From: ovek AT arcticnet DOT no (Ove Kaaven)
Newsgroups: comp.os.msdos.djgpp
Subject: Allegro perspective-correct texture mapping
Date: Mon, 03 Mar 1997 12:41:00 GMT
Organization: Vplan Programvare AS
Lines: 40
Message-ID: <5fg3s0$hob$1@troll.powertech.no>
NNTP-Posting-Host: alwayscold.darkness.arcticnet.no
To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp

Okay, for some reason, my workspace recently got invaded by a friend,
who seemed to be interested in making games. So it was time to put my
copy of the excellent game library Allegro to work. After having spent
a couple of days trying it out by making a shoot-the-target program
(which he says made a "hit" on the school lab he installed it on), we
moved on to Allegro 2.2's 3D features.
After 2-3 days of work, I wrote the necessary code for arbitrary
texture mapping with view volume clipping etc. However, there is a
couple of issues I was wondering about.

1. Using POLYTYPE_PTEX, polygon3d_f bombs with SIGFPE when looking at
my (vertical) wall at extremely small angles (wall and camera ray
almost parallel). Here's a traceback, just in case.

Floating Point exception at eip=00018166
eax=0a090209 ebx=0134302b ecx=00000001 edx=01970922 esi=0009e004
edi=000c2784
ebp=00083cd0 esp=00083c88 program=C:\DJGPP\SNIPER\SNIPER.EXE
[some irrelevant selectors]

Call frame traceback EIPs:
  0x00018166   __poly_scanline_ptex+514
  0x000066ab   _triangle+3135

I realize that this is probably a floating-point overflow, but is
there a good way around it?

2. The perspective-correct mapping is also somewhat slower than what's
appropriate for games like Doom or Duke3D. By looking at the Allegro
sources, it seems to be calculating scanlines the 'traditional' way.
Are there plans to provide optimized routines for horizontal and
vertical surfaces, or even better, the method described in zed3d
(letting the logical scanlines you map onto be slanted themselves) to
have a very fast perspective mapper?

3. What should I give Shawn for his excellent library?

4. Where should I post this to keep it from being off-topic?