6 ms·
Nine – seemingly impossible C64 demo
- kbelder 2y agoNeat. Having more than 8 sprites on screen is not hard, it just requires some well-timed interrupts to move a sprite from above the current raster location to below it, so that the same sprite is drawn multiple times. But I'm not sure about the end of the demo where they are all rotating on the same line. I guess it could be done with interrupts occurring partway through the horizontal scanline, but I didn't think they could be that precise?
- o11c 2y agoYoutube comments said something about "multiple apparent sprites per actual sprite, with stretching"?
- pwrrr 2y agoYeah, you can x and/or y expand a sprite, then change it to make it look like it contains more data by cleverly drawing the sprite because 1 pixel becomes 2 pixels wide in expanded mode.
- egypturnash 2y agoNope, that’s not it. There’s a video out there already that dissects it and figures out the trick.
- peterleiser 2y agoI don't know the trick, but take a look at the "7". It looks like it's made from the vertical line in "4" and a portion of another number.
- badc0ffee 2y agoThis is true, but not relevant to the trick.
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- boltzmann-brain 2y agoyou can't multiplex sprites so that more than 8 appear on the same line. the secret is to pretend you have a 9th sprite by emulating it with characters that look exactly like one of the sprites would.
- egypturnash 2y agoThat stops working when you open the borders and put sprites outside the area the video chip will render characters/bitmaps.
- eru 2y agoYes, but the demo is really cheeky about that. The borders are open when you can still do the normal multiplexing, because there's lots of vertical separation between the sprites. Later, when they put the sprites together on the same line, the borders are closed.
- jchw 2y agoBut in this case, that (alone) can't be it because all 9 sprites clip (smoothly) in and out of the border. So they've really gotten cheeky with this one.
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- Dylan16807 2y agoI don't like how you worded this post. You know the demo's point is having all 9 go at the same time, so it's quite misleading to start off describing something that sounds like the demo's point, but is not the demo's point, and saying it's "not hard". Your second paragraph mostly corrects that false impression, but I dislike that you gave a false impression to start with.
- Underphil 2y agoThis is an extremely common type of comment on articles like this. The commenter has to ensure everyone knows their credentials before offering any sort of praise to the creator. I also despise this type of self-serving comment.
- flohofwoe 2y ago9 sprites on screen at the same time is trivial (this is known as sprite multiplexing), 9 sprites on the same raster line is the actual 'aha effect' because the C64 can only do 8 sprites per line.
- Dylan16807 2y agoI'm not sure why you replied to me? Are you nitpicking that I said "go at the same time" because I think it's clear enough in context.
- pinoy420 2y agoAll you need is a little “Just”(TM) and everything is possible. Typical HN comment minimising the immense effort required to do %thing% due to their own arrogance.
- flohofwoe 2y agoThe VIC-II reads the data for each sprite at hardwired horizontal positions outside the visible area, so this sort of 'horizontal multiplexing' shouldn't work.
- aappleby 2y agoEight hardware sprites and one blitted to the background every frame?
- erik 2y agoThat works until you see what appears to be 9 sprites on the same scan line in the border region.
- boltzmann-brain 2y agoVIC II doesn't have that functionality. the first commodore that could do this was the Amiga series.
- badc0ffee 2y agoNot quite. Drawing a full sprite-sized bitmap would not be so smooth on the C64.
- pwrrr 2y agoThey are in the border, Only sprites, ghostbyte (a repeating byte only shown in black. It is the last byte of the chosen videobank) and background color can be displayed there.
- siltcakes 2y agoAs soon as I saw who made this, I cranked my speakers and wasn't disappointed! Linus is amazing. I recommend his cover of The Moon from DuckTales: https://linusakesson.net/chipophone/ducktalesmoon.php https://linusakesson.net/chipophone/ducktalesmoon.php
- jetbalsa 2y agoI'm a huge fan of his A Mind is born demo 256 byte demo he did awhile back, freaking amazing https://linusakesson.net/scene/a-mind-is-born/index.php https://linusakesson.net/scene/a-mind-is-born/index.php
- userbinator 2y agoDiscussion of it showed up on HN too: https://news.ycombinator.com/item?id=26511266 https://news.ycombinator.com/item?id=26511266 I like how the title really invites you to pun it as "a mind is blown", because mine sure was when I first watched it.
