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glFeedbackBuffer(3G)
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glFeedbackBuffer - controls feedback mode
void glFeedbackBuffer(
GLsizei size,
GLenum type,
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 feedback
data.
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 produced 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. 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
option 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_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 floatingpoint
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.
glFeedbackBuffer(), 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 glEnd().
glGet() 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
glBegin(3), glLineStipple(3), glPassThrough(3), glPolygonMode(3), glRenderMode(3), glSelectBuffer(3)
glFeedbackBuffer(3G)
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