8 ms·

I can't believe my eyes. 256 colors on CGA?! HOW?!

logicallee 12y ago
the most impressive ascii art effect I ever saw was during movement. the minute it stops the motion blur goes away. in effect, I concluded I was looking at temporal antialiasing-like effects. (forget which demo it was, some 3d thing.) I wonder how much you could improve the effect (i.e. the woman after the 4:00 minute mark) by making it move. From the way it's drawn I'm not sure if you could get even 2 fps though.
- ajenner 12y ago
  I wanted to do some more impressive animation with the 1K color mode, but ran out of time unfortunately. One tricky part about doing animation with this mode is that it suffers from the "CGA snow" bug (visual glitching when writing to video memory). To avoid that by writing to video memory only during the vertical retrace/overscan period means that it would take about a quarter of a second to update the entire screen.
  - TheLoneWolfling 12y ago
    Is the CGA snow bug deterministic? Or at least model-able?
    - ajenner 12y ago
      Totally deterministic. I think there's even an emulator which models it (though without a cycle-exact emulation of the CPU the snow won't be in the same places as on real hardware).
ekianjo 12y ago
The Amiga had such tricks as well to display 4096 colors at the same time on screen while it could only display up to 32 specs wise. Not sure how they do it on PC, but there's probably a way to achieve it as well.
- frooxie 12y ago
  The 4096 color HAM mode was always part of the spec.
  - ekianjo 12y ago
    yes, it is, but it can only be used in very specific conditions (you cannot have the colors where you want all over the place), it's a pure hack.
    - vardump 12y ago
      You could either set any pixel color to any of 16 palette entries OR hold red & green from left side pixel, modify blue, hold red & blue, modify green, etc. You could also change those 16 colors for every scanline (horizontal line). Overall there was a lot of control, but software such as Deluxe Paint did not support full possibilities of HAM mode. Oh and, btw, EHB (extra halfbrite) mode has 64 colors. 32 colors palette + second 32 colors with brightness halved. http://en.wikipedia.org/wiki/Amiga_Halfbrite_mode http://en.wikipedia.org/wiki/Amiga_Halfbrite_mode
- chipsy 12y ago
  CGA's "Composite" mode contains output artifacts, a trait also found in other early microcomputer graphics like those of the Apple II or Atari 800; with careful use of dithering, CGA can be expanded to a larger effective gamut[0] although most games wouldn't be as ambitious as this demo. It's not as well-known or used as the 4-color modes because it wasn't supported by later PC graphics adapters, nor was it in the spec of the standard monitors of the era. You can see this system struggle to reproduce the demo's intended look: [1] [0] Example: https://www.youtube.com/watch?v=TfVe9l77zLU https://www.youtube.com/watch?v=TfVe9l77zLU [1] https://www.youtube.com/watch?v=aibZKrXc8Nk https://www.youtube.com/watch?v=aibZKrXc8Nk
pja 12y ago
