MirBSD manpage: glFeedbackBuffer(3)



     glFeedbackBuffer - controls feedback mode


     void glFeedbackBuffer( GLsizei size,
                            GLenum type,
                            GLfloat *buffer )


     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.


     The glFeedbackBuffer function controls feedback. Feedback,
     like selection, is a GL mode. The mode is selected by cal-
     ling glRenderMode with GL_FEEDBACK. When the GL is in feed-
     back 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 infor-
     mation 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

     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

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     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, fol-
     lowed 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 imple-
     mentation 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 ver-
     tices. 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 | feedbackI-

          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 | 4dColor-

          2d <- value value

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          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.
     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 coordi-
     nates are generated, if texture coordinate generation is
     enabled. They are always transformed by the texture matrix.


     glFeedbackBuffer, when used in a display list, is not com-
     piled into the display list but is executed immediately.

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     When the GL_ARB_multitexture extension is supported,
     glFeedbackBuffer returns only the texture coordinates of
     texture unit GL_TEXTURE0_ARB.


     GL_INVALID_ENUM is generated if type is not an accepted

     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.


     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


     glBegin(3G), glLineStipple(3G), glPassThrough(3G),
     glPolygonMode(3G), glRenderMode(3G), glSelectBuffer(3G)

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