GLFEEDBACKBUFFER(3G)UNIX Programmer's Manual 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 cal-
ling glRenderMode with GL_FEEDBACK. When the GL is in feed-
back 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 infor-
mation 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
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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, fol-
lowed 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 imple-
mentation 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 ver-
tices. 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 | feedbackI-
tem
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 | 4dColor-
Texture
2d <- value value
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GLFEEDBACKBUFFER(3G)UNIX Programmer's Manual GLFEEDBACKBUFFER(3G)
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 coordi-
nates 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 com-
piled into the display list but is executed immediately.
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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)
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