- nonane 2y agoHere is a video where Martin Piper uses a C64 debug tools to figure out how Nine was created: https://m.youtube.com/watch?v=Ik1vsMM2EuY https://m.youtube.com/watch?v=Ik1vsMM2EuY .
- iamevn 2y agoThis is probably a really basic question but why does the wizard need to stop being drawn with characters when the sprites move out past the left/right borders? Is this a hardware restriction where those lines aren't able to have text on them when there are sprites on the edges?
- FLT8 2y agoI think it's probably about the timing. I'm not an expert, but I believe that opening the side borders requires cycle exact timing at both edges of the screen, and when you have characters enabled on the screen, the VIC-II graphics chip ends up 'stealing' memory cycles from the CPU every 8 scanlines to fetch character and font data from RAM. These lines are known as 'badlines'. Across an entire scanline I think you normally get something like 63 6510 cpu cycles to do 'work' in, but only 23 if you hit a badline - keeping in mind that some instructions take multiple cycles to execute. This probably makes the timing difficult or impossible to manage with the characters turned on.
- phire 2y agoIt's not just that the 'badlines' steal 40 cycles. They steal a solid block of 40 cycles that cover most of the screen, from end of hblank until just before the right border starts. This blog post [1] has a nice interactive demo showing badline timings. During a badline it's simply impossible write to the VIC-II's registers during the left border. Though, this seems to indicate it's still possible to open the right border during a bad line, but it's a 1 cycle window (maybe 2). [1] https://nurpax.github.io/posts/2018-06-19-bintris-on-c64-part-5.html https://nurpax.github.io/posts/2018-06-19-bintris-on-c64-par...
- pimlottc 2y agoJust guessing here, but does it have anything to do with the fact that 9 is the same as 6 rotated?
- boltzmann-brain 2y agono. there are 9 separately moving objects and them being visually similar or even the same doesn't mean much
- TheRealPomax 2y agoCan we take a moment to appreciate that the music starts in 10/8, too? Because that's just the cherry on top.
- donohoe 2y agoI assume it is 8 sprites but the "6" is also used to represent the "9" and jumped (and rotated) from position to position?
- JKCalhoun 2y agoThat was my guess — but I'm not familiar with the C-64 and its sprite limitations.
- badc0ffee 2y agoThe limit on the C64 is, nominally, 8 sprites. Widely known tricks exist to increase that to 8 per raster line, by IIRC repositioning them during horizontal retrace. This demo actually shows 9 "sprites" on the same raster line using some clever tricks like low-res stretched sprites combined with custom characters. It all makes sense once you know the tricks, but the really impressive thing is how smooth it is. Which I guess is why it features a magician.
- pwrrr 2y agoYou can't change the sprites every rasterline. Sprites are 21 pixels in height in unexpanded mode. So you can only change the sprite after displaying all 21 lines, unless you use sprite-crunching, where the limit is still several lines. Linus has an article on sprite-crunching (mind-blowingly complex).
- jll29 2y ago
- aqueueaqueue 2y agoRabbit holed to this https://linusakesson.net/music/withering-bytes/index.php https://linusakesson.net/music/withering-bytes/index.php Thia is .... Something else!
- noman-land 2y agoThis dude's work is incredible. His entire YouTube channel is amazing. My personal recent favorite, The Commodordion: "A fully functional 8-bit accordion with bellows made out of floppy disks." Legend. https://www.youtube.com/watch?v=EBCYvoC4muc https://www.youtube.com/watch?v=EBCYvoC4muc
- junon 2y agoI want a Savant remix of that tune. It was so good.
- relistan 2y agoWow
- neuroelectron 2y agoI probably don't understand it completely but from what I gathered from Martin Piper's video is that the spite locations and data are updated during the scan lines allowing the renderer to reuse the same hardware sprites before the scanline is complete.
