Re: Triangles vs. Trapezoids

On Apr 29, Keith Packard wrote:
 > You're talking about doubling (probably) the amount of code, and also 
 > requiring a lot of additional testing to make sure it works right in every 
 > case.  I'm not willing to place that burden on every implmentation, 
 > especially for a single pixel.  I am willing to require that apps 
 > tesselate carefully to get the right answer; they'll probably do that in 
 > any case.

Uncle!

The code is bad enough without solving for the intersection. I won't
ask for that again.

I've made some progress. Here's some stream of consciousness:

I suppose that since we're talking about precise trapezoids here, I
must maintain precision enough to compute pixel coverage within 1 part
in 2**16, (assuming maximum of 16 bits of alpha)?

I've got the code figured out for line walking I think. Bresenham
never really applies since in one dimension I walk a full pixel while
in the other I walk some number of partial pixels, (so a binary
decision variable doesn't help). I think I can preserve full precision
by using a 64-bit fixed-point slope, (or inverse slope to keep it
within [0, 1.0]).

The one case I still need to think about precision is in clipping the
initial left/right lines to the top line before starting the iterative
walking. Maybe I'll just have to use the same iterative walk to extend
from the points I'm given until I hit the top line.

I don't know if that makes any sense at all. Maybe some code to look
at tomorrow.

-Carl

-- 
Carl Worth                                        
USC Information Sciences Institute                 [email protected]
3811 N. Fairfax Dr. #200, Arlington VA 22203		  703-812-3725