OpenGL man pages
glFeedbackBuffer - controls feedback mode
void glFeedbackBuffer( GLsizei size, GLenum type, GLfloat *buffer )
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.
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.
The GL is taken out of feedback mode by calling glRenderMode with a
parameter value other than GL_FEEDBACK. When this is done while the GL is
in feedback mode, glRenderMode returns the number of entries placed in the
feedback array. The returned value never exceeds size. If the feedback
data required more room than was available in buffer, glRenderMode returns
a negative value.
While in feedback mode, each primitive that would be rasterized generates a
block of values that get 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 glPolyMode
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.
glFeedbackBuffer, when used in a display list, is not compiled into the display list but rather is executed immediately.
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
glBegin, glLineStipple, glPassThrough, glPolygonMode, glRenderMode, glSelectBuffer
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Last Edited:
Fri Dec 6 11:18:03 EST 1996
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AFV
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