[Coco] Artifacting (again)

[Robert Gault]

Mark McDougall wrote:
> Hi,
>
> I'm looking for some information on basic (simple, 4-colour) artifacting
> on the Coco 1/2 PMODE 4 screen - NOT interested in the Coco 3 at this
> point.
>
> I've seen the threads in which John has been asking about artifacting,
> but he's after the "extra" colours in particular. I just want to know
> about the back/white/red/blue scheme, and the links tend to "gloss over"
> the information I'm looking for.
>
> Trouble is of course that I don't have a Coco1/2 to try this on either.
>
> Is it simply something like this (starting on even columns) :
>
> 0 0 = black
> 1 1 = white(buff)
> 0 1 = red
> 1 0 = blue
>
> Do both PAL and NTSC produce artifacts? And are there any differences
> between PAL/NTSC machines? Do you get artifacting on the green/black
> PMODE 4 screen (SCREEN 1,0)?
>
> TIA
> Regards,
>

It might help you to know that you do not need to keep in mind which pixels are 
odd or even. Basic will do that for you with a little trick.

Normally you select your PMODE, turn on video with SCREEN, and then draw your 
pictures. However, there is nothing to prevent you from inserting extra PMODE 
commands.
Let's say you want to use artifacts. Just select a normal 4 color PMODE, draw in 
the colors you want, and then select PMODE4 before using the SCREEN command. You 
will automatically have on the screen either pairs of pixels, odd pixels, or 
even pixels and thus get the four colors black, white, red, and blue.

Given that the Coco 1/2 have a 50/50 chance of booting with red=odd pixels, you 
can also add automatic selection. Just use algebraic color selection instead of 
fixed color selection with numbers.
Draw a colored block in either color2 or color3 of the PMODE3 set. Ask the user 
to hit one of two keys depending on the color seen, and make C2 and C3 equal to 
the appropriate values 2 or 3 depending on results. Then every COLOR, DRAW, 
LINE, or PAINT command should use C2 and C3 instead of the numbers 2 and 3.

Here is a very brief example of controlled artifact drawing.

10 PMODE3:PCLS0
11 C2=3:C3=2:REM These values would be set with a test each time the program is run.
20 COLOR C2,0
30 LINE(0,0)-(255,191),PSET
40 COLOR C3,0
50 LINE(255,0)-(0,191),PSET
60 PMODE4,1:SCREEN1,1
70 GOTO70