glFeedbackBuffer

glFeedbackBuffer
: controls feedback mode.

C Specification
|
Parameters
|
Description
|
Notes
|
Errors
|
Associated Gets
|
See Also

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 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

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.

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

.
http://www.talisman.org/opengl-1.1/Reference/glFeedbackBuffer.html