I believe you can manipulate the colour bleed between pixels on the composite output from a high resolution mode to achieve high-colour output. (They actually have a 1024-colour mode in part of the demo.) There are probably a bunch of weird limitations on the adjacent colours you can achieve, but it’s an impressive effect regardless! (Here’s the obligatory wikipedia page: https://en.wikipedia.org/wiki/Composite_artifact_colors https://en.wikipedia.org/wiki/Composite_artifact_colors It sounds like the demo developers have taken this effect & turned it up to 11.)
- Zardoz84 12y ago
  I remember that "extra colours" could be archived on a ZX Spectrum using the bleed of the RF or composite output.
draugadrotten 12y ago
On Atari ST, it was possible to change the palette colors in midflight, and using that trick to display more colors on screen. The technique was used by Spectrum 512 and Quantum Paint and many demos. "QPs 512 mode is pretty straightforward; its only color limitation being that it can display a maximum of 40 colors on a single scan line. Mode 4K uses a special technique called "interlacing" in order to display a supposed 4,096 "colors -- http://www.atarimagazines.com/startv3n2/quantumpaint.html http://www.atarimagazines.com/startv3n2/quantumpaint.html see also http://www.atari-wiki.com/index.php/ST_Picture_Formats http://www.atari-wiki.com/index.php/ST_Picture_Formats
- ghostberry 12y ago
  You could also go beyond that by changing the image 50/60 times a second, which would then blur together. Enabling you to display colours that aren't even in the ST's palette.
- aerique 12y ago
  I'm not aware of the specifics anymore but by using well-placed NOPs when drawing a scanline you could make the borders disappear on the Atari ST thus getting a higher resolution. This was one of the many advantages the Amiga had over the Atari ST: being able to use the whole screen while the ST had a screen like a letterboxed movie except on all four sides of the screen.
  - qrmn 12y ago
    Toggling the register to change between 50/60Hz mode at the right time, specifically, could reset the counter in the ST's Video Shifter and trick it to carry on drawing screen when it should have been outputting blank border. Top and bottom borders were however much easier, because you could open them with just one carefully-timed interrupt each (I used Timer-B, which was linked to horizontal blank and counted lines, if I remember!). Described as far back as the B.I.G. Demo (check the scrolltext). Opening the left and right borders however required doing it for each line, I recall, which uses a lot more CPU time. (Unless, of course, there's a trick I don't know!) Spectrum 512 uses NOP timings to swap the palette at regular intervals throughout the screen; the "4096 colour interlaced" mode just flickered between one colour and another on alternate blanks to give the visual impression of flickery intermediates (before the STe came out, which used the high bit of each nybble to actually have 4-bit-per-channel palettes of 16). That technique, in turn, came from the C64 scene, as did the border trick, though I think they wrote the screen address? What's old is new again: plenty of lower-end TN LCD panels pull the same colour trickery to fake 8-bit colour from 6-/7- bit panels (or, reportedly, 10-bit deep colour from 8-bit ones in some cases). This demo is crazy. I don't think CGA even gives them a VBL to hang off! Wonderful.

