glFeedbackBuffer(3G)	       OpenGL Reference		  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.	The GL is taken	out of
     feedback mode by calling 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 get 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



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glFeedbackBuffer(3G)	       OpenGL Reference		  glFeedbackBuffer(3G)



     written for bitmaps and pixel rectangles.	Feedback occurs	after polygon
     culling and glPolyMode 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



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glFeedbackBuffer(3G)	       OpenGL Reference		  glFeedbackBuffer(3G)



	  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.

  ______________________________________________________________________________
  |____________________|_____________|_______|_________|________________________|
  |	  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 rather 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.







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glFeedbackBuffer(3G)	       OpenGL Reference		  glFeedbackBuffer(3G)



ASSOCIATED GETS
     glGet with	argument GL_RENDER_MODE


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
















































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