[Coco] [blog] Stupid VDG Tricks

Brendan Donahe brendan at polylith.com
Mon Jul 30 17:30:42 EDT 2012


I read your blog with interest.  Steve Spiller and I have a piggy-back
modification for the 6847 working fairly well now (with 2 caveats that need
some quality time with the logic-analyzer) with an FPGA sending B&W output
to a VGA monitor.

I have a silly question - what is the reason you chose a semigraphics mode
for one of the 2 fields vs. a 2-color (which is 4-color with artifacting -
black, white, red, blue, for example) hi-res graphics mode (like PMODE 4)?
I wouldn't think it would be necessary to align pixels between the 2 fields
if you're dithering anyway...?

The method of switching CSS on a row-by-row basis is honestly not one I had
considered for what I've been doing, although I know of other systems
(Atari 2600, Atari 8-bit, etc.) where programmers employ similar methods.
We have to use a ping-pong buffer to capture video to handle the dissimilar
VGA and CoCo clock rates without tearing (it's not quite 2:1 for the
640x480 we chose), but should be capturing CSS with each pixel so in theory
it should not be affected.  However, I'm not capturing A/S, A/G, and GM to
keep with each field of data and need to fix this.

More to the point, however, is that I'd be interested in running your code
as an test case.  I look forward to your follow-up post.


On Sun, Jul 29, 2012 at 11:00 PM, David Roper <ebonhand09 at gmail.com> wrote:

> Very interesting read - thanks, John!
> - Dave
> On 30 July 2012 11:27, John W. Linville <linville at tuxdriver.com> wrote:
> > For those who aren't aware of what I've been doing lately with the
> > original CoCo (i.e. not a CoCo3), you might be interested in taking
> > a look at a blog I've been keeping:
> >
> >         http://vdgtricks.blogspot.com/
> >
> > I've been getting what I think are quite impressive results with
> > converting still images (e.g. digital photos) for display on the CoCo.
> > Again, this is on the original 6847-equipped CoCo platform [*],
> > not a CoCo3.
> >
> > John
> >
> > * The images in the photos were displayed on a TDP-100 hooked to a 27"
> > Sony CRT television.
> > --
> > John W. Linville                Someday the world will need a hero, and
> you
> > linville at tuxdriver.com                  might be all we have.  Be ready.
> >
> > --
> > Coco mailing list
> > Coco at maltedmedia.com
> > http://five.pairlist.net/mailman/listinfo/coco
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