values that were in effect when it was created.

GLfloat color_vector[3]={0.0,0.0,0.0};
glNewList(1,GL_COMPILE);
   glColor3fv(color_vector);
glEndList();
color_vector[0]=1.0;

Not all OpenGL commands can be stored and executed from within a display list. Generally, commands that pass parameters by reference or that return a value can't be stored in a display list, since the list might be called outside the scope of where the parameters are originally defined. If such commands are called when making a display list, they're executed immediately and aren't stored in the display list. Here are the OpenGL commands that aren't stored in a display list (also, note that glNewList() generates an error if it's called while you're creating a display list). Some of these commands haven't been described yet; you can look in the index to see where they're discussed.

glDeleteLists() glIsEnabled()

glFeedbackBuffer() glIsList()

glFinish() glPixelStore()

glFlush() glReadPixels()

glGenLists() glRenderMode()

glGet*() glSelectBuffer()

To understand more clearly why these commands can't be stored in a display list, remember that when you're using OpenGL across a network, the client may be on one machine and the server on another. After a display list is created, it resides with the server, so the server can't rely on the client for any information related to the display list. If querying commands, such as glGet*() or glIs*(), were allowed in a display list, the calling program would be surprised at random times by data returned over the network. Without parsing the display list as it was sent, the calling program wouldn't know where to put the data. Thus, any command that returns a value can't be stored in a display list. Other routines - such as glFlush() and glFinish() - can't be stored in a display list because they depend on information about the client state. Finally, commands that change a state value maintained by the client can't be stored in a display list.

OpenGL Programming Guide