OpenGL Programming Guide

Chapter 12Selection and Feedback

Feedback

The Feedback Array

In feedback mode, each primitive that would be rasterized (or each call to glDrawPixels() or glCopyPixels(), if the raster position is valid) generates a block of values that's copied into the feedback array. The number of values is determined by the type argument to glFeedbackBuffer(), as listed in Table 12-1 . Use the appropriate value for the type of primitives you're drawing: GL_2D or GL_3D for unlit two- or three-dimensional primitives, GL_3D_COLOR for lit, three-dimensional primitives, and GL_3D_COLOR_TEXTURE or GL_4D_COLOR_TEXTURE for lit, textured, three- or four-dimensional primitives.

Each block of feedback values begins with a code indicating the primitive type, followed by values that describe the primitive's vertices and associated data. Entries are also written for pixel rectangles. In addition, pass-through markers that you've explicitly created can be returned in the array; the next section explains these markers in more detail. Table 12-2 shows the syntax for the feedback array; remember that the data associated with each returned vertex is as described in Table 12-1 . Note that a polygon can have n vertices returned. Also, the x, y, z coordinates returned by feedback are window coordinates; if w is returned, it's in clip coordinates. For bitmaps and pixel rectangles, the coordinates returned are those of the current raster position. In the table, note that GL_LINE_RESET_TOKEN is returned only when the line stipple is reset for that line segment.

Table 12-2 : Feedback Array Syntax

Primitive Type	Code	Associated Data
Point	GL_POINT_TOKEN	vertex
Line	GL_LINE_TOKEN or GL_LINE_RESET_TOKEN	vertex vertex
Polygon	GL_POLYGON_TOKEN	n vertex vertex ... vertex
Bitmap	GL_BITMAP_TOKEN	vertex
Pixel Rectangle	GL_DRAW_PIXEL_TOKEN or GL_COPY_PIXEL_TOKEN	vertex
Pass-through	GL_PASS_THROUGH_TOKE N	a floating-point number

OpenGL Programming Guide