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.
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.
NOTES
glFeedbackBuffer, when used in a display list, is not
compiled into the display list but is executed immediately.
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
SEE ALSO
glBegin, glLineStipple, glPassThrough, glPolygonMode,
glRenderMode, glSelectBuffer