[Coco] The myth of the Coco3 256 color mode :)

[Robert Gault]

There have been several recent threads about generating multi-color graphics 
screens on the Coco. One was for the Coco1 or 2, semigraphics was mentioned, and 
also the 256 color mode on the Coco3 prototype board. Also there is the flicker 
procedure where several different graphics screens are rapidly switched to yield 
a blended image.

While all the above are of interest, there is one Coco3 mode which provides 
close to 256 colors at one time which may soon be lost to us forever. It works 
on composite monitors only (which are a dying breed) and is not supported by any 
emulator. The mode was discussed in a thread on coco3.com and that site has been 
problematic of late and is frequently off the air.
http://www.coco3.com/community/2009/02/256-color-mode-composite-mode-artifacting/comment-page-1/#comments
There are 7 pages of comments.

I don't want to see this mode lost in time and mythology as happened with the 
supposed mode on the prototype board. :) So here is a short demo of the 
technique for a Coco3 and CMP monitor.
10 HSCREEN4:PALETTE0,0:PALETTE1,16:PALETTE2,32:PALETTE3,48
20 M=&H60000:POKE&HFFD9,0
30 FOR Y=0TO3
40 FOR X=315 TO 331 STEP 4
50 FOR C=1TO3
60 HCOLORC,0:HLINE(X+20*(C-1)-Y,71+10*Y)-(X+20*(C-1)-Y,80+10*Y),PSET
70 NEXTC,X,Y
80 GOTO80

By rights, the above should give just vertical gray lines on a CMP monitor. 
Instead, you see four rows of three shaded colors, blue, purple, red-orange, and 
green. It is possible to create a pattern that appears to yield 256 colors, 
although depending on your CMP monitor some will seem duplicates.

I've written a Basic-ML pair of programs that will load a .bmp image that 
displays perhaps 256 colors without any flickers associated with screen swaps. 
The following restrictions apply.

* This works only on a TV/composite monitor. It will not work with an emulator 
or RGB monitor.
* BMP images are limited to 256 colors. The width
* must be 256 pixels or less.
* With a 128K Coco3, the total image must be 65K bytes or less. With a
* 512K Coco3, the total must be 458K bytes or less.
* The screens used are 640x225x4.

You need to use a PC graphics program to convert an image to the above 
conditions and place the image on a .dsk or real Coco disk. The program will 
work nicely with Drivewire.
I can send a program disk on request but this list does not like "attachments".

All of the above resulted from a discussion started by Potatohead (who are you?) 
and a Coco program called GrafExpress.

Robert