questions about drawing polygons

[Thant Tessman]

In article <9753 at watcgl.waterloo.edu>, jdchrist at watcgl.waterloo.edu (Dan Christensen) writes:
> I have a couple of questions regarding the drawing of filled polygons
> on an Iris 4D/120GTX.
> 
> 1) Using the fastest settings of the lighting model (single light source
>    at infinity, viewer at infinity), how many shaded polygons can be
>    drawn per second?  (I know that this is a difficult question, but a
>    rough figure would be useful).
> 

Check the sales literature.  SGI quotes lit polygons.  (I'm no good with 
numbers.)  Realize that the number doesn't include clearing the screen.  Also,
since on the GTX the graphics are CPU limited, indexing into data will be 
slower than running a pointer through the data.  Indexing requires a multiply
and an add.  Incrementing a pointer is just an add.  Also, using the z-buffer
for polygons under a certain size (50x50 pixels?) doesn't cost you anything.


> 2) I am trying to draw a large mesh of shaded triangles.  The GT
>    Library has routines for drawing triangular meshes.  These
>    routines look like they would speed up drawing significantly because
>    only one third of the lighting calculations need to be done and one
              ^^^^^ 
One half.  In a single strip, each point is sent down once instead of twice
(not including the end points).  In a connected row of strips each point is 
sent down twice instead of four times.

>    third of the data needs to be put into the pipeline, compared to
>    drawing each triangle separately.  The problem I found with these
>    routines is that you have no control over the order in which the
>    vertices are drawn so that backface removal becomes useless.
>    Turning off backface removal results in an overall slowing in the
>    drawing.  Is there any way to specify the order in which the
>    vertices are drawn?  It would be a shame if there isn't because
>    these routines look like they could speed things up greatly.  Is
>    there a way to do the same thing manually?  Could the program draw
>    each vertex and save the colours calculated by the hardware and then
>    draw the polygons with the lighting model turned off, using the
>    precalculated colours?
> 
> Thanks for any help.
> 
> ----
> Dan Christensen, Computer Graphics Lab,	         jdchrist at watcgl.uwaterloo.ca
> University of Waterloo, Waterloo, Ont.	         jdchrist at watcgl.waterloo.edu

Triangle-meshes can be used with backface removal.  The 'front' of the tmesh 
is determined by the first triangle and preserved for the rest of the mesh,
even if swaptmesh is used.

The 'swaptmesh' command is a way of manipulating which edge of the previous 
triangle you want to connect the new triangle to.

There is one bug that shows up when backfaced tmeshes get clipped.  Sometimes
the individual clipped triangles will randomly disappear or appear, but this 
is being fixed.

thant at sgi.com "there are 336 dimples on the standard golf ball"