14

Computer and game consoles switched gradually from single-bitplane graphics (2 colours per character) to 2-bitplane graphics (4 colours per character), to eventually 4-bitplane graphics (16 colours per character).

Some early Super NES games internally uses 3-bitplane graphics (allowing 8 colours per character, and a graphical quality between 2 and 4-bitplane graphics) and de-compresses it to 4-bitplanes in order to save ROM. I wonder if any system ever used 3-bitplane graphics natively.

  • Are text mode implementations doing just that offtopic for you? And CGA's RGBI mimics that were arguably not true 4 bit color? – rackandboneman Apr 27 '17 at 22:13
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    I'm not sure you're using the term "bitplane" correctly here. A bitplane is a a two dimensional array of bits, just like a 2 colour bitmap. The difference is while a bitmap is the complete image, a bitplane is just one "plane" of an image. Each plane holds only one bit of colour. A N-bitplane image is made up of N separate bitplanes, each stored separately that when combined form the image. Most computers and game consoles didn't use this format. Instead they packed 2 or 4 bits together sequentially in some form, instead of taking each bit from separate planes. – Ross Ridge Apr 29 '17 at 6:47
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    @RossRidge that's right. In fact, I don't think there were any consoles that supported true bitplanes other than the CD-32 which was just an Amiga. – cbmeeks Sep 5 '17 at 17:28
  • Some arcade boards such as Robotron 2084 (and similar hardware like Defender, Joust, and others) used a RRRGGGBB type scheme in 8 bits. – LawrenceC Jun 17 at 22:13
23

The Amiga hardware supported only bitplane graphics, and you could have as many or as few bitplanes as you desired, up to certain bandwidth and archtectural limits. A neat trick of the Amiga's video hardware is to superimpose two three-bitplane screens and scroll them independently, which is obviously pretty useful for games.

I tended to use an eight-colour Workbench on my old Amiga 4000, as that was a reasonable compromise between having enough colours to render pretty icons, and yet not use up so much precious video bandwidth that window movements were slow.

  • 4
    Also, IIRC, the Amiga could scroll individual bitplanes if using a well-timed copper list. This technique was used in games like Agony. Basically, one bitplane (2 color) was used statically but the one color was changed at almost every scanline while the other two bitplanes scrolled. This was then stacked behind the other three bitplanes for what looked like three layers of scrolling. The Amiga is such an awesome computer. :-) – cbmeeks May 3 '17 at 15:27
9

Windows 1.x and 2.x use only three of the four planes on an EGA or VGA, because they use a 1-bit true-colour model with dithering.

5

While not exactly using "bitplane graphics" in a strict sense (see @RossRidge's comment), the Sinclair QL used 3 bits/pixel in its "hi"-color mode (8 colors + a flash bit). I do suspect, however, that this was rather due to a "design accident" than done deliberately: They ran out of pins on the video ULA in the design process and couldn't get the 4th colour pin out to the DAC, thus ended up using the "4th bit plane" for a rather useless flash (blinking) mode that didn't need a separate ULA pin.

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    The BBC Micro similarly had at most 8 colors + flash bit, due to its choice of digital RGB monitors with only 1 bit per color channel, though it used 1, 2 or 4 bits per pixel with 'chunky' rather than bitplane graphics. – user3570736 Apr 30 '17 at 9:39
  • Assuming it followed the Sinclair Spectrum model it used 8 bits for an attribute block (8x8 pixels) - 3 for foreground colour, 3 for background, 1 for bright and 1 for flash - and then used 1 bit per pixel for foreground/background, so depending how you want to count, 1.25 bits per pixel, four thirds of a bit per colour channel or seven bit colour – JCRM Feb 5 '18 at 0:47
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    @JCRM Th QL didn't follow the Spectrum model of a separate attribute plane. Each pixel had its own color, either 2 or 4 bits/pixel. – tofro Feb 5 '18 at 7:08
4

Another oddity: The Programmed Symbols card for the IBM 3270 PC has three three-plane fonts, giving 768 8-colour tiles that can be used to create graphical images.

3

The Compucolor II, from 1977, had an 8-color (3-bit) display, arranged as 32 lines of 64 characters.

Graphic resolution was 128x128, implying a graphics mode that displayed 8 pixels (4 x 2) in each character cell. I seem to remember that color could only be specified per character cell, which meant all 8 pixels in a cell had to be the same color. So, there weren't three bitmapped planes, but a three-bit attribute array specifying the color of each character cell.

3

The Sanyo MBC550 had a 640x200 bitmapped display with 3 planes.

The MBC video RAM is composed of three planes - green, red and blue. The green plane occupies main memory, and its position varies; writes to port 10h set its address:

Value | Address
======+========
    4 | 0C000h
    5 | 1C000h     (other values have not been tested)
    6 | 2C000h
    7 | 3C000h
======+========

The red and blue planes appear to have fixed locations of F0000h and F4000h respectively.

3

PP01 had 3-plane videoRAM organization: red, green, blue regions, 8KB each (resolution 256×256 pixels), pixel addressable.

The access was done cleverly in two modes: either the whole videoRAM has been mapped into 64KB CPU address space, or (the default), only one block (thus saving addressable RAM), and writes to this block are copied to one or both other two blocks (selected by a "colo(u)r register" value)

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