backbu(3G)

NAME
     backbu, frontb - enable and disable drawing to the back or front buffer

FORTRAN 77 SPECIFICATION
     subroutine backbu(b)
     logical b

     subroutine frontb(b)
     logical b

PARAMETERS
     b    is either .TRUE. or .FALSE..

           .TRUE. enables updating in the back/front bitplane buffer.

           .FALSE. disables updating in the back/front bitplane buffer.

DESCRIPTION
     The GL supports four separate framebuffers:  pop-up, overlay, underlay,
     and normal.  Three of these framebuffers, overlay, underlay, and normal,
     can be configured in double buffer mode.  When so configured, a
     framebuffer includes two color bitplane buffers:  one visible bitplane
     buffer, called the front buffer, and one non-visible bitplane buffer,
     called the back buffer.  The commands swapbu and mswapb interchange the
     front and back buffer assignments.

     By default, when a framebuffer is configured in double buffer mode,
     drawing is enabled in the back buffer, and disabled in the front buffer.
     frontb and backbu enable and disable drawing into the front and back
     buffers, allowing the default to be overriden.  It is acceptable to
     enable neither front nor back, either front or back, or both front and
     back simultaneously.  Note, for example, that z-buffer drawing continues
     to update the z-buffer with depth values when neither the front buffer
     nor the back buffer is enabled for drawing.

     frontb and backbu state is maintained separately for each of the overlay,
     underlay, and normal framebuffers.  Calls to these routines affect the
     framebuffer that is currently active, based on the current drawmode.

     backbu is ignored when the currently active framebuffer is in single
     buffer mode.  frontb is also ignored when the currently active
     framebuffer is in single buffer mode, unless zdraw is enabled for that
     framebuffer (see zdraw).

     After each call to gconfi, backbu is enabled and frontb is disabled.

SEE ALSO
     drawmo, double, getbuf, gconfi, single, swapbu, zdraw, rightb, leftbu
NOTE
     IRIS-4D G, GT, and GTX models, the Personal Iris, Indigo Entry, Indy, XL,
     XS, XS24, XZ, Elan and Extreme systems do not support double buffer
     operation in the overlay and underlay framebuffers.  IRIS-4D
     RealityEngine model does not support double buffer operation in the
     underlay framebuffer.

     At most four color buffers, such as front-left, front-right, back-left,
     back-right, and zdraw, can be rendered into simultaneously on the IRIS-4D
     RealityEngine model.