Index for Section 3G

Alphabetical listing for G

Bottom of page

glFeedbackBuffer(3G)
NAME
  glFeedbackBuffer - controls feedback mode

SYNOPSIS
  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. 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 option 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 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_TOKENvertex 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							   2

			x, y

  GL_3D							   3

			x, y, z

  GL_3D_COLOR			       k		   3 + k

			x, y, z

  GL_3D_COLOR_TEXTURE		       k	 4	   7 + k

			x, y, z,
  GL_4D_COLOR_TEXTURE		       k	 4	   8 + k

			x, y, z, w

  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 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 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().

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

Index for Section 3G

Alphabetical listing for G

Top of page