GLFEEDBACKBUFFER() MachTen Programmer’s Manual GLFEEDBACKBUFFER()

NAME
glFeedbackBuffer - controls feedback mode

C SPECIFICATION
void glFeedbackBuffer( GLsizei size,
GLenum type,
GLfloat *buffer )

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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. Feed-
back, 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 infor-
mation 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 glRen-
derMode 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 maxi-
mum, 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 prim-
itive’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 indi-
cating 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 | feed-
backItem

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 poly-
gon. 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.

center box tab(:) ; c | ci | ci | ci | ci c | c | c | c |
c . 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 genera-
tion is enabled. They are always transformed by the tex-
ture matrix.

NOTES
glFeedbackBuffer, when used in a display list, is not com-
piled 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 glRen-
derMode is called with argument GL_FEEDBACK before glFeed-
backBuffer is called at least once.

GL_INVALID_OPERATION is generated if glFeedbackBuffer is
executed between the execution of glBegin and the corre-
sponding execution of glEnd.

ASSOCIATED GETS
glGet with argument GL_RENDER_MODE

SEE ALSO
glBegin, glLineStipple, glPassThrough, glPolygonMode,
glRenderMode, glSelectBuffer

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