Z-buffering Problems...

[Glenn Randers-Pehrson, WMB]

 > From: zyda at NPS-CS.ARPA (michael zyda)
 > Subject: Z-buffering Problems...
 > IRIS 4D/70G Z-Buffering Problems...
 > 
 >   Problem: At the intersection of the sea and land you cannot 
 > rely on the Z-buffer to always draw the land over the water 
 > or even to consistently draw it the same.
 >   
 >   This solution is not very acceptable. Does anyone out there
 > have similar experiences?"
 > 

	Yes.  On a 2500T, 3130, and 3030, I attempted to do a
hidden-line wireframe by drawing the filled polygons in a background color
and then drawing unfilled polygons.  It did not work very well.  I
complained to SGI and was told it was due to the resolution of the
geometry engine calculations.  I think the exact quote was that "It
works as designed". [i.e. not necessarily as specified in the manual].

Here's a copy of my e-mail to SGI:

Glenn Randers-Pehrson <glennrp at brl.arpa>
Date:     Thu, 3 Dec 87 12:21:54 EST
From:     Glenn Randers-Pehrson (WMB)  <glennrp at BRL.ARPA>
To:       dave at sgi.com
Subject:  polly()

	Re: HOTLINE log 1824, 3 Dec 87

        program pollytest
c       demonstrates that poly() does not outline polygons properly
c       in conjuntion with polf() in z-buffer mode.  Tests polly()
c       by Glenn Randers-Pehrson, which attempts to do this properly.
        dimension square(3,4)
        integer near,far
        character*1 key
        data near,far/x'c000',x'3fff'/
      call ginit
      call greset
      call cursof
        call setdep(near,far)
        call ortho (0., 1.0, 0.0, 1.0, 1.0,-1.0)
        call zbuffe(.true.)
        call color(0)
        call clear
        do 200 mode=0,1
        call zclear
        kol=1
        do 100 k=1,2
        x=.025*k + .5*mode
        y=.025*k +  .1
        z=k-1.5 + x*.01
        square(1,1)=x
        square(2,1)=y
        square(3,1)=z+.01
        square(1,2)=x+.3
        square(2,2)=y
        square(3,2)=z+.01
        square(1,3)=x+.3
        square(2,3)=y+.3
        square(3,3)=z+.02
        square(1,4)=x
        square(2,4)=y+.3
        square(3,4)=z
        call color(kol)
        call polf(4,square)
        call color(3)
        call cmov2(x,.5)
        if(mode.eq.1)then
          if(k.eq.1)call charst('polly()',7)
          call color(2)
          call polly(4,square)
        else
          if(k.eq.1)call charst('poly()',6)
          call color(2)
          call poly(4,square)
        endif
        kol=4
 100    continue
 200    continue
 300    continue
        read(*,'(a1)')key
        call fhalt(1)
        end
      subroutine polly(n,xyz)
c     FORTRAN version of poly() that uses zbuffer consistently
c     with polf(), for use in making outlined polygons
c      by Glenn Randers-Pehrson <glennrp at brl.arpa>
c         U.S. Army Ballistic Research Laboratory
c         Aberdeen Proving Ground, MD 21005-5066
      dimension xyz(3,n)
      dimension temp(3,2)
      do 10 j=1,3
        temp(j,1)=xyz(j,n)
   10 continue
      do 40 i=1,n
        do 20 j=1,3
          temp(j,2)=xyz(j,i)
   20   continue
        call polf(2,temp)
        do 30 j=1,3
          temp(j,1)=temp(j,2)
   30   continue
   40 continue
      return
      end