ST Demos (2)

Again, we assume no previous experience of any type of hardware programming, although the odd bit of assembler, STOS or GFA would not go amiss... also the explanation of bitplanes really needs you to be comfortable with the idea of binary numbers...  

2. Scrollers

Scrollers were the staple diet of the old ST demo. Although they do all look a bit passe now in the times of heavy 3D, the well-programmed scroller was still a fair technical achievement when you consider how much data has to be shifted. I've chosen this as one of the first topics after some of the really hardcore fancy stuff because at its heart there are some of the core problems of handling graphics on the ST which we will need later to discuss some of the fancier bits of ST demos - in this way, we will also discuss how screens are stored on the ST and how we need to carefully manage data and plan everything clearly.

The basic problem with scrollers

In 1985 when the ST first came out, the machine's 8Mhz processor was seen as a major advance in the speed of computers. The big problem was that the increase in processor power was more than matched by an increase in the amount of screen data needed to draw the screen. Added to the fact that the demos all needed to run at 50 frames every second to look smooth (as opposed to down to 5 per second for games) and this meant that some tricks would be needed to put something decent on the screen.

Let's take a simple example: the ST's screen is always 32K in size assuming no borders are removed. At 8Mhz, each frame of the screen "lasts" for exactly 160,256 processor cycles (each scan line lasting 512 cycles with 313 scanlines in total - see last issue's article for more info about scanlines.) To copy one area of memory 32K in size to another, as fast as possible, only using the processor, requires almost exactly this amount of time. In fact to do this we need to access 64K of data: 32K for the source and 32K for the destination.

So simply copying a whole screen locks up the system completely. If we want other effects or music, or fiddle about with what we are copying, then it would be impossible to do smoothly.

Storing the screen

Clearly we need to find some ways round this. One is to not copy the whole screen, and this is where bitplanes come in:-


This is an explanation of how a low resolution 16 colour screen is stored in the ST's memory, and seems confusing to the uninitiated.

(1) There are 200 rows of pixels and 320 pixels in each row.

(2) Each pixel takes a value from 0 to 15 (hence 16 colours) and since this is a computer it is used in binary format i.e. 0000 for zero, 0001 for one, 0010 for two and so on, up to 1111 or fifteen. So to represent 16 colours we need 4 computer "bits" to represent it - 4 bits being half a computer "byte".

(3) The screen is stored in horizontal lines. If there are 320 pixels, this means there are 320 * 1/2 = 160 bytes for each line.

(4) Now the strange bit: in the memory itself we break each line up into groups of 16 pixels. Let's take the top left 16 pixels of the first row. The values of the pixels (top left first) we shall take to be 0, 1, 2, 3... 15. Or in binary:

0000 0001 0010 0011 0100 0101 0110 0111 1000 1001 1010 1011 1100 1101 1110 1111

 What is NOT done is that we take the 4 bits of the first pixel and the 4 bits of the second pixel and make a byte by glueing them together.

(5) Instead, we take the lowest (i.e. last) "bit" of all 16 values in the chunk and make 2 bytes by glueing them all together. If we look at the line of bit data above, then this will make 01010101-01010101. This makes up the first two bytes of the screen as it is stored in memory.

(6) We repeat this for the third, second and first bits of data in turn. In the end we have the following bit patterns:

0,1 01010101 01010101
2,3 00110011 00110011
4,5 00001111 00001111
6,7 00000000 11111111

In this way the pairs of bytes are separated from one another. Each chunk is called a "bitplane" since it refers to all the same bits of a group of many pixels.

Advantages of bitplanes

At first sight this looks like a really stupid way to store a screen. First, why not do it the easy way? This is really a technical problem because it is easier for the video processor to decode the data. Also the same technique is used to decode medium and high resolution screens (i.e. splitting into chunks of 16 bits)

Second, there is the cunning advantage that we can draw 16 pixels all at once by simply altering two consecutive bytes of memory. If we take the above example, let us alter bytes 0 and 1 to 11111111-11111111. This will alter pixel 0,2,4...14 all in one go. If we used the easy way to store a screen, we would need to access all 8 bytes of data, so the bitplane technique is quicker.

Disadvantages of bitplanes

As with all things, there are difficulties with this approach. Firstly, if we want to move something by one pixel, we must alter (or "shift") the data around before we draw it to the screen, and this eats up processor time. Secondly, any piece of code that accesses any pixels not in the same chunk of 16 must take this into account. Therefore it must be specially adapted and this takes time and effort. The blitter in the STe made this much easier for programmers, but came too late to be of any real help - most people had normal STs by then.

Using bitplanes

You can now see that programming bitplanes is, well, a bit of a pain. A great deal of thought is needed to get any efficiency from an ST. Let's look at some of the techniques used in scrollers to do this.

