OpenGL Programming Guide

Chapter 12Selection and Feedback

Selection

The Hit Record

In selection mode, a primitive that intersects the viewing volume causes a selection hit. Whenever a name-stack manipulation command is executed or glRenderMode() is called, OpenGL writes a hit record into the selection array if there's been a hit since the last time the stack was manipulated or glRenderMode() was called. With this process, objects that share the same name - for example, an object that's composed of more than one primitive - don't generate multiple hit records. Also, hit records aren't guaranteed to be written into the array until glRenderMode() is called.

In addition to primitives, valid coordinates produced by glRasterPos() can cause a selection hit. In the case of polygons, no hit occurs if the polygon would have been culled.

Each hit record consists of four items, in order:

• The number of names on the name stack when the hit occurred.
  
  

• Both the minimum and maximum window-coordinate z values of all vertices of the primitives that intersected the viewing volume since the last recorded hit. These two values, which lie in the range [0,1], are each multiplied by 232-1 and rounded to the nearest unsigned integer.
  
  

• The contents of the name stack at the time of the hit, with the bottommost element first.

When you enter selection mode, OpenGL initializes a pointer to the beginning of the selection array. Each time a hit record is written into the array, the pointer is updated accordingly. If writing a hit record would cause the number of values in the array to exceed the size argument specified with glSelectBuffer(), OpenGL writes as much of the record as fits in the array and sets an overflow flag. When you exit selection mode with glRenderMode(), this command returns the number of hit records that were written (including a partial record if there was one), clears the name stack, resets the overflow flag, and resets the stack pointer. If the overflow flag had been set, the return value is -1.

OpenGL Programming Guide