Re: Sega Multigame and Vector Generator Card...

On Tue, 16 Dec 1997 15:00:38 -0800, Clay Cowgill <clayc@diamondmm.com> wrote:

>>> Moving "fractional" portions of pixels/dots/whatever is handy for
>>> rate-control and makes really fluid movement easy from the software side.
>>> Just gives you some easy dynamic range to play with. No real reason to
>>> "need" it though.
>>
>>Well ya, you need fractional pixels/dots to get variable slope without
>>using bresenham.
>
>Well, sorta-- you need it for calculation, but not necessarily for
>plotting. (Anything smaller than the discernable resolution of the tube
>could be left out...)
>
>>> Actually, I bet the dot size of the (focused) beam on a 19" raster tube is
>>> probably larger than the lower bit or two of resolution from a 12bit DAC.
>>> (Not to mention the shadow-mask.)
>>
>>Good point. But some may want to run on a B/W monitor and then you need
>>all the resolution you can get. That wouldn't be me though.

If you watch the slow floating of a ship on Space War you can see the ship
*jump* from one position to the next as it moves across the screen. It's very
subtle but noticeable. The Cinematronics uses a 1024 x 768 (or thereabouts)
grid for the starting points of it's vectors -- or about the resolution of 10bit
DACs. Since it's noticeable on a 19" screen it would sure be nice to have
higher resolutions available for the possibilities of larger screens.

The jumps are very small using the 10bit resolution, 12bits would be 1/4 of that
jump and would be very hard to discern. I think it would also be smaller than
the thickness of the trace -- the jumps on Space War seem to be about the same
size as the trace. The disadvantage of 12bits is that your clock rate must be 4
times faster than the asteroids DVG to maintain the same slew rate -- probably
not a problem. I wasn't able to find any serial DACs (I didn't do all that
thorough of a search) that worked at this speed, you might have to stick with
parallel loaded DACs.

<snip>

>>It just hit me! The 2nd order system (using ddx & ddy) would allow
>>another feature: Variable intensity along a line. Rather than using
>>them to draw a curve, you could use them to simply alter the speed
>>of a straight line draw. It'd be cool to have vectors that fade to
>>black (as the beam speeds up).

This might not work. I was comparing the speed of the vector generators with
the slew rates of the monitors, and as it is they run the monitors pretty close
to their maximum speed. Unfortunately you wouldn't be able to fade a trace by
drawing faster simply because you can't draw any faster. If the ability to draw
faster was available I'm sure Atari (or whoever) would have already taken
advantage of it. I imagine the speed of the monitors was used to set the speed
of the DVG and AVG, and not the other way around.

-Zonn

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Received on Wed Dec 17 10:46:03 1997