fglFeedbackBuffer(3G)

NAME
     fglFeedbackBuffer - controls feedback mode


FORTRAN SPECIFICATION
     SUBROUTINE fglFeedbackBuffer( INTEGER*4 size,
                                   INTEGER*4 type,
                                   CHARACTER*8 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 fglFeedbackBuffer function controls feedback.  Feedback, like
     selection, is a GL mode.  The mode is selected by calling fglRenderMode
     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.

     fglFeedbackBuffer 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.  fglFeedbackBuffer must
     be issued before feedback mode is enabled (by calling fglRenderMode with
     argument GL_FEEDBACK).  Setting GL_FEEDBACK without establishing the
     feedback buffer, or calling fglFeedbackBuffer while the GL is in feedback
     mode, is an error.

     When fglRenderMode 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, fglRenderMode returns a negative value.  To take the
     GL out of feedback mode, call fglRenderMode 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 fglPolygonMode 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 fglPassThrough command can be used to insert a marker into the
     feedback buffer.  See fglPassThrough.

     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
     texture matrix.

NOTES
     fglFeedbackBuffer, 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, fglFeedbackBuffer
     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 fglFeedbackBuffer is called while
     the render mode is GL_FEEDBACK, or if fglRenderMode is called with
     argument GL_FEEDBACK before fglFeedbackBuffer is called at least once.

     GL_INVALID_OPERATION is generated if fglFeedbackBuffer is executed
     between the execution of fglBegin and the corresponding execution of
     fglEnd.

ASSOCIATED GETS
     fglGet with argument GL_RENDER_MODE
     fglGet with argument GL_FEEDBACK_BUFFER_POINTER
     fglGet with argument GL_FEEDBACK_BUFFER_SIZE
     fglGet with argument GL_FEEDBACK_BUFFER_TYPE


SEE ALSO
     fglBegin, fglLineStipple, fglPassThrough, fglPolygonMode, fglRenderMode,
     fglSelectBuffer