OpenGL Programming Guide

Chapter 12Selection and Feedback

Selection

Typically, when you're planning to use OpenGL's selection mechanism, you first draw your scene into the framebuffer and then you enter selection mode and redraw the scene. Once you're in selection mode, however, the contents of the framebuffer don't change until you exit selection mode. When you exit, OpenGL returns a list of the primitives that would have intersected the viewing volume (remember that the viewing volume is defined by the current modelview and projection matrices and any clipping planes you've specified, as explained in "Additional Clipping Planes." ) Each primitive that intersects the viewing volume causes a selection hit. The list of primitives is actually returned as an array of integer-valued names and related data - the hit records - that correspond to the current contents of the name stack. You construct the name stack by loading names onto it as you issue primitive drawing commands while in selection mode. Thus, when the list of names is returned, you can use it to determine which primitives might have been selected on the screen by the user.

In addition to this selection mechanism, OpenGL provides a utility routine designed to simplify selection in some cases by restricting drawing to a small region of the viewport. Typically, you use this routine to determine which objects are drawn near the cursor, so that you can identify which object the user is picking. You can also delimit a selection region by specifying additional clipping planes; see "Additional Clipping Planes" for more information about how to do this. Since picking is a special case of selection, selection is described first in this chapter, and then picking.

The Basic Steps
Creating the Name Stack
The Hit Record
A Selection Example
Picking
Hints for Writing a Program That Uses Selection

OpenGL Programming Guide