- yumraj 2y agoI didn’t get. Can someone please explain what’s impossible about it?
- themadturk 2y agoThe problem is presented in the intro screen: The Commodore 64 only has eight independent sprites, yet this demo showed what appear to be nine sprites.
- yumraj 2y agoThanks!!
- panic 2y agoBy putting all the sprites on one horizontal line in the top “border”, the demo also proves it’s not using any of the standard tricks to fake extra sprites. The only thing that the C64 will draw up there are real sprites (not character/bitmap graphics), and you can’t multiplex sprites in the same horizontal line of pixels.
- mdp2021 2y agoNot really the point: that is a tongue-in-cheeck trick, as we know how to solve that problem - but it takes using different stripes of the screen, whereas in the demo they are before 2:00 all put in the same stripe... At that point we go "Ah." (It's very normal in illusionism: the practitioner states something that puts you in the relaxed mood of participation to a game you know - then actually surprises you.)
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- thih9 2y agoIs there a text explanation of the trick - for those that cannot watch a video?
- exitb 2y agoThe demo shows nine separate sprites, even though C64 supports showing only eight. It starts off by separating them vertically, which is a common trick, but then moves them all to the same raster lines and overlapping locations, which is much more surprising.
- nottorp 2y agoBy "cannot" i assume you mean you watched the demo video but your life is too short to waste it on video explanations of assembly tricks that are much better done in text? :)
- account42 2y agoOr he's somewhere where enabling audio is not an option.
- human3047628272 2y agoSome people have sight problems, or are even totally blind. But they still use the Internet.
- nottorp 2y agoYeah but the OP sounds like they saw the demo and are wondering how it's done now.
- stavros 2y agoI don't know about the GP, but for me it meant "I have ten seconds to understand this curiosity, but not 32 minutes. If someone wanted to spend ten seconds explaining to the rest of us, I'd be grateful".
- masswerk 2y agoThere are some basic debugging notes by Martin Piper: https://github.com/martinpiper/DebuggingDetails/blob/main/Nine%20-%20LFT.txt https://github.com/martinpiper/DebuggingDetails/blob/main/Ni... For better understanding: The demo starts with a magician character drawn in the center of the screen, announcing the trick in text (some of this is big text achieved by overlaying sprites). Then, 9 sprites, each in the form of a colored number from 1 to 9, exit the magician's hat to rotate around him (in the notes screen #1), then, the screen gradually widens horizontally beyond the usual borders and the circle of the 9 apparent sprites spreads out over the entire screen (screen #2). (At this point, the display of the magician, which was clearly drawn by PETSCII characters, before, shouldn't be viable anymore.) The swirling numbers transition to a narrow ellipse near the top of the screen (screen #3), which then transition to the very top of the screen, above the normal display area, a zone, which is available for sprites only, showing the sprites in a linear rotation (screen #4). Meaning, these must be sprites and these 9 apparent sprites are drawn at the same scan-lines. (So there is no way this could have been achieved by conventional, vertical multiplexing. This is the highlight, the big trick of the magician's show.) At the end, the sprites return to their initial circling motion and vanish, one by one, into the magician's hat. The demo is a combination of various tricks and artful timing, which enables perfect transitions between them.
- yuchi 2y agoPosted it few days ago but didn’t get any traction. All Linus’ work on retro computing is fenomenal. Please have a look at its channel, a lot of awesome content there.
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- nerdralph 2y agoMy guess is it is chasing the beam. Change the border color from black to your sprite color, then back to black again. The change takes a minimum of 4 cycles (store absolute), which I think explains the thickness of the numbers. Some calculations involving the PAL scan line frequency could confirm if 4 cycles is quick enough. edit for further detail: This would be used for one of the numbers, with the other 8 using sprites.
- nerdralph 2y agoAs mentioned in other posts, the trick is using multi-color sprites to look like multiple single-color sprites. I did the scan line frequency calculations, and my though of changing the border color would not work. In one cycle the scanline advances 8 pixels, so toggling the border color would be a minimum of 32 pixels in width.