overla(3G)

NAME
     overla - allocates bitplanes for display of overlay colors

FORTRAN 77 SPECIFICATION
     subroutine overla(planes)
     integer*4 planes

PARAMETERS
     planes   expects the number of bitplanes to be allocated for overlay
              colors.  Acceptable values are 0, 2, 4, and 8.

DESCRIPTION
     The IRIS physical framebuffer is divided into four separate GL
     framebuffers:  normal, popup, overlay, underlay.  Because a single
     physical framebuffer is used to implement the four GL framebuffers,
     bitplanes must be allocated among the GL framebuffers.  overla specifies
     the number of bitplanes to be allocated to the overlay framebuffer.
     overla does not take effect immediately.  Rather, it is considered only
     when gconfi is called, at which time all requests for bitplane resources
     are resolved.

     While only one of the four GL framebuffers can be drawn to at a time (see
     drawmo), all four are displayed simultaneously.  The decision of which
     bitplane to display at each pixel is made based on the contents of the
     four framebuffers at that pixel location, using the following
     hierarchical rule:

     if           the popup pixel contents are non-zero

     then         display the popup bitplanes

     else if      overlay bitplanes are allocated AND
                  the overlay pixel contents are non-zero

     then         display the overlay bitplanes

     else if      the normal pixel contents are non-zero OR
                  no underlay bitplanes are allocated

     then         display the normal bitplanes

     else         display the underlay bitplanes

     Thus images drawn into the overlay framebuffer appear over images in the
     normal framebuffer, and images drawn into the underlay framebuffer appear
     under images in the normal framebuffer.  Popup images appear over
     everything else.

     The default configuration of the overlay framebuffer is: single buffer,
     color map mode.  To make a change to this configuration other than to
     change the bitplane size, the drawing mode must be OVERDR.  For example,
     the overlay framebuffer can be configured to be double buffered by
     calling double while draw mode is OVERDR.

     On models that cannot support overlay and underlay bitplanes
     simultaneously, calling overla with a non-zero argument forces underl to
     zero.  When simultaneous overlay and underlay operation is supported,
     calling overla may have no effect on the number of underlay bitplanes.

SEE ALSO
     double, drawmo, gconfi, getgde, single, underl

NOTES
     This routine is available only in immediate mode.

     IRIS-4D G, GT, and GTX models, Personal Iris, Indy, XL, XS, XS24, XZ,
     Elan and Extreme systems support only single buffered, color map mode
     overlay bitplanes.

     The Personal Iris supports 0 or 2 overlay bitplanes.  There are no
     overlay or underlay bitplanes in the minimum configuration of the
     Personal Iris.

     IRIS-4D60G and 4D70G support 2 overlay bitplanes and 2 pop-up bitplanes.

     The Indy and XL supports 0 or 8 overlay and 2 pop-up bitplanes.

     IRIS-4DG, GT, and GTX models, XS, XS24, and Elan support 0, 2, or 4
     overlay bitplanes.  Because 4-bitplane allocation reduces the popup
     framebuffer to zero bitplanes, however, its use is strongly discouraged.
     The window manager cannot operate properly when no popup bitplanes are
     available.

     IRIS-4D VGX models support 0, 2, 4, or 8 overlay bitplanes, either single
     or double buffered, in color map mode only.  The 4 and 8 bitplane
     allocations utilize the alpha bitplanes, which must be present, and which
     therefore are unavailable in draw mode NORMAL.

     IRIS Entry does not support overlay bitplanes.

     Use getgde to determine the maximum number of bitplanes supported for the
     overlay framebuffer.

     Iris Entry does not support overla so getgde returns zero for the number
     of overlay bitplanes on Iris Entry systems.


BUGS
     The Personal Iris does not support shade model GOURAU in the overlay
     framebuffer.