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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 selec‐
       tion, 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 num‐
       ber  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	 poly‐
       gons  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 ren‐
       ders polygons by performing this decomposition.

       The glPassThrough command can be used to insert a marker into the feed‐
       back 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 | pixel‐
	      Rectangle | 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 num‐
       ber 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 coor‐
       dinate 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 argu‐
       ment 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)
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