5 ms·

Thanks for the correction. I will update the drawings asap. Since you seem to know the machine well, maybe you can shed some light on something i was unable to

Thanks for the correction. I will update the drawings asap. Since you seem to know the machine well, maybe you can shed some light on something i was unable to clarify: What is the fastest way to clear a framebuffer to a desired color on an Amiga. My hypethesis is that it was done with the Blitter, by disabling all DMA entries "doing something else". Do you have any idea?

quincunx 7y ago
To set the framebuffer to a single color, consider that there are actually up to 6 "framebuffers" (one for each bitplane.) Consequently, it is to either set all bits for a given bitplane to 1 or to set them all to 0. To control the actual word that is blitted without DMA, you would use BLTADAT, BLTBDAT or BLTCDAT registers to control the corresponding blitter input word with DMA disabled (so the blitter does not write into these registers itself.) A common technique to avoid having to blit each bitplane independently (esp. for BOB sprites where the blits are small and plentiful) is to interleave the bitplanes by row using the BPL1MOD and BPL2MOD register values; these values are added to each bitplane pointer address at the end of each row to move the pointer to the start of the next row; consequently a single blit operation can capture all bitplanes as in memory a row can consist of all row bits belonging to bitplane 0, all row bits belonging to bitplane 1, all row bits belonging to bitplane 2 and so on. If this is the bitplane layout used then a single blit can only set the values to 0 or 15 (all 0's or all 1's for 4 bitplanes enabled); note however that the blitter too has modulo registers (BLTA/B/CMOD for each input and BLTDMOD for the destination) so perhaps best examine how fancy the programmer got with it (eg. multiple blits to clear the screen to a specific color even if the bitplanes are interleaved using BLTDMOD.) Obviously I'm a bit rusty given it's been a few years, but I hope this helps. I'm a fan of your work, thank you.
- fabiensanglard 7y ago
  Thank you so much, I am going to amend the "fill" section. Are you ok with being credited as "quincunx" or you prefer something else?
  - quincunx 7y ago
    That's very kind, quincunx is fine, or no credit is fine too. Glad I could be of help.
- fabiensanglard 7y ago
  I had no idea about the BPL1MOD and BPL2MOD. Now that i know they exist I don't see why a game developer would not use them. I will try to double check with Eric Chahi but this is so much more efficient that it has to be the way he drew polygons (instead of drawing into a scrap buffer and blitting four times, he used BPL*MOD to draw and blit only once).
  - Flow 7y ago
    Unless I'm wrong, a drawback of using interleaved bitplanes to do fewer but larger blits is that the mask for the sprite shape gets bigger too. If you do several blits, one per bitplane, you get to use the same mask data, but when using interleaved data the mask needs the same shape data * number of bitplanes involved. It's still a pretty small price to pay compared to Atari ST/ZX Spectrum preshifted bitmaps.
    - fabiensanglard 7y ago
      Does that mean you cannot use the Blitter to draw lines and fill area as described in the article?
      - quincunx 7y ago
        You'd use a single multi-bitplane interleaved fill-blit if the background surrounding it did not need to be preserved (eg. no mask requirements as per Flow - demos tend to do this for its speed). I'd imagine Another World creates an off-screen single-bit deep bitmap and uses that as a mask. I sent you an email if you'd like to explore more, however the original programmer probably just remembers what he actually did. Also note that non-overlapping polygons can be grouped together in both approaches by line-blitting with XOR destination (adjacent polygons share an edge and so the edge is XOR blitted twice); so there are substantial speed-ups to be had.
jacobush 7y ago
Half-brite was not in the Amiga 1000 and early Amiga 500 and 2000. The later “ECS” chipset has included “halfbrite”. Most games did not take advantage.
- egypturnash 7y ago
  Do you know of any games that used halfbrite? I can’t think of a single game that did. I think there were like three games that used HAM mode, if you filter out “displaying a scan of the box art while the rest of the game loads”. Knights of the Crystallon, Pioneer Plague, and maybe Mindscape?
  - jacobush 7y ago
    At least Pinball Dreams did and googling suggests Cannon Fodder. I suppose a few more must be in there. Edit: https://retrocomputing.stackexchange.com/a/196/4616 https://retrocomputing.stackexchange.com/a/196/4616 Also The Settlers Black Crypt - http://hol.abime.net/126 http://hol.abime.net/126 Links - http://hol.abime.net/892 http://hol.abime.net/892 Universe - https://en.wikipedia.org/wiki/Universe_%281994_video_game%29 https://en.wikipedia.org/wiki/Universe_%281994_video_game%29
  - Fr0styMatt88 7y ago
    Also interesting is the game 'Universe', which boasted achieving 256 colours on ECS (edit: a comment on the StackOverflow page linked below says it works on OCS as well). Some quick Googling before asking you guys about it as I was planning to turned up this: https://retrocomputing.stackexchange.com/questions/44/how-to-obtain-256-arbitrary-colors-with-limitation-of-64-per-line-in-amiga-ecs https://retrocomputing.stackexchange.com/questions/44/how-to... The Amiga was such an interesting machine. It's too bad I never got into assembly programming back then.
  - krige 7y ago
    I know of one game that actually used HAM (specifically HAM6, the original OCS version) during gameplay and it seemed to run very fast as well - it was a PD game called Zdzisław Bohater Galaktyki 3D and it's, well, not very serious [0]. [0] https://www.youtube.com/watch?v=IV5UPk8ZQZw https://www.youtube.com/watch?v=IV5UPk8ZQZw
vidarh 7y ago
To maximize performance you'd tend to combine the blitter with a M68k loop, typically using MOVEM to maximize the number of bytes copied per instruction. The reason for this is that the blitter can not utilize every single memory cycle. So if you want to maximize graphics throughput you'll need to carefully balance the work between the blitter and CPU depending on which CPU.
- Flow 7y ago
  There's also a NASTY bit you can set so the Blitter gets the bus cycles it want, any CPU access will have to wait for a spare cycle. I'm not sure how much faster using that bit will make Blitter graphic operations in practise. I suppose when using 6 bitplanes a small speedup of like 10-20% could be seen.
deleted 7y ago
[deleted]
agumonkey 7y ago
so fun to see both of you discuss in depth like old magicians

