OpenGL Programming Guide

Chapter 12Selection and Feedback

Selection

The Basic Steps

To use the selection mechanism, you need to perform the following steps.

1. Specify the array to be used for the returned hit records with glSelectBuffer().
   
   

2. Enter selection mode by specifying GL_SELECT with glRenderMode().
   
   

3. Initialize the name stack using glInitNames() and glPushName().
   
   

4. Define the viewing volume you want to use for selection. Usually, this is different from the viewing volume you used to draw the scene originally, so you probably want to save and then restore the current transformation state with glPushMatrix() and glPopMatrix().
   
   

5. Alternately issue primitive drawing commands and commands to manipulate the name stack so that each primitive of interest has an appropriate name assigned.
   
   

6. Exit selection mode and process the returned selection data (the hit records).

The following paragraphs describe glSelectBuffer() and glRenderMode(). In the next section, the commands to manipulate the name stack are described. void glSelectBuffer(GLsizei size, GLuint *buffer);

Specifies the array to be used for the returned selection data. The buffer argument is a pointer to an array of unsigned integers into which the data is put, and size indicates the maximum number of values that can be stored in the array. You need to call glSelectBuffer() before entering selection mode.

GLint glRenderMode(GLenum mode);

Controls whether the application is in rendering, selection, or feedback mode. The mode argument can be one of GL_RENDER (the default), GL_SELECT, or GL_FEEDBACK. The application remains in a given mode until glRenderMode() is called again with a different argument. Before entering selection mode, glSelectBuffer() must be called to specify the selection array. Similarly, before entering feedback mode, glFeedbackBuffer() must be called to specify the feedback array. The return value for glRenderMode() has meaning if the current render mode (that is, not the mode parameter) is either GL_SELECT or GL_FEEDBACK: The return value is the number of selection hits or the number of values placed in the feedback array when either mode is exited; a negative value means that the selection or feedback array has overflowed. You can use GL_RENDER_MODE with glGetIntegerv() to obtain the current mode.

OpenGL Programming Guide