[Coco] More colors on CoCo 1?

[John Kowalski]

At 11:27 AM 24/01/2007 -0600, CoCoList for Color Computer Enthusiasts wrote:
   
>>> Some amount of flicker is a given with this kind of thing.  The point is
>>> just to see if it could be done.  If you would superimpose a G6R with a
>>> dithered high-res bitmap over the color screen you could break up the
>>> 4-pixel long blocks.  33%-50% of the time (depending on # of screens)
>>> many of the big blocks of color would be broken up into high-res
>>> pixels.  I don't know whether that would have any effect on perceived
>>> flicker, but it would add detail.

It's a tricky thing to judge without trying it, but I'd guess it would add
some detail but the flicker would be significant if it's going to flip
between two entirely different looking screens.

The perceived flicker could be significantly reduced if the two screens were
similar looking.  One way to pull that off could be to switch between
graphics modes each scan line and alternate pages & even/odd switching
between video frames.


I sort of missed some of this thread, but, is what we're aiming for simply
some way to coax more colors out of the CoCo 1/2 using something similar to
Hicolor on CoCo 3?

If so, there's a relatively easy way to get 10 "solid" colors out of PMODE3
(and probably PMODE4) simply by switching between two screens that contain
the same image, but with the dither offset between the two.

Quickie BASIC demonstration:

10 PCLEAR8:FORS=1TO5STEP4:PMODE3,S:SCREEN1,0:PCLS
20 FORG=0TO3:COLORG:LINE(G*64,0)-(G*64+63,191),PSET,BF:NEXT:NEXT
30 FORG=0TO3:FORY=G*48 TOG*48+47STEP2:FORX=0TO255STEP4
40 PMODE3,1:PSET(X,Y,G):PSET(X+2,Y+1,G)
50 PMODE3,5:PSET(X+2,Y,G):PSET(X,Y+1,G):NEXT:NEXT:NEXT
60 PMODE3,1:SCREEN1,0:A=TIMER::::::::PMODE3,5:SCREEN1,0
70 IFA=TIMER THEN70 ELSE60

Lines 10 to 50 are for drawing a simple color chart into both PMODE3 screens.

The "::::::::" in line 60 is for timing purposes to help switching between
PMODE3 screens on alternating video frames.  The CoCo is expected to be in
slow (.89Mhz) mode.
I only tried this on my CoCo3.  There may be timing differences on the CoCo2
that muck up the screen switch timing.  :(  If so, try to adjust it so that
it cleanly switches between both PMODE3 screens each alternating video
field.  PAL systems would probably need a number of extra :::s added.

In this case, the usual Red,Green,Yellow and Blue of PMODE3 become:
Red,Brown,Orange,Yellow,yellow-green,Green,Cyan,Blue,Purple and Grey.

                                         John Kowalski (Sock Master)
                                         http://www.axess.com/twilight/sock/