State changes for post-draw GL callbacks

Kent Watsen (watsen++at++netcom.com)
Sat, 31 Dec 1994 08:00:46 -0800 (PST)

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My applications often require 2-D symbology to be overlayed
on top of a simulation channel. The 2-D symbology could be
a VAPS or a DWB object, but when its trivial in nature I
like to use home-brewed GL code. A typical GL overlay call
would assume an orthogonal view in a unit channel between
-1 and 1. Since Performer is a 3-D application, I must
performer the transformations myself. An example of this
follows: (please excude psuedocode as I have neither manual
not code with me :)

postdraw ()
{
pfPushState();
pfBasicState();
something to overwrite pizels in z-buffer
ortho2(-1.0, 1.0, -1.0, 1.0);

pushmatrix();
create a 4x4 identity matrix
loadmatrix();

update_overlay ( overlay_data );

popmatrix();
reset z-buffer
pfPopState();
}

This code works just fine, but I wonder if the state and
matrix changes are seriously affecting frame-rate in the
most time-critical section of the rendering loop. It
should be noted that this procedure if sometimes repeated
(up to four times) for each channel, with up to six
channels, 20 times a second.

Kent Watsen
DCS Corporation
Simulation Lab
703.683.8430 x369

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