Managing State Variables with Display Lists

A display list can contain calls that change the value of OpenGL state variables. These values change as the display list is executed, just as if the commands were called in immediate mode and the changes persist after execution of the display list is completed. As previously seen in Example 7-2 and in Example 7-6, which follows, the changes to the current color and current matrix made during the execution of the display list remain in effect after it has been called.

Example 7-6 : Persistence of State Changes after Execution of a Display List

glNewList(listIndex,GL_COMPILE);

   glColor3f(1.0, 0.0, 0.0);

   glBegin(GL_POLYGON);

      glVertex2f(0.0,0.0);

      glVertex2f(1.0,0.0);

      glVertex2f(0.0,1.0);

   glEnd();

   glTranslatef(1.5,0.0,0.0);

glEndList();

So if you now call the following sequence, the line drawn after the display list is drawn with red as the current color and translated by an additional (1.5, 0.0, 0.0):

glCallList(listIndex);

glBegin(GL_LINES);

   glVertex2f(2.0,-1.0);

   glVertex2f(1.0,0.0);

glEnd();

Sometimes you want state changes to persist, but other times you want to save the values of state variables before executing a display list and then restore these values after the list has executed. Remember that you cannot use glGet*() in a display list, so you must use another way to query and store the values of state variables.

You can use glPushAttrib() to save a group of state variables and glPopAttrib() to restore the values when you're ready for them. To save and restore the current matrix, use glPushMatrix() and glPopMatrix() as described in "Manipulating the Matrix Stacks" in Chapter 3. These push and pop routines can be legally cached in a display list. To restore the state variables in Example 7-6, you might use the code shown in Example 7-7.

Example 7-7 : Restoring State Variables within a Display List

glNewList(listIndex,GL_COMPILE);

   glPushMatrix();

   glPushAttrib(GL_CURRENT_BIT);

   glColor3f(1.0, 0.0, 0.0);

   glBegin(GL_POLYGON);

      glVertex2f(0.0,0.0);

      glVertex2f(1.0,0.0);

      glVertex2f(0.0,1.0);

   glEnd();

   glTranslatef(1.5,0.0,0.0);

   glPopAttrib();

   glPopMatrix();

glEndList();

If you use the display list from Example 7-7, which restores values, the code in Example 7-8 draws a green, untranslated line. With the display list in Example 7-6, which doesn't save and restore values, the line is drawn red, and its position is translated ten times (1.5, 0.0, 0.0).

Example 7-8 : The Display List May or May Not Affect drawLine()

void display(void)

{

   GLint i;

   glClear(GL_COLOR_BUFFER_BIT);

   glColor3f(0.0, 1.0, 0.0);  /* set current color to green   */

   for (i = 0; i < 10; i++)

      glCallList(listIndex);  /* display list called 10 times */

   drawLine();        /* how and where does this line appear? */

   glFlush();

}

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 may be more efficient than immediate mode for switching among various lighting, lighting-model, and material-parameter settings. You might also use display lists for stipple patterns, fog parameters, and clipping-plane equations. In general, you'll find 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 7-9 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 7-9 : 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);