ST Demos (3)

Distorters, more scrollers, and sprites

3.1. More scrollers

Now I know scrollers have been banned under the infamous Representation of the Demo Watching Public Act (1992) which declared them illegal on the grounds of having to read greetings to the same old people again and again. Unfortunately they did not have the delight of reading Quazar's full serialisation of Proust's "Remembrance of Things Past" in their fantastic 6-disk "Eat my Trousers" demo, or they would have changed their minds.

No, the real "interest" of the scroller was not the words - after all, we were usually dealing with 16 year-olds for whom English was not the native language (even the British coders, who conversed in assembly) - but the silly things you could do to a piece of text. We shall look at some of the inventive tricks they used, starting with some simple stuff:
 

3.1 Benders

(yes, that does say Benders)

Best Example: Omega's part in the SoWatt demo (1990)

A "bender" in demo-speak is something that distorted, usually sideways or up and down. Most people would call it a distorter, but I quite fancy taking this as an excuse to be gratuitously obstruse.
 

Preshifting

Part Two dealt with one of the main tricks of distortion: that of pre-shifting, although in a slightly different way. In distortion, the thing that's quite difficult is shifting graphic data sideways. Moving it in blocks of 16 pixels is easy, because that is the way the screen is stored. Moving it in steps of less than 16 pixels is hard, because we need to shift each individual piece of graphic data sideways (in itself quite slow on a 68000 processor) and because to make one chunk of shifted data on the screen, we need to take two pieces of source data, shift them, then combine the results into one destination chunk.

With demo makers hating waste as they do, they were reluctant to use lots of processor time doing all this shifting. The clever trick is to make 16 copies of the sprite and store them all in memory BEFORE the demo started. Then all you needed to do was simply copy all the graphic areas you wanted. Voila! No shifting.

3.2. Distorters

The same principle is used in all those distorting logos in old ST demos. A preshifted logo rather than a sprite or chunk of scroller buffer was used, and the shift amount on each scanline was changed, usually by looking it up in a pre-calculated table. Not much more to say about the standard horizontal distorters, really.
 
3.1.2 Word Benders and Byte Benders
Just being able to move a block of graphics sideways wasn't much of a technical achievement. Wibbling them up and down was the next decent improvement to be made. The idea is very simple: just get the data out in the same way, but according to table data, draw each chunk on the scanline above or below as required. Here's a very simple assembler example:

  move.w (a0)+,d0      ;get the chunk of gfx data
  move.w (a0)+,d1      ;get the offset onto the scrn
  move.w d0,(a1,d1.w)  ;draw it
  addq.w #8,a1         ;move to next chunk along

where the offset d1 would be –160 for a line above, 0 for the same line and so on. This was easily optimised using self-modifying code to:

  move.w (a0)+,xxx1(a1) ;or use movem.w (a0)+
  move.w (a0)+,xxx2(a1)
  move.w (a0)+,xxx3(a1)

where the offsets xxx1, xxx2, xxx3 were pre-written each frame by taking them from a table and adding 8 to each successive value.

"Byte"benders just put a byte ie. 8 pixels of data onto the screen each time instead of 16. For copy more than one bitplane, there was a very nice trick to this – the 'movep' instruction. Instead of a normal 'move' this placed 8 bits at the effective address, the next 8 bit at address+2, the next at address+4. It was intended to be used for writing to hardware peripherals, but it comes in useful because you can access up to 4 bitplanes all in one go, for example:

  movem.l (a0)+,d1/d2/d3/d4 ;4-plane gfx
  movep.l d1,xxx1(a1)
  movep.l d2,xxx2(a1)
  movep.l d3,xxx3(a1)
  movep.l d4,xxx4(a1)

Barrel scrollers and distorters; 4 bit benders

The barrel scroller was the one that appeared to wrap itself round a central object. This in itself was quite easy, but came in handy with the 4-bit bender. To create the barrel effect, all you need to do is copy selected scanlines of the scroller buffer to give the impression of circular distortion. (A similar effect in distorters was used to make them seem like they were 'squashing' horizontally.)

This comes in useful for the 4-bit bender, because it's not a '4-bit' bender in the sense of byte and word benders above. In fact there was no bending at all: all that was done was the font graphics used were pre-distorted (ie 'sheared' vertically every 4 pixels) to give the impression that the whole scroller was distorted. Cunning.
 

Horizontal squishing

By now you might have guessed how horizontal squishing might work: yes, it's all precalculated frames of the logo, just like pre-shifting.
 

3.3. Sprites

Yes, sprites were all preshifted as well. That's why there were so few animated sprites in demos (they ate quite a lot of memory) and why they were often small but with thousands of the little blighters. However, even sprites had their tricks..
 

Self-coded sprite routines

Just like the time wasted on coding a hardware scroll routine, much effort was put into the perfect sprite routine. The biggest way of speeding the extra ounce of speed from your routines was not just to pre-shift the little darlings, but to also use a routine that would 'write' another routine that would draw the sprite:

This had the advantage of:

Although the speed improvements gained were individually tiny, this was the only way you could get several big sprites onto the screen at once.

The supreme example of this is an Overlanders screen from the European Demos. Not only did it use this technique, but it was a fullscreen demo. So as well as making an intelligent sprite routine, the coders had to make an instruction cycle-counter which would alter the video registers to create the overscan effects as well!

Tat
 
 

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