the most impressive ascii art effect I ever saw was during movement. the minute it stops the motion blur goes away. in effect, I concluded I was looking at temporal antialiasing-like effects. (forget which demo it was, some 3d thing.) I wonder how much you could improve the effect (i.e. the woman after the 4:00 minute mark) by making it move. From the way it's drawn I'm not sure if you could get even 2 fps though.

ajenner 12y ago
I wanted to do some more impressive animation with the 1K color mode, but ran out of time unfortunately. One tricky part about doing animation with this mode is that it suffers from the "CGA snow" bug (visual glitching when writing to video memory). To avoid that by writing to video memory only during the vertical retrace/overscan period means that it would take about a quarter of a second to update the entire screen.
- TheLoneWolfling 12y ago
  Is the CGA snow bug deterministic? Or at least model-able?
  - ajenner 12y ago
    Totally deterministic. I think there's even an emulator which models it (though without a cycle-exact emulation of the CPU the snow won't be in the same places as on real hardware).

ekianjo 12y ago

The Amiga had such tricks as well to display 4096 colors at the same time on screen while it could only display up to 32 specs wise. Not sure how they do it on PC, but there's probably a way to achieve it as well.

frooxie 12y ago
The 4096 color HAM mode was always part of the spec.
- ekianjo 12y ago
  yes, it is, but it can only be used in very specific conditions (you cannot have the colors where you want all over the place), it's a pure hack.
  - vardump 12y ago
    You could either set any pixel color to any of 16 palette entries OR hold red & green from left side pixel, modify blue, hold red & blue, modify green, etc. You could also change those 16 colors for every scanline (horizontal line). Overall there was a lot of control, but software such as Deluxe Paint did not support full possibilities of HAM mode. Oh and, btw, EHB (extra halfbrite) mode has 64 colors. 32 colors palette + second 32 colors with brightness halved. http://en.wikipedia.org/wiki/Amiga_Halfbrite_mode http://en.wikipedia.org/wiki/Amiga_Halfbrite_mode
chipsy 12y ago
CGA's "Composite" mode contains output artifacts, a trait also found in other early microcomputer graphics like those of the Apple II or Atari 800; with careful use of dithering, CGA can be expanded to a larger effective gamut[0] although most games wouldn't be as ambitious as this demo. It's not as well-known or used as the 4-color modes because it wasn't supported by later PC graphics adapters, nor was it in the spec of the standard monitors of the era. You can see this system struggle to reproduce the demo's intended look: [1] [0] Example: https://www.youtube.com/watch?v=TfVe9l77zLU https://www.youtube.com/watch?v=TfVe9l77zLU [1] https://www.youtube.com/watch?v=aibZKrXc8Nk https://www.youtube.com/watch?v=aibZKrXc8Nk

pja 12y ago

I believe you can manipulate the colour bleed between pixels on the composite output from a high resolution mode to achieve high-colour output. (They actually have a 1024-colour mode in part of the demo.) There are probably a bunch of weird limitations on the adjacent colours you can achieve, but it’s an impressive effect regardless! (Here’s the obligatory wikipedia page: https://en.wikipedia.org/wiki/Composite_artifact_colors https://en.wikipedia.org/wiki/Composite_artifact_colors It sounds like the demo developers have taken this effect & turned it up to 11.)

Zardoz84 12y ago
I remember that "extra colours" could be archived on a ZX Spectrum using the bleed of the RF or composite output.

draugadrotten 12y ago

On Atari ST, it was possible to change the palette colors in midflight, and using that trick to display more colors on screen. The technique was used by Spectrum 512 and Quantum Paint and many demos. "QPs 512 mode is pretty straightforward; its only color limitation being that it can display a maximum of 40 colors on a single scan line. Mode 4K uses a special technique called "interlacing" in order to display a supposed 4,096 "colors -- http://www.atarimagazines.com/startv3n2/quantumpaint.html http://www.atarimagazines.com/startv3n2/quantumpaint.html see also http://www.atari-wiki.com/index.php/ST_Picture_Formats http://www.atari-wiki.com/index.php/ST_Picture_Formats

ghostberry 12y ago
You could also go beyond that by changing the image 50/60 times a second, which would then blur together. Enabling you to display colours that aren't even in the ST's palette.
aerique 12y ago
I'm not aware of the specifics anymore but by using well-placed NOPs when drawing a scanline you could make the borders disappear on the Atari ST thus getting a higher resolution. This was one of the many advantages the Amiga had over the Atari ST: being able to use the whole screen while the ST had a screen like a letterboxed movie except on all four sides of the screen.
- qrmn 12y ago
  Toggling the register to change between 50/60Hz mode at the right time, specifically, could reset the counter in the ST's Video Shifter and trick it to carry on drawing screen when it should have been outputting blank border. Top and bottom borders were however much easier, because you could open them with just one carefully-timed interrupt each (I used Timer-B, which was linked to horizontal blank and counted lines, if I remember!). Described as far back as the B.I.G. Demo (check the scrolltext). Opening the left and right borders however required doing it for each line, I recall, which uses a lot more CPU time. (Unless, of course, there's a trick I don't know!) Spectrum 512 uses NOP timings to swap the palette at regular intervals throughout the screen; the "4096 colour interlaced" mode just flickered between one colour and another on alternate blanks to give the visual impression of flickery intermediates (before the STe came out, which used the high bit of each nybble to actually have 4-bit-per-channel palettes of 16). That technique, in turn, came from the C64 scene, as did the border trick, though I think they wrote the screen address? What's old is new again: plenty of lower-end TN LCD panels pull the same colour trickery to fake 8-bit colour from 6-/7- bit panels (or, reportedly, 10-bit deep colour from 8-bit ones in some cases). This demo is crazy. I don't think CGA even gives them a VBL to hang off! Wonderful.