glFeedbackBuffer(3G) OpenGL Reference glFeedbackBuffer(3G)NAMEglFeedbackBuffer - 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
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glFeedbackBuffer(3G) OpenGL Reference glFeedbackBuffer(3G)
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
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glFeedbackBuffer(3G) OpenGL Reference glFeedbackBuffer(3G)
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.
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.
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glFeedbackBuffer(3G) OpenGL Reference glFeedbackBuffer(3G)
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
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