glFeedbackBuffer man page on Oracle
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GLFEEDBACKBUFFER(3G) OpenGL Manual GLFEEDBACKBUFFER(3G)
NAME
glFeedbackBuffer - controls feedback mode
C SPECIFICATION
void glFeedbackBuffer(GLsizei size, GLenum type, GLfloat * buffer);
PARAMETERS
size
Specifies the maximum number of values that can be written into
buffer.
type
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.
buffer
Returns the feedback data.
DESCRIPTION
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 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.
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.
NOTES
glFeedbackBuffer, when used in a display list, is not compiled into the
display list but is executed immediately.
glFeedbackBuffer returns only the texture coordinate of texture unit
GL_TEXTURE0.
ERRORS
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().
ASSOCIATED GETS
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
SEE ALSO
glBegin(), glLineStipple(), glPassThrough(), glPolygonMode(),
glRenderMode(), glSelectBuffer()
COPYRIGHT
Copyright © 1991-2006 Silicon Graphics, Inc. This document is licensed
under the SGI Free Software B License. For details, see
http://oss.sgi.com/projects/FreeB/.
AUTHORS
opengl.org
opengl.org 04/29/2014 GLFEEDBACKBUFFER(3G)
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