NAME

glFeedbackBuffer - controls feedback mode

C SPECIFICATION

void ggllFFeeeeddbbaacckkBBuuffffeerr( GLsizei _s_i_z_e,
                       GLenum _t_y_p_e,
                       GLfloat _*_b_u_f_f_e_r )

delim $$

PARAMETERS

_s_i_z_e
Specifies the maximum number of values that can be written into _b_u_f_f_e_r.
_t_y_p_e
Specifies a symbolic constant that describes the information that will be returned for each vertex. GGLL__22DD, GGLL__33DD, GGLL__33DD__CCOOLLOORR, GGLL__33DD__CCOOLLOORR__TTEEXXTTUURREE, and GGLL__44DD__CCOOLLOORR__TTEEXXTTUURREE are accepted.
_b_u_f_f_e_r
Returns the feedback data.

DESCRIPTION

The ggllFFeeeeddbbaacckkBBuuffffeerr function controls feedback. Feedback, like selection, is a GL mode. The mode is selected by calling ggllRReennddeerrMMooddee with GGLL__FFEEEEDDBBAACCKK. 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.

ggllFFeeeeddbbaacckkBBuuffffeerr has three arguments: _b_u_f_f_e_r is a pointer to an array of floating-point values into which feedback information is placed. _s_i_z_e indicates the size of the array. _t_y_p_e is a symbolic constant describing the information that is fed back for each vertex. ggllFFeeeeddbbaacckkBBuuffffeerr must be issued before feedback mode is enabled (by calling ggllRReennddeerrMMooddee with argument GGLL__FFEEEEDDBBAACCKK). Setting GGLL__FFEEEEDDBBAACCKK without establishing the feedback buffer, or calling ggllFFeeeeddbbaacckkBBuuffffeerr while the GL is in feedback mode, is an error.

When ggllRReennddeerrMMooddee 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 _s_i_z_e. If the feedback data required more room than was available in _b_u_f_f_e_r, ggllRReennddeerrMMooddee returns a negative value. To take the GL out of feedback mode, call ggllRReennddeerrMMooddee with a parameter value other than GGLL__FFEEEEDDBBAACCKK.

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 ggllPPoollyyggoonnMMooddee 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 ggllPPaassssTThhrroouugghh command can be used to insert a marker into the feedback buffer. See ggllPPaassssTThhrroouugghh.

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 _t_y_p_e. 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 <- GGLL__PPOOIINNTT__TTOOKKEENN vertex

lineSegment <- GGLL__LLIINNEE__TTOOKKEENN vertex vertex | GGLL__LLIINNEE__RREESSEETT__TTOOKKEENN vertex vertex

polygon <- GGLL__PPOOLLYYGGOONN__TTOOKKEENN n polySpec

polySpec <- polySpec vertex | vertex vertex vertex

bitmap <- GGLL__BBIITTMMAAPP__TTOOKKEENN vertex

pixelRectangle <- GGLL__DDRRAAWW__PPIIXXEELL__TTOOKKEENN vertex | GGLL__CCOOPPYY__PPIIXXEELL__TTOOKKEENN vertex

passThru <- GGLL__PPAASSSS__TTHHRROOUUGGHH__TTOOKKEENN 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. GGLL__PPOOIINNTT__TTOOKKEENN, GGLL__LLIINNEE__TTOOKKEENN, GGLL__LLIINNEE__RREESSEETT__TTOOKKEENN, GGLL__PPOOLLYYGGOONN__TTOOKKEENN, GGLL__BBIITTMMAAPP__TTOOKKEENN, GGLL__DDRRAAWW__PPIIXXEELL__TTOOKKEENN, GGLL__CCOOPPYY__PPIIXXEELL__TTOOKKEENN and GGLL__PPAASSSS__TTHHRROOUUGGHH__TTOOKKEENN are symbolic floating-point constants. GGLL__LLIINNEE__RREESSEETT__TTOOKKEENN is returned whenever the line stipple pattern is reset. The data returned as a vertex depends on the feedback _t_y_p_e.

The following table gives the correspondence between _t_y_p_e and the number of values per vertex. _k is 1 in color index mode and 4 in RGBA mode.

center tab(:);
lb lb cb cb cb
l l c c c.
_
Type:Coordinates:Color:Texture:Total Number of Values
_
GGLL__22DD:_x, _y:::2
GGLL__33DD:_x, _y, _z:::3
GGLL__33DD__CCOOLLOORR:_x, _y, _z:$k$::$3 ~+~ k$
GGLL__33DD__CCOOLLOORR__TTEEXXTTUURREE:_x, _y, _z,:$k$:4:$7 ~+~ k$
GGLL__44DD__CCOOLLOORR__TTEEXXTTUURREE:_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

ggllFFeeeeddbbaacckkBBuuffffeerr, when used in a display list, is not compiled into the display list but is executed immediately.

When the GGLL__AARRBB__mmuullttiitteexxttuurree extension is supported, ggllFFeeeeddbbaacckkBBuuffffeerr returns only the texture coordinates of texture unit GGLL__TTEEXXTTUURREE00__AARRBB.

ERRORS

GGLL__IINNVVAALLIIDD__EENNUUMM is generated if _t_y_p_e is not an accepted value.

GGLL__IINNVVAALLIIDD__VVAALLUUEE is generated if _s_i_z_e is negative.

GGLL__IINNVVAALLIIDD__OOPPEERRAATTIIOONN is generated if ggllFFeeeeddbbaacckkBBuuffffeerr is called while the render mode is GGLL__FFEEEEDDBBAACCKK, or if ggllRReennddeerrMMooddee is called with argument GGLL__FFEEEEDDBBAACCKK before ggllFFeeeeddbbaacckkBBuuffffeerr is called at least once.

GGLL__IINNVVAALLIIDD__OOPPEERRAATTIIOONN is generated if ggllFFeeeeddbbaacckkBBuuffffeerr is executed between the execution of ggllBBeeggiinn and the corresponding execution of ggllEEnndd.

ASSOCIATED GETS

ggllGGeett with argument GGLL__RREENNDDEERR__MMOODDEE
ggllGGeett with argument GGLL__FFEEEEDDBBAACCKK__BBUUFFFFEERR__PPOOIINNTTEERR
ggllGGeett with argument GGLL__FFEEEEDDBBAACCKK__BBUUFFFFEERR__SSIIZZEE
ggllGGeett with argument GGLL__FFEEEEDDBBAACCKK__BBUUFFFFEERR__TTYYPPEE

SEE ALSO

ggllBBeeggiinn((33GG)), ggllLLiinneeSSttiippppllee((33GG)), ggllPPaassssTThhrroouugghh((33GG)), ggllPPoollyyggoonnMMooddee((33GG)), ggllRReennddeerrMMooddee((33GG)), ggllSSeelleeccttBBuuffffeerr((33GG))