quincunx 7y ago

To set the framebuffer to a single color, consider that there are actually up to 6 "framebuffers" (one for each bitplane.) Consequently, it is to either set all bits for a given bitplane to 1 or to set them all to 0. To control the actual word that is blitted without DMA, you would use BLTADAT, BLTBDAT or BLTCDAT registers to control the corresponding blitter input word with DMA disabled (so the blitter does not write into these registers itself.) A common technique to avoid having to blit each bitplane independently (esp. for BOB sprites where the blits are small and plentiful) is to interleave the bitplanes by row using the BPL1MOD and BPL2MOD register values; these values are added to each bitplane pointer address at the end of each row to move the pointer to the start of the next row; consequently a single blit operation can capture all bitplanes as in memory a row can consist of all row bits belonging to bitplane 0, all row bits belonging to bitplane 1, all row bits belonging to bitplane 2 and so on. If this is the bitplane layout used then a single blit can only set the values to 0 or 15 (all 0's or all 1's for 4 bitplanes enabled); note however that the blitter too has modulo registers (BLTA/B/CMOD for each input and BLTDMOD for the destination) so perhaps best examine how fancy the programmer got with it (eg. multiple blits to clear the screen to a specific color even if the bitplanes are interleaved using BLTDMOD.) Obviously I'm a bit rusty given it's been a few years, but I hope this helps. I'm a fan of your work, thank you.

