[Coco] Re: CoCo to SVGA

[Roger Taylor]

At 06:09 PM 10/30/2004, you wrote:
>In a message dated 10/29/04 2:40:15 AM Eastern Daylight Time,
>Torsten at Dittel.info writes:
>
> > You're right. If not so, I'd be able to program an interlaced mode
> >  doubling the vertical resolution by flipping video pages. However, I
> >  heard that Sockmaster was able to do something like this on a CoCo3
> >  using a bug of the early GIMEs (I just tried it on a CoCo2).
>
>Neat trick -- even lines in one image page and odds on another!
>But creating the split image would be a pain (well, splitting a given image
>file would be easy, but drawing lines and circles in real time would be, ah,
>interesting.
>--Mike K.


My old Super-MAC Macintosh picture viewer does this.  The image is 
"shredded" in half so that the even lines make up image #1 and the odd 
lines make up image #2.  Even though both fields/frames display at the same 
vertical offset from the top of the screen, a grayscale effect is achieved 
from the flickering of the two images.  Since any pixel in either image can 
be black or white, you get the following combinations:

black, black
black, white
white, black
white, white

This allows 3 shades to be displayed (black, gray, white).

Projector-3 borrows the same idea but then does away with the flickering 
and just generates solid pixels of black, gray, or white to display MAC 
images nicely.

If Super-MAC was coded to take advantage of some kind of GIME flaw that 
causes the vertical offset on some monitors to move up or down the screen 
every so slightly, then one of the fields (flickered frames) could be done 
this way to cause a real nice interlaced video mode.  This would give twice 
the vertical height of normal video.  A 640x400 image might be 
displayable.  Keep in mind that the height of the screen wouldn't change, 
but you'd get ~better~ vertical resolution.

--
Roger Taylor