GLFEEDBACKBUFFER(3G)													      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  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 tex-
       ture 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.

       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)

															      GLFEEDBACKBUFFER(3G)