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glFeedbackBuffer

Chapter 6

151

in the feedback buffer. It can also occur after polygons with more than three edges are

broken up into triangles, if the GL implementation renders polygons by performing this

decomposition.

The glPassThrough command can be used to insert a marker into the feedback buffer.

See glPassThrough.

Following is the grammar for the blocks of values written into the feedback buffer. Each

primitive is indicated with a unique identifying value followed by some number of

vertices. Polygon entries include an integer value indicating how many vertices follow. A

vertex is fed back as some number of ﬂoating-point values, as determined by type. Colors

are fed back as four values in RGBA mode and one value in color index mode.

feedbackList ← feedbackItem feedbackList | feedbackItem

point ← GL_POINT_TOKEN vertex

lineSegment ← GL_LINE_TOKEN vertex vertex | GL_LINE_RESET_TOKEN vertex

vertex

polygon ← GL_POLYGON_TOKEN n polySpec

polySpec ← polySpec vertex | vertex vertex vertex

bitmap ← GL_BITMAP_TOKEN vertex

pixelRectangle ← GL_DRAW_PIXEL_TOKEN vertex |GL_COPY_PIXEL_TOKEN

vertex

passThru ← GL_PASS_THROUGH_TOKEN value

vertex ← 2d | 3d | 3dColor | 3dColorTexture | 4dColorTexture

2d ← value value

3d ← value value value

3dColor ← value value value color

3dColorTexture ← value value value color tex

4dColorTexture ← value value value value color tex

color ← rgba | index

rgba ← value value value value

index ← value

tex ← value value value value

value is a ﬂoating-point number, and n is a ﬂoating-point integer giving the number of

vertices in the polygon. GL_POINT_TOKEN, GL_LINE_TOKEN,

GL_LINE_RESET_TOKEN, GL_POLYGON_TOKEN, GL_BITMAP_TOKEN,

GL_DRAW_PIXEL_TOKEN, GL_COPY_PIXEL_TOKEN and

GL_PASS_THROUGH_TOKEN are symbolic ﬂoating-point constants.

GL_LINE_RESET_TOKEN is returned whenever the line stipple pattern is reset. The

data returned as a vertex depends on the feedback type.

HP b2600 Reference Guide

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