BLITTER MANIA
I'm going to try to explain how I've had the idea to use the blitter of the STE
to drive its hardware and how I implemented it in two demo screens.
First of all, it's advisable to explain briefly how the Blitter works or what it
does in general. Basically the blitter allows operations with rectangular screen
contents. For example you can copy a bitmap from one two-dimensional buffer into
another. To achieve that the blitter has registers which allow to describe
source and destination areas.
$FFFF8A20.w Source X Increment Register
The source address gets incremented by this value every time a word is
transferred, this however doesn't occur at the end of each line where the value
from the following register is added instead.
$FFFF8A22.w Source Y Increment Register
The value from this register is added once at the end of each line to
allow operations for any rectangular bitmap area.
$FFFF8A2E.w Destination X Increment Register
Same as Source X but this one is applied to the destination address.
$FFFF8A30.w Destination Y Increment Register
Same as Source Y Increment but it's applied to the destination address.
For more information refer to Paranoids great blitter tutorial in Alive 6 (
http://alive.atari.org/alive6/ste3.html ).
One special thing about these increment registers is the fact, that they are
signed. Which means you can decrease them instead of increasing. So, we are able
to use the blitter to scroll the source buffer, simply by setting vertical
destination increase by a negative value to get back to the beginning of the
destination buffer after each line, which allows an easy implementation of a
circular buffer.
Imagine you want to do a plasma effect. As you know plasma is done by
synchronized changes of a colour register. The effect will change the colour of
a monochrome area to whatever you write to the colour register. If you want to
do several plasmas on the same scanline you need to arrange the data in a
special way to gain speed while writing them to the hardware, also it's a good
idea to use adjacent colours for that reason.
To speed up the critical part (writing to the hardware) we copy colours from
several source buffers into an intermediate buffer which will contain the
pattern for the next frame. The data has to be written interleaved, eg. col1,
col2, col1, col2 etc.
This was the basic idea I implemented in 'Tribute to TOS' for the Atari-Forum
- Demo. The main difference was using palette registers instead of a
destination- line, resulting in plasma type areas. I've set the blitter to copy
a colour filled buffer onto two adjacent palette registers $FFFF8242.w and
$FFFF8244.w and since destination X increment register was set to 2 and
destination Y increment Register was set to -2 (remember the blitter doesn't
increment x after the last word of a line) this buffer was circular, which
means that all source data - no matter how much will come - is written to
these two colours only. C-Rem provided special graphics to maximise the
effect. This way, we got a plasma on two hardware registers, which results in
4 different plasma areas on the screen: One green, one yellow, one red, and
the last one looking like the colour palette of the Atari logo from TOS 1.62.
Right after completing this screen, I thought about other ways to use this
technique, still using two registers. I asked myself how the STE-shifter could
be used to distort the screen with the Blitter. For a start, I was using the
following hardware scrolling registers:
$FFFF820E.w Line-width-Register (ammount of additional words to standard line)
$FFFF8264.w HScroll Register (pixel offset from 0 to 15)
There is a special thing about these two registers. The value of the Line-width
-Register depends on the value of HScroll. If HScroll is set to zero we need to
add 4 to the value of the Line-width-Register. Beside the screenbuffer needs to
be wider than a normal screen. If HScroll is set to 1-15 we need to subtract 4
from the additional word ammount.
I've started to code this effect during a beer-code party in Grenoble with
Strider, Shazz, C-Rem and Gizmo, reusing code from 'Tribute to TOS' to save
time.
First results were terribly disappointing; I've especially managed to completely
confuse my poor STE shifter. I then realized it was safer to set HScroll to
everything but zero to keep a constant value in the Line-width-Register.
Numerous beers later I managed to code what Shazz and I named "The most
complicated way to draw a picture on ST". Nonetheless the shifter was holding
out, and what was left was trying to distort the picture using offset register.
Of course, it didn't work on our first try, and I had to seek the right timing,
for the NOP, to modify the HScroll-Register while the screen was scanned.
Further, it is important to note that the blitter copys one word in two NOPs,
and so 2 words in 4 NOPs, corresponding not only to my objective to drive the 2
registers but to get the necessary time for the shifter to draw 16 pixels. Some
would say it was pure luck, but I'd rather think the STE is just a perfect
machine! The effect was working, I just had to let it run and drink some beers.
A few days later, I thought it was a pity that 50% of the blitter power was
wasted to set a constant value to the 'length of a line' register, and then I
asked myself how to make the most of my code. By measuring distance between
blitter registers and palette registers, I realized we could change one colour
every 16 pixels, allowing us to display a plasma while distorting the screen.
The effect was now fully functional, finding a interesting distort and
optimising a bit was the only things left, enough to keep me busy during
HunoParty II at Saint Villers les Pots. (Nft: what a nice name!)
So it is, this effect is now being finalized, looking for a suitable piece of
artwork, a sweet music and it will be visible by everyone!
In the very last second a screenshot of this really colourful screen arrived at
Alive HQ and we had some trouble to convert it down to just sixteen colours, the
original snapshot had approximately ten times more. As you probably can spot it
will be used as an intro for Atari Legend, or better for a CD from them.
Tobe / MJJ Prod (roughly) translated by RaZ/TheLAB
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