At 12:29 06.05.98 +1000, you wrote:
>OK, there are a couple of things in Eckhard's post I am not familiar with.
>Could somebody please explain what or how to achieve...
>
>a) small playfields (ie: 12x12 mentioned here)
That referred to the playfield of the game Plotting and not
to the TIA playfield graphics. I suggested to use the 6 sprites
trick and then display two symbols in every 8 pixel sprite.
That would give a horizontal resolution of 12 symbols.
>b) "the 6 sprites" routine
This trick has been used in many games, like Bob Colbert's
Okie Dokie. You might want to check the archive or Bob's
website for a better explanation and some source code.
What you do is basically, that you set up both players to display
three copies each with close spacing. They would look like p0p1p0p1p0p1.
Now you just change the graphics data on the fly, so that you
get six different sprites or 48 pixels of graphics.
The problem is, that you can only preset the graphics for
two sprites and then you have only three registers in the 6507
to hold the data to write out at the right time.
The trick is to enable the vertical delay for both player
graphics, which turns on two extra graphic registers that the
TIA updates in a special way. When you write data to GRP0, the
new data is written to GRP0-1 and the data from GRP1-1 is copied
to GRP1-2. When you write data to GRP1, the new data is written
to GRP1-1 and the data from GRP0-1 is copied to GRP0-2. VDEL
selects which of the two registers to use. That way you can
preset 2 1/2 sprites before the display starts, write the
3 registers of the 6507 to the graphic registers on the fly
and move the data for the last sprite from GRP1-1 to the
displayed GRP1-2 by writing any data to GRP0.
Ciao, Eckhard Stolberg
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