glFeedbackBuffer
GLFEEDBACKBUFFER(3G) 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 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.
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 feed-
back 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 rect-
angles. 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 trian-
gles, 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 identify-
ing value followed by some number of vertices. Polygon entries
include an integer value indicating how many vertices follow. A ver-
tex 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 | pixel-
Rectangle | 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 num-
ber 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 coor-
dinate generation is enabled. They are always transformed by the tex-
ture matrix.
NOTES
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.
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(3G), glLineStipple(3G), glPassThrough(3G), glPolygonMode(3G),
glRenderMode(3G), glSelectBuffer(3G)
GLFEEDBACKBUFFER(3G)
