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<STRONG>NAME</STRONG>
<STRONG>glFeedbackBuffer</STRONG> - controls feedback mode
<STRONG>C</STRONG> <STRONG>SPECIFICATION</STRONG>
void <STRONG>glFeedbackBuffer</STRONG>( GLsizei <EM>size</EM>,
GLenum <EM>type</EM>,
GLfloat *<EM>buffer</EM> )
<STRONG>PARAMETERS</STRONG>
<EM>size</EM> Specifies the maximum number of values that can be
written into <EM>buffer</EM>.
<EM>type</EM> Specifies a symbolic constant that describes the
information that will be returned for each vertex.
<STRONG>GL_2D</STRONG>, <STRONG>GL_3D</STRONG>, <STRONG>GL_3D_COLOR</STRONG>, <STRONG>GL_3D_COLOR_TEXTURE</STRONG>, and
<STRONG>GL_4D_COLOR_TEXTURE</STRONG> are accepted.
<EM>buffer</EM> Returns the feedback data.
<STRONG>DESCRIPTION</STRONG>
The <STRONG>glFeedbackBuffer</STRONG> function controls feedback. Feedback,
like selection, is a GL mode. The mode is selected by
calling <STRONG>glRenderMode</STRONG> with <STRONG>GL_FEEDBACK</STRONG>. 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.
<STRONG>glFeedbackBuffer</STRONG> has three arguments: <EM>buffer</EM> is a pointer
to an array of floating-point values into which feedback
information is placed. <EM>size</EM> indicates the size of the
array. <EM>type</EM> is a symbolic constant describing the
information that is fed back for each vertex.
<STRONG>glFeedbackBuffer</STRONG> must be issued before feedback mode is
enabled (by calling <STRONG>glRenderMode</STRONG> with argument <STRONG>GL_FEEDBACK</STRONG>).
Setting <STRONG>GL_FEEDBACK</STRONG> without establishing the feedback
buffer, or calling <STRONG>glFeedbackBuffer</STRONG> while the GL is in
feedback mode, is an error.
When <STRONG>glRenderMode</STRONG> 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 <EM>size</EM>. If
the feedback data required more room than was available in
<EM>buffer</EM>, <STRONG>glRenderMode</STRONG> returns a negative value. To take the
GL out of feedback mode, call <STRONG>glRenderMode</STRONG> with a parameter
value other than <STRONG>GL_FEEDBACK</STRONG>.
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 <STRONG>glPolygonMode</STRONG> 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 <STRONG>glPassThrough</STRONG> command can be used to insert a marker
into the feedback buffer. See <STRONG>glPassThrough</STRONG>.
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
<EM>type</EM>. 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 <- <STRONG>GL_POINT_TOKEN</STRONG> vertex
lineSegment <- <STRONG>GL_LINE_TOKEN</STRONG> vertex vertex |
<STRONG>GL_LINE_RESET_TOKEN</STRONG> vertex vertex
polygon <- <STRONG>GL_POLYGON_TOKEN</STRONG> n polySpec
polySpec <- polySpec vertex | vertex vertex vertex
bitmap <- <STRONG>GL_BITMAP_TOKEN</STRONG> vertex
pixelRectangle <- <STRONG>GL_DRAW_PIXEL_TOKEN</STRONG> vertex |
<STRONG>GL_COPY_PIXEL_TOKEN</STRONG> vertex
passThru <- <STRONG>GL_PASS_THROUGH_TOKEN</STRONG> 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
<EM>value</EM> is a floating-point number, and <EM>n</EM> is a floating-point
integer giving the number of vertices in the polygon.
<STRONG>GL_POINT_TOKEN</STRONG>, <STRONG>GL_LINE_TOKEN</STRONG>, <STRONG>GL_LINE_RESET_TOKEN</STRONG>,
<STRONG>GL_POLYGON_TOKEN</STRONG>, <STRONG>GL_BITMAP_TOKEN</STRONG>, <STRONG>GL_DRAW_PIXEL_TOKEN</STRONG>,
<STRONG>GL_COPY_PIXEL_TOKEN</STRONG> and <STRONG>GL_PASS_THROUGH_TOKEN</STRONG> are symbolic
floating-point constants. <STRONG>GL_LINE_RESET_TOKEN</STRONG> is returned
whenever the line stipple pattern is reset. The data
returned as a vertex depends on the feedback <EM>type</EM>.
The following table gives the correspondence between <EM>type</EM>
and the number of values per vertex. <EM>k</EM> is 1 in color index
mode and 4 in RGBA mode.
______________________________________________________________________________
| <EM>type</EM> | <EM>coordinates</EM> | <EM>color</EM> | <EM>texture</EM> | <EM>total</EM> <EM>number</EM> <EM>of</EM> <EM>values</EM> |
<EM>|</EM>____________________<EM>|</EM>_____________<EM>|</EM>_______<EM>|</EM>_________<EM>|</EM>________________________|
| <STRONG>GL_2D</STRONG> | <EM>x</EM>, <EM>y</EM> | | | 2 |
| <STRONG>GL_3D</STRONG> | <EM>x</EM>, <EM>y</EM>, <EM>z</EM> | | | 3 |
| <STRONG>GL_3D_COLOR</STRONG> | <EM>x</EM>, <EM>y</EM>, <EM>z</EM> | k | | 3+k |
|<STRONG>GL_3D_COLOR_TEXTURE</STRONG> | <EM>x</EM>, <EM>y</EM>, <EM>z</EM>, | k | 4 | 7+k |
|<STRONG>GL_4D_COLOR_TEXTURE</STRONG> | <EM>x</EM>, <EM>y</EM>, <EM>z</EM>, <EM>w</EM> | k | 4 | 8+k |
<EM>|</EM>____________________<EM>|</EM>_____________<EM>|</EM>_______<EM>|</EM>_________<EM>|</EM>________________________|
Feedback vertex coordinates are in window coordinates,
except <EM>w</EM>, 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.
<STRONG>NOTES</STRONG>
<STRONG>glFeedbackBuffer</STRONG>, when used in a display list, is not
compiled into the display list but is executed immediately.
<STRONG>ERRORS</STRONG>
<STRONG>GL_INVALID_ENUM</STRONG> is generated if <EM>type</EM> is not an accepted
value.
<STRONG>GL_INVALID_VALUE</STRONG> is generated if <EM>size</EM> is negative.
<STRONG>GL_INVALID_OPERATION</STRONG> is generated if <STRONG>glFeedbackBuffer</STRONG> is
called while the render mode is <STRONG>GL_FEEDBACK</STRONG>, or if
<STRONG>glRenderMode</STRONG> is called with argument <STRONG>GL_FEEDBACK</STRONG> before
<STRONG>glFeedbackBuffer</STRONG> is called at least once.
<STRONG>GL_INVALID_OPERATION</STRONG> is generated if <STRONG>glFeedbackBuffer</STRONG> is
executed between the execution of <STRONG>glBegin</STRONG> and the
corresponding execution of <STRONG>glEnd</STRONG>.
<STRONG>ASSOCIATED</STRONG> <STRONG>GETS</STRONG>
<STRONG>glGet</STRONG> with argument <STRONG>GL_RENDER_MODE</STRONG>
<STRONG>SEE</STRONG> <STRONG>ALSO</STRONG>
<STRONG>glBegin</STRONG>, <STRONG>glLineStipple</STRONG>, <STRONG>glPassThrough</STRONG>, <STRONG>glPolygonMode</STRONG>,
<STRONG>glRenderMode</STRONG>, <STRONG>glSelectBuffer</STRONG>
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