1. The chunky scroller and the bitplane scroller

This is the old scroller made up of small blocks, usually 8x8 or similar. To get size, you could load one line of graphics into the processor, and copy them several time, thus requiring less time to copy an amount of data. The resulting 68000 code usually looked something like this:

 move.w (a0)+,d0    ;get a word of data, stick in d0
 move.w d0,(a1)     ;copy to row 0
 move.w d0,160(a1)  ;copy to row 1
 move.w d0,320(a1)  ;copy to row 2
 move.w d0,480(a1)  ;copy to row 3
 move.w d0,640(a1)  ;copy to row 4

and so on, where a0 pointed to the source graphic and a1 the destination. The other big trick was to only use one bitplane (see the above explanation) In this way, to cover a whole screen you only need to copy 8K of data onto the screen (i.e. one quarter) and if the repeated "chunky" trick is used, perhaps only fetch 1K of data to copy on. This is opposed to reading a full screen of 32K from a background and writing another 32K. Hence we have had to copy about 15% of the data of our first example. Most demos desperateyl tried to hide their 1-colour shabbiness by using gaudy rasters (see last issue) or other cunning bitplane effects.

2. Shifting graphics

This is all very well, but what happened if your scroller moved at a speed of less than 16 pixels? In this case we need to alter the graphics put on the screen. This is very costly, but possible in some cases. If you have the famous "BIG Demo" by TeX, look at their "Big Scroller" - it moves one pixel at a time. Luckily in this case there is a very useful instruction, the ROXL instruction, which is perfect for this application. It would shift the data you wanted by one place to the left, and store the bit that was shifted out at the left hand side into a special register: the "X" or Extended Register. In the following ROXL instruction, the X bit would be shifted in at the far right of the next data you shifted. All you needed to do was draw one column of pixels at the right hand side of the screen and the scroller was complete.

This is all that there is to the "ROXL" scroller. Here it is in its assembler glory:

Screen data:
+136     +144     +152   X reg Assembler code
----     ----     ----   ----- --------------

00101010 10101110 10101110 0   roxl.w 152(a0) ;do the last chunk of
                                              ;16 on a row
00101010 10101110 01011100 1   roxl.w 144(a0) ;do the chunk to its
00101010 01011101 01011100 1   roxl.w 136(a0) ;and left again...
01010101 01011101 01011100 0   ..etc..
Despite its simplicity, the roxl was still quite slow. What happens if your scroller moves 4 or 8 pixels left at a time? Doing the "roxl" trick 4 times meant you usually ran out of processor time, unless your scroller was slow. We need to find way round this - the answer was buffering.

3. Buffering

The concept of a "buffer" is an old one in computing, and really just means a piece of storage space. If we look at a scroller that moves at 2 pixels each frame, then it takes 8 frames for it to move one whole "chunk" of the screen (since 2 pixels * 8 frames = 16 pixels.) Instead of shifting all this data each frame, if we store 8 copies of the line of scrolltext somewhere in memory, each shifted 2 pixels left from the last, we can simply copy each one of these buffers in turn to the screen. The only shifting we need to do is when we add parts of the new letter to the right hand end of the buffer!

To demonstrate, here's an example. Each "character" in the example represents 2 pixels of the buffers we use. The splits for each 16 pixels are denoted by the spaces:

Buffer 0:
Buffer 1:
Buffer 2:
Buffer 3:
Buffer 4:
Buffer 5:
Buffer 6:
Buffer 7:

If in frame one we copy buffer 0, then in frame two copy buffer 1 to the screen, the scroller moves along at two pixels. When we get to buffer 7, we then copy buffer 0, only this time copy 8*2 pixels = 16 pixels = one data word from its start, and tag some new slice of graphics on to the end of the buffer. Voila! A much faster method of scrolling buffers. In this way, scrollers using more bitplanes or larger areas of the screen can be achieved.

You may well have noticed that this takes up a lot of memory, and this is one of the main principles of demos: more memory --> faster demos. The other main idea here is that you always appear to be doing more work than is really the case!

4. More tricks with bitplanes

There is one other aspect to bitplanes which involves a fair bit of cunning thinking. It is concerned with the careful use of colour palettes when bitplanes overlap. Let's say we are using a 1-bitplane scroller, while the other three bitplanes are being occupied by an 8-colour picture (we can split bitplanes like this to give new effects)

The scroller goes in the 4th bitplane, that is it governs the most significant bit of the colour of each pixel, of value %1000 or 8. If a pixel of this bitplane is set, then the colour of that pixel MUST lie in the range %1000 to %1111 (8 to 15) no matter what lies in the other 3 bitplanes. Look at the example below:

Bitplane 0   0101010101010101 Picture 1
Bitplane 1   0011001100110011 Picture 2
Bitplane 2   0000111100001111 Picture 3
Bitplane 3   0000111111110000 Scroller

Resulting        1111  11
Colour       0123234589014567

Now, if we use a random palette all the colours will look ugly and you won't be able to read the scroller. But if we set all the colours 8 to 15 to the same value (let's say white) then the scroller will appear to run over the top of the picture without them interfering with one another.

Similarly, if we want the scroller to run behind the picture, the colours of 0 to 7 should be copied to those of 8 to 15, with the exception of 8 (ie. where the picture is empty) which should be set to the scroller colour.

That's about it for bitplanes; these tricks are used all over the place in demos, especially those "one million bitplane scroller" screens in fashion a few years ago.

Page maintained by Steven Tattersall