[Coco] 256 color mode

[Joel Ewy]

Allen Huffman wrote:
> Where can I find information on the 256-color composite mode?
>
> 		-- Allen
>
>   

Since coco3.com is hacked right now I'll try to give you a synopsis:  
This is just the regular 640x200x4-color mode where each byte represents 
4 pixels.  Just like in PMODE 4, adjacent pixels of different colors 
produce artifact colors.  But where PMODE 4 has only black and white, 
the 640x200x4 mode has more shades of gray.  So if PMODE 4 artifacting 
can produce black, white, red, and blue, this mode can produce 
intermediate shades.  Or something along those lines.

This technique treats each byte, which would ordinarily be 4 pixels, as 
a single, 8-bit pixel.  So the effective resolution is about 160x200 
with 8 bits of color.  The "pixels" are really patterns of 4 adjacent 
pixels, but taken together they produce an artifact color.  Some have 
questioned whether there are really 256 distinct colors visible, but it 
definitely looks like an 8-bit color display of yore.  It makes me think 
of an old Paradise 8-bit VGA board. 

Of course, since these are artifact colors, the effect only appears on 
an NTSC monitor or TV.  But it is quite good.  The only better displays 
I've seen for reproducing color photos on a CoCo are the flicker modes 
used in Projector 3 and Sockmaster's Hi-Color display.  But the 
advantage of the artifact colors is that there is no flicker, no 
re-stuffing of the palette registers, or anything else -- it's just a 
straight-up CoCo screen.  That means it can easily be used in games or 
paint programs, or animations, where the others can't (or at least never 
have been...)

The real trick is to build a palette that renders a useful distribution 
of artifact colors, and that's what Potatohead, Jason Law, Briza, and 
Robert Gault worked on.  They came up with several workable ones that 
have a good distribution of colors.  In theory, I suppose, you could 
have a number of different palettes weighted toward different color 
distributions and programmatically select the one that most closely 
matches the source image.  The problem is that there doesn't appear to 
be a way to predict what artifact colors a given palette will produce, 
so that part has to be done experimentally.

But one of the palettes the guys found has a good range of colors and 
can fairly convincingly render arbitrary images.  One of them made a 
.PAL file for a shareware Windows paint program whose name escapes me at 
the moment.  I loaded it up in The GIMP and it works just fine there as 
well.  I used that to convert some pictures -- mostly photos I got off 
of flickr -- for the slideshow demo.

The demo is a bootable NitrOS-9 disk image that (IIRC) goes straight 
into the slideshow.  You need a 512K CoCo 3 with an NTSC monitor, and 
there are two disk images:  one version requires a 720K disk drive, and 
the other works with a Drivewire 3 server.  I had meant to make a 
version that fit on multiple 360K disk images, but haven't gotten around 
to it.

I guess I should digitize a run-through of the slideshow and put it on 
youtube for those who don't have the equipment to run the demo...

JCE

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