fabiensanglard 7y ago
Thank you so much, I am going to amend the "fill" section. Are you ok with being credited as "quincunx" or you prefer something else?
- quincunx 7y ago
  That's very kind, quincunx is fine, or no credit is fine too. Glad I could be of help.
fabiensanglard 7y ago
I had no idea about the BPL1MOD and BPL2MOD. Now that i know they exist I don't see why a game developer would not use them. I will try to double check with Eric Chahi but this is so much more efficient that it has to be the way he drew polygons (instead of drawing into a scrap buffer and blitting four times, he used BPL*MOD to draw and blit only once).
- Flow 7y ago
  Unless I'm wrong, a drawback of using interleaved bitplanes to do fewer but larger blits is that the mask for the sprite shape gets bigger too. If you do several blits, one per bitplane, you get to use the same mask data, but when using interleaved data the mask needs the same shape data * number of bitplanes involved. It's still a pretty small price to pay compared to Atari ST/ZX Spectrum preshifted bitmaps.
  - fabiensanglard 7y ago
    Does that mean you cannot use the Blitter to draw lines and fill area as described in the article?
    - quincunx 7y ago
      You'd use a single multi-bitplane interleaved fill-blit if the background surrounding it did not need to be preserved (eg. no mask requirements as per Flow - demos tend to do this for its speed). I'd imagine Another World creates an off-screen single-bit deep bitmap and uses that as a mask. I sent you an email if you'd like to explore more, however the original programmer probably just remembers what he actually did. Also note that non-overlapping polygons can be grouped together in both approaches by line-blitting with XOR destination (adjacent polygons share an edge and so the edge is XOR blitted twice); so there are substantial speed-ups to be had.

jacobush 7y ago

Half-brite was not in the Amiga 1000 and early Amiga 500 and 2000. The later “ECS” chipset has included “halfbrite”. Most games did not take advantage.

egypturnash 7y ago
Do you know of any games that used halfbrite? I can’t think of a single game that did. I think there were like three games that used HAM mode, if you filter out “displaying a scan of the box art while the rest of the game loads”. Knights of the Crystallon, Pioneer Plague, and maybe Mindscape?
- jacobush 7y ago
  At least Pinball Dreams did and googling suggests Cannon Fodder. I suppose a few more must be in there. Edit: https://retrocomputing.stackexchange.com/a/196/4616 https://retrocomputing.stackexchange.com/a/196/4616 Also The Settlers Black Crypt - http://hol.abime.net/126 http://hol.abime.net/126 Links - http://hol.abime.net/892 http://hol.abime.net/892 Universe - https://en.wikipedia.org/wiki/Universe_%281994_video_game%29 https://en.wikipedia.org/wiki/Universe_%281994_video_game%29
- Fr0styMatt88 7y ago
  Also interesting is the game 'Universe', which boasted achieving 256 colours on ECS (edit: a comment on the StackOverflow page linked below says it works on OCS as well). Some quick Googling before asking you guys about it as I was planning to turned up this: https://retrocomputing.stackexchange.com/questions/44/how-to-obtain-256-arbitrary-colors-with-limitation-of-64-per-line-in-amiga-ecs https://retrocomputing.stackexchange.com/questions/44/how-to... The Amiga was such an interesting machine. It's too bad I never got into assembly programming back then.
- krige 7y ago
  I know of one game that actually used HAM (specifically HAM6, the original OCS version) during gameplay and it seemed to run very fast as well - it was a PD game called Zdzisław Bohater Galaktyki 3D and it's, well, not very serious [0]. [0] https://www.youtube.com/watch?v=IV5UPk8ZQZw https://www.youtube.com/watch?v=IV5UPk8ZQZw

vidarh 7y ago

To maximize performance you'd tend to combine the blitter with a M68k loop, typically using MOVEM to maximize the number of bytes copied per instruction. The reason for this is that the blitter can not utilize every single memory cycle. So if you want to maximize graphics throughput you'll need to carefully balance the work between the blitter and CPU depending on which CPU.

Flow 7y ago
There's also a NASTY bit you can set so the Blitter gets the bus cycles it want, any CPU access will have to wait for a spare cycle. I'm not sure how much faster using that bit will make Blitter graphic operations in practise. I suppose when using 6 bitplanes a small speedup of like 10-20% could be seen.

deleted 7y ago

[deleted]

agumonkey 7y ago

so fun to see both of you discuss in depth like old magicians