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 ar‐
       ray 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 buf‐
       fer.  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.

              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 giv‐
       ing 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 ver‐
       tex 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
     ─────────────────────────────────────────────────────────────────────────────

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 be‐
       tween 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, glLineStipple, glPassThrough, glPolygonMode, glRenderMode,
       glSelectBuffer

                                                           GLFEEDBACKBUFFER(3G)