[Coco] Higher graphics mode (256 color mode?)

[Arthur Flexser]

Do these sorts of weird undocumented graphics effects tend to work the same
with an emulator?  Just curious.

Art

On Wed, Jul 15, 2015 at 5:43 PM, William Astle <lost at l-w.ca> wrote:

> I wonder if this isn't something similar to the "64 column VDG" screen I
> detailed some time back, which works by setting the coco2 text screen
> and fiddling with the GIME registers.
>
> If my findings from then hold for the "VDG" graphics modes, then setting
> a particular bit in one of the GIME registers (I can't remember which
> one off the top of my head) should double the horizontal resolution.
> That would give a 512 pixel wide "pmode 4" screen. It could also involve
> a mismatch between the "VDG" graphics mode and the "SAM" graphics mode
> settings which only makes it look like 4 areas by virtue of running the
> data fetch counter at the wrong rate so it fetches the same data
> multiple times to make up a full pixel row.
>
> The only thing I doubt is that it is actually 1024 pixels across, based
> simply on the characteristics of the NTSC signal. If someone could
> demonstrate the mode, it would be interesting. I'd try some experiments
> but my coco3 is in a storage locker on the other side of the city and I
> have nowhere to set it up.
>
> On 2015-07-15 15:25, Arthur Flexser wrote:
> > P.S  I didn't mean for my last message to seem negative.  This could be
> an
> > interesting contribution, if in fact the mode is truly 1024 pixels wide,
> > even if there is no way to uncouple the 4 areas.  Maybe could be used for
> > special effects in a game in situations where it doesn't matter if each
> > quarter of the screen is identical to the others, and assuming that the
> > 1024 resolution is appreciably sharper than 512 pixels on any type of
> > monitor usable with a CoCo.
> >
> > Art
> >
> > On Wed, Jul 15, 2015 at 5:08 PM, Arthur Flexser <flexser at fiu.edu> wrote:
> >
> >> A couple of questions occur to me:
> >>
> >> 1.  You say the graphics are repeated 4 times across the screen.  Could
> >> there be any way in this mode to change one of the 4 areas without
> >> similarly affecting the other three?  (I tend to doubt it.)
> >>
> >> 2.  Are we sure that this is really 1024 pixel resolution?  It could be,
> >> for example, that the 4 areas are in fact only reproducing every 2nd or
> >> every 4th horizontal pixel of the original screen.  Indeed, maybe the 4
> >> areas are not entirely identical, but 1/4 resolution and each offset by
> 1
> >> pixel horizontally from the adjacent area to yield the same number of
> >> pixels as were there in the original screen.
> >>
> >> 3.  Why exactly might this have anything to do with a 256-color mode?
> >>
> >> Art
> >>
> >> On Wed, Jul 15, 2015 at 4:49 PM, Paul Wittkoski via Coco <
> >> coco at maltedmedia.com> wrote:
> >>
> >>> Hi everyone,
> >>>
> >>> At one time, I found a higher graphics mode that is not documented with
> >>> the
> >>> CoCo 3. It was 1024 pixels across and the pictures in pmode graphics
> could
> >>> be seen 4 times across the screen. I am unable to pull the exact method
> >>> from
> >>> my computer since my 5.25" floppy drive blew a chip some time back.
> But,
> >>> someone may be able to find it again from what I remember what I did.
> It
> >>> may
> >>> have been odd thinking at the time but I wondered if pmode and hscreen
> >>> could
> >>> be used at the same time. Following the machine code for pmode and
> hscreen
> >>> in the unravelled series I found a specific poke to turn on pmode and
> >>> another one for hscreen. When I used them together on a cm-8 monitor it
> >>> produce a screen with 1024 pixel resolution across. I am not sure if
> this
> >>> would also lead to the 256 color mode if someone messed around with it.
> >>> Interesting enough, I have not ever heard anyone else describe what I
> have
> >>> done. Maybe that's how Tandy hid the 256 color mode so well; right
> under
> >>> our
> >>> noses with pmode. I hope this information is useful to someone in some
> way
> >>> after all of these years.
> >>>
> >>>
> >
>
>
> --
> Coco mailing list
> Coco at maltedmedia.com
> https://pairlist5.pair.net/mailman/listinfo/coco
>