Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!cmcl2!rna!dan
From: dan@rna.UUCP (Dan Ts'o)
Newsgroups: comp.graphics
Subject: Re: DBW_render for SUN 3 ?????
Summary: modified for 8-bit pixels
Message-ID: <631@rna.UUCP>
Date: 8 Jun 89 05:32:00 GMT
References: <93@unmvax.unm.edu> <62500002@uicbert.eecs.uic.edu> <9029@xanth.cs.odu.edu>
Organization: Rockefeller University - Neurobiology
Lines: 63

In article <9029@xanth.cs.odu.edu>,  writes:
> daniel@unmvax.unm.edu writes:
> >Has anyone converted the public domain ray trace program called DBW_render
> >to run on a SUN workstation?
> 
> Ofer Licht <ofer@gandalf.berkeley.edu> has done just that.  His
> modified DBW_Render is available via anonymous ftp from
> xanth.cs.odu.edu as /amiga/dbw.zoo.

	I picked this program up and started playing with it. I don't claim
to know much about it. However, it seemed designed for the Amiga (obviously)
and only wanted to output 4bits per color. I decided I wanted a full 8-bits
and started to probe into it. I don't know if the calculations are performed
to at least 8 bit accuracy, although I'd be surprised if they weren't, so
I just arranged to dump out more bits per color per pixel.

In ray.h:

#define BPP         4                   /* Bits per pixel */

should be changed to

#define BPP         8                   /* Bits per pixel */

Unfortunately, this was not enough. It seems that some code in main() also
assumes 4bits/color:

In ray.c, line 458:

                modulo = (((modulo >> 2) + 1) % PPW) << 2;

should be changed to something like

                i = 8 * sizeof (short);
                modulo = (modulo + (i/PPW))%i;

Unfortunately, the code also seems to use 0xFF (decimal 255) as a marker for
unknown pixels. But MAXGRAY-1 at 8bits is now also 255. The proper change
would be to choose another marker such as 32768 (or eliminate markers).
I punted and set MAXGRAY-1 to be 254:

In ray.c, line 294:

        d_maxgray = (float) MAXGRAY;

was changed to

    if (MAXGRAY == (0xFF + 1))
        d_maxgray = (float) (MAXGRAY - 1);
    else
        d_maxgray = (float) MAXGRAY;

Now it seems to produce 8-bit/color. Of course the format of the output
file *.tmp is now different, having twice as many bytes. The difference in
image quality is apparent, even on 8-bit FB's (not 24bit FB's) with dithering
such that each color only gets 6 values. Must be the extra info to dither with.

				Cheers,
				Dan Ts'o		212-570-7671
				Dept. Neurobiology	dan@rna.rockefeller.edu
				Rockefeller Univ.	...cmcl2!rna!dan
				1230 York Ave.		rna!dan@nyu.edu
				NY, NY 10021		tso@rockefeller.arpa
							tso@rockvax.bitnet