OpenGL Programming Guide

Chapter 12Selection and Feedback

Feedback

Using Markers in Feedback Mode

Feedback occurs after transformations, lighting, polygon culling, and interpretation of polygons by glPolygonMode(). It might also occur after polygons with more than three edges are broken up into triangles (if your particular OpenGL implementation renders polygons by performing this decomposition). Thus, it might be hard for you to recognize the primitives you drew in the feedback data you receive. To help yourself parse the feedback data, call glPassThrough() as needed in your sequence of drawing commands to insert a marker. You might use the markers to separate the feedback values returned from different primitives, for example. This command causes GL_PASS_THROUGH_TOKEN to be written into the feedback array, followed by the floating-point value you pass in as an argument. void glPassThrough(GLfloat token);

Inserts a marker into the stream of values written into the feedback array, if called in feedback mode. The marker consists of the code GL_PASS_THROUGH_TOKEN followed by a single floating-point value, token. This command has no effect when called outside of feedback mode. Calling glPassThrough() between glBegin() and glEnd() generates a GL_INVALID_OPERATION error.

OpenGL Programming Guide