GLenum type, GLfloat *buffer )
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glFeedbackBuffer has three arguments: buffer is a pointer to an array of floating-point values into which feedback information is placed. size indicates the size of the array. type is a symbolic constant describing the information that is fed back for each vertex. glFeedbackBuffer must be issued before feedback mode is enabled (by calling glRenderMode with argument GL_FEEDBACK). Setting GL_FEEDBACK without establishing the feedback buffer, or calling glFeedbackBuffer while the GL is in feedback mode, is an error.
When glRenderMode is called while in feedback mode, it returns the number of entries placed in the feedback array, and resets the feedback array pointer to the base of the feedback buffer. The returned value never exceeds size. If the feedback data required more room than was available in buffer, glRenderMode returns a negative value. To take the GL out of feedback mode, call glRenderMode with a parameter value other than GL_FEEDBACK.
While in feedback mode, each primitive, bitmap, or pixel rectangle that would be rasterized generates a block of values that are copied into the feedback array. If doing so would cause the number of entries to exceed the maximum, the block is partially written so as to fill the array (if there is any room left at all), and an overflow flag is set. Each block 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 bitmaps and pixel rectangles. Feedback occurs after polygon culling and glPolygonMode interpretation of polygons has taken place, so polygons that are culled are not returned 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
feedbackItem <- point | lineSegment | polygon | bitmap | pixelRectangle | passThru
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.
The following table gives the correspondence between type and the number of values per vertex. k is 1 in color index mode and 4 in RGBA mode.
| Type | Coordinates | Color | Texture | Total Number of Values |
| GL_2D | x, y | 2 | ||
| GL_3D | x, y, z | 3 | ||
| GL_3D_COLOR | x, y, z | $k$ | $3 ~+~ k$ | |
| GL_3D_COLOR_TEXTURE | x, y, z, | $k$ | 4 | $7 ~+~ k$ |
| GL_4D_COLOR_TEXTURE | x, y, z, w | $k$ | 4 | $8 ~+~ k$ |
Feedback vertex coordinates are in window coordinates, except w, which is in clip coordinates. Feedback colors are lighted, if lighting is enabled. Feedback texture coordinates are generated, if texture coordinate generation is enabled. They are always transformed by the texture matrix.
When the GL_ARB_multitexture extension is supported, glFeedbackBuffer returns only the texture coordinates of texture unit GL_TEXTURE0_ARB.
GL_INVALID_VALUE is generated if size is negative.
GL_INVALID_OPERATION is generated if glFeedbackBuffer is called while the render mode is GL_FEEDBACK, or if glRenderMode is called with argument GL_FEEDBACK before glFeedbackBuffer is called at least once.
GL_INVALID_OPERATION is generated if glFeedbackBuffer is executed between the execution of glBegin and the corresponding execution of glEnd.