OpenGL Programming Guide

Chapter 12Selection and Feedback

Selection

Creating the Name Stack

As mentioned in the previous section, the name stack forms the basis for the selection information that's returned to you. To create the name stack, first you initialize it with glInitNames(), which simply clears the stack, and then you add integer names to it as you issue corresponding drawing commands. As you might expect, the commands to manipulate the stack allow you to push a name onto it (glPushName()), pop a name off of it (glPopName()), and replace the name on the top of the stack with a different one (glLoadName()). Example 12-1 shows what your name-stack manipulation code might look like with these commands.

Example 12-1 : Creating a Name Stack

glInitNames();
glPushName(-1);

glPushMatrix();   /* save the current transformation state */

    /* create your desired viewing volume here */

    glLoadName(1);
    drawSomeObject();
    glLoadName(2);
    drawAnotherObject();
    glLoadName(3);
    drawYetAnotherObject();
    drawJustOneMoreObject();

glPopMatrix ();   /* restore the previous transformation state*/

In this example, the first two objects to be drawn have their own names, and the third and fourth objects share a single name. With this setup, if either or both of the third and fourth objects causes a selection hit, only one hit record is returned to you. You can have multiple objects share the same name if you don't need to differentiate between them when processing the hit records. void glInitNames(void);

Clears the name stack so that it's empty.

void glPushName(GLuint name);

Pushes name onto the name stack. Pushing a name beyond the capacity of the stack generates the error GL_STACK_OVERFLOW. The name stack's depth can vary among different OpenGL implementations, but it must be able to contain at least sixty-four names. You can use the parameter GL_NAME_STACK_DEPTH with glGetIntegerv() to obtain the depth of the name stack.

void glPopName(void);

Pops one name off the top of the name stack. Popping an empty stack generates the error GL_STACK_UNDERFLOW.

void glLoadName(GLuint name);

Replaces the value on the top of the name stack with name. If the stack is empty, which it is right after glInitNames() is called, glLoadName() generates the error GL_INVALID_OPERATION. To avoid this, if the stack is initially empty, call glPushName() at least once to put something on the name stack before calling glLoadName().

Calls to glPushName(), glPopName(), and glLoadName() are ignored if you're not in selection mode. You might find that it simplifies your code to use these calls throughout your drawing code, and then use the same drawing code for both selection and normal rendering modes.

OpenGL Programming Guide