glFeedbackBuffer(3G)glFeedbackBuffer(3G)NAMEglFeedbackBuffer - controls feedback mode
GLfloat *buffer );
Specifies the maximum number of values that can be written into buffer.
Specifies a symbolic constant that describes the information that will
be returned for each vertex. GL_2D, GL_3D, GL_3D_COLOR,
GL_3D_COLOR_TEXTURE, and GL_4D_COLOR_TEXTURE are accepted. Returns the
The glFeedbackBuffer() function controls feedback. Feedback, like
selection, is a GL mode. The mode is selected by calling glRenderMode()
with GL_FEEDBACK. When the GL is in feedback mode, no pixels are pro‐
duced by rasterization. Instead, information about primitives that
would have been rasterized is fed back to the application using the GL.
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. glFeed‐
backBuffer() 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 option is
set. Each block begins with a code indicating the primitive type, fol‐
lowed by values that describe the primitive's vertices and associated
data. Entries are also written for bitmaps and pixel rectangles. Feed‐
back 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 ren‐
ders 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
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_TOKENvertex 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 ver‐
tex depends on the feedback type.
The following table gives the correspondence between type and the num‐
ber of values per vertex. k is 1 in color index mode and 4 in RGBA
Type Coordinates Color Texture Total Number of
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 coordi‐
nate generation is enabled. They are always transformed by the texture
NOTESglFeedbackBuffer(), when used in a display list, is not compiled into
the display list but is executed immediately.
When the GL_ARB_multitexture extension is supported, glFeedbackBuffer()
returns only the texture coordinates of texture unit GL_TEXTURE0_ARB.
GL_INVALID_ENUM is generated if type is not an accepted value.
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
ASSOCIATED GETSglGet() with argument GL_RENDER_MODE
glGet() with argument GL_FEEDBACK_BUFFER_POINTER
glGet() with argument GL_FEEDBACK_BUFFER_SIZE
glGet() with argument GL_FEEDBACK_BUFFER_TYPE
SEE ALSOglBegin(3), glLineStipple(3), glPassThrough(3), glPolygonMode(3),