Re: [stella] Some (newbie) precisions about Missiles and Ball

Subject: Re: [stella] Some (newbie) precisions about Missiles and Ball
From: "Eckhard Stolberg" <Eckhard_Stolberg@xxxxxx>
Date: Fri, 7 May 2004 21:28:18 +0200
- Both Missiles and Ball can have 1, 2, 4 or 8 clock
width. Right ?

This is correct.

- Missiles can be displayed 1, 2 or 3 times on a same
scanline : what is the clock width corresponding to
the close, medium and wide space ? is it 8, 24 and 56
clocks ?

This is all correct too. But you have to keep in mind,
that sprite copying only works for the missiles and
players together. So if you set missile0 to 2 copies
with close spacing, player0 will be 2 copies with
close spacing too.

- The Missiles size can be x2 and x4 : does this mean
they can be 1x2 to 8x4 clocks width ?

No, this is for the players only. Missile width is
set with bits D4 and D5 only and can be 1, 2, 4 or 8
clocks, as you mention above.

- I haven't understand how works VDELBL. Anyone can
explain me simply what to do with this register ?

With the ENABL register you can turn on and off the
displaying of the ball. Only that there are actually
two ENABL register, ENABL-1 and ENABL-2. When you
write to ENABL, the data gets written to ENABL-1.
When you write to GRP0 the data from ENABL-1 gets
copied into ENABL-2. The VDELBL register lets you
chose which of the internal ENABL registers gets
evaluated for displaying the ball.

The idea behind this (and the VDELPx registers) is
that you can update the graphics for player0 in one
scanline and for player1 in the next, but still be
able to have single scanline motion for these objects.

Ciao, Eckhard Stolberg

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