[Coco] hardware scrolling

[Torsten Dittel]

Simon,

this looks really impressive. And I thought I had squeezed anything out of 
the CoCo2 way back then... ;-)

Regarding the "yawn, had all allready been there..." which came up here 
recently:

I claim I have seen a lot of VDG mode switching tricks on the CoCo and 
tried out a lot myself (e.g. my Dragonfire "intro" with mixed modes, smooth 
hi-res scrolling both horizontally and vertically and on-border segment 
coloring and animation, http://www.6809.org.uk/tmp/da/pal_coco_probe/pal-
dragonfire-1.png ). Let's assume you're not copying bytes in memory to show 
horizontal scrolling, and do not achieve this by keeping several "shifted" 
images in the memory, and do just "vertical page flipping" like in a flip-
book: I have never seen what you're showing in the last video as an effect 
of SAM/VDG programming.

And what's even more impressive is that you can obviously truely "draw" on 
the right border (not sure how difficult it is for a standard monitor to 
catch such a video signal - some PAL color TV's did neither like the 
Dragonfire intro thing because of a weird toggling of the color burst 
signal).

If you ever manage to show a stable picture (without garbage), which is 
wider than 256 pixel, I'm the first one to applaud.

I think the concerns which came up are related to the amount of CPU load it 
takes to bring the SAM/VDG to what it's showing. If you have to write 
several bytes per scanline to the involved registers to get a stable 
picture (e.g. triggered by a Fast Sync interrupt), the CPU can hardly do 
anthing else but showing that mode. This means you could not use it for an 
action type game, however, there are programs where it could be useful, and 
if it is "just" a demo / intro / slideshow etc., it would be still a great 
achievement, assuming you could get rid of the garbage (e.g. fill the 
involved memory areas with with monocolored pixels).

Currently, it looks like all you're showing is in "PMODE4,1:SCREEN1,0" 
(light green on dark green). I assume it would be even more fun to extend 
this to other modes including color (or to the SCREEN1,1 variant).

Please don't stop having fun figuring this out - might be boring for some 
of the guys who started with a CoCo3, but I'm a heavily interested 
follower...

Best regards,
Torsten