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Last Update : Sunday 20 March 2022 at 17 h 23

Manual n° 1

1Kraftwerk ========== How much Kraftwerk can you fit in a kilobyte? Let's find out! 1K intro for Amstrad CPC Planned for Forever 2020 "other 8bit" competition, but that didn't happen. I wanted to finish and polish it a bit during the trip to the party and during the party itself. So you get something not as good as it could have been, I guess. I hope that's ok. Code by PulkoMandy Visuals and Music from Kraftwerk, adjusted to fit the constraints Some random notes from 2020 --------------------------- Two years ago I released my previous 1K intro, 1Kusai. It used the CPC firmware floating point routines to compute B├ęzier curves and render a picture. Originally I wanted to add some music, but I ran out of time to squeeze that in. I kept the idea of better music in 1K intros in my head, however. This year the theme for Forever is "Robots" and the Kraftwerk song sounds like a good match. So let's do something with it? I started with some visuals to have a rough idea of the size budget for the music. I took a picture of some live Kraftwerk concert and traced out some outlines. This time I used only straight lines, so the code to draw the things was a lot simpler and smaller. After a first session of work I had the thing rendering, already under the 1K limit and with several ideas to further optimize it (more on that later if I try them). I also drafted the music in Arkos Tracker, even if it was clear that none of the players provided with it would be suitable (they are designed, at best, for 4K intros, and I'm not doing one of these). Arkos Tracker players are of course quite well optimized, so what could be the plan for a more compact one? The answer, of course, once again lies in the CPC firmware (which I'm already using for drawing lines and filling shapes). The CPC BASIC has commands to manage sound (mainly SOUND, ENV and ENT). These allow to define volume and tone "enveloppes" (variations over time) and synchronize the 3 sound channels together. This lays the base for a music player: notes, instruments, and timing. The only thing that remains is some kind of "pattern" handler to feed the data to the firmware. I made something very simple, each channel has a single pattern with a loop point (which allows for some progressive buildup of the music, and then play the main part in a loop). For now, I didn't need something like patterns. The data for the music is not super efficient (9 bytes per note), but it is very repetitive so it packs quite well. The firmware routines are not perfect, but with some creativity, the limitations can be used at our advantage. For example, the volume enveloppe can add and subtract arbitrary values to the volume, but it does not clamp it to the range the AY can handle. This means if you're not careful, it will rollover, and what you wanted to be silent will now be maximum volume. But using that cleverly makes for a cheap way to repeat a note multiple times. Size stats (approximate, as the code changed a little after the music turned out even smaller) ---------------------------------------------------------------------------------------------- Amstrad standard binary file header: 128 bytes ZX7 packer code: 89 bytes Space remaining for packed data and code: 896 bytes Space used by shapes and shape drawing code: 725 compressed bytes Space left for music after this: 171 bytes Possible optimizations ---------------------- - Use relative lines instead of absolute. Allows to use 1byte instead of 2 for each coordinate, and repetitive shapes (eg. the consoles) will pack a lot better because most of the lines will be identical. However, hiding control bits in the MSB will not work as there will not be enough space. The least significant bit can be used because the firmware works in a 640x400 space and scales down to 320x200 when drawing, so adding or removing 1px from a coordinate will not have any visible effect. Alternatively, an escape value (0x80?) can be used instead, and that may even end up being smaller because the code to extract and mask the control bits is not that simple, after all. This could save about 200 (!) bytes here, but the code has to be a little more complex (but probably not 200-byte complex). - It would also be possible to keep absolute coordinates but have them remain < 256 bytes by setting an ORIGIN for each polygon and then using absolute commands. This way we can do away with the negative coordinates, which keeps the code simpler. - Use rendez-vous in the music instead of adding silent notes. Rendez-vous costs just one byte, so if it can save a 9 byte note entry, that's quite great. - Think about how to factorize the music player code, repeating the same code 3 times for each channel isn't great. Lessons learned --------------- Very repetitive data compresses much better than z80 code. Don't hesitate to have the same sequence of bytes repeated multiple times if that makes your z80 code even slightly smaller. This will compress very well. I have a good toolchain for converting vector graphics from SVG (not perfect, it still needs some handtweaking, for example for determining a good start point for the fill). I don't have anything for the music, however. Should probably think about it, because hand-converting it to assembler is not that fun. NEVER edit the generated data for SVG files by hand. It may seem easier at first, but simplifying the source data allows to re-run the conversion pipeline when you want to make a change in the encoding format. The ZX7 compressor seems not very efficient at first, but it starts to perform better when getting closer to the 1K limit. So filling the first bytes goes fast, but filling the last remaining bytes fortunately goes a bit slower.

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