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I haven't been able to find an easily comprehensible description of the super hi-res, dithered 16-color, 640x200 mode of the Apple ][gs.

I can tell from the references I did locate, this is a hardware supported mode of the VGC. So, the dithering effect is done in the hardware, not via software.

It seems that only 2 bits-per-pixel are used to create the sixteen colors on screen, but some of the colors aren't dithered (text foreground and background, for instance).

How does this work, how do you program it, and what are the limitations on palettes and adjacent pixels?

9

The Apple IIgs Hardware reference explains it:

In 640-pixel mode, color selection is more complicated. The 640 pixels in each horizontal line occupy 160 adjacent bytes of memory, each byte representing 4 pixels that appear side-by-side on the screen. The 16 colors in the palette are divided into four groups of 4 colors each. The first pixel in each horizontal line can select one of 4 colors from the third group of 4 in the palette. The second pixel selects from the fourth group of 4 colors in the palette. The third pixel selects from the first group of 4 colors, and the fourth pixel selects from the second group, as shown in Table 4-21. The process repeats for each successive group of 4 pixels in a horizontal line. Thus, even though a given pixel can be one of 4 colors, different pixels in a line can take on any of the 16 colors in a palette.

------------------------------------------------------
                 Palette                      Palette
Pixel    Value   color       Pixel    Value   color
------------------------------------------------------
3         0      0           1         0      8       
          1      1                     1      9       
          2      2                     2      10       
          3      3                     3      11       
4         0      4           2         0      12       
          1      5                     1      13       
          2      6                     2      14       
          3      7                     3      15       

The Apple IIgs Toolbox Reference Volume II explains the standard 640 mode palette:

------------------------------------------------------------------------
                       Minipalette                         Minipalette
Offset  Color   Value  offset       Offset  Color   Value  offset
------------------------------------------------------------------------
0       Black   000    0            8       Black   000    0            
1       Blue    00F    1            9       Blue    00F    1            
2       Yellow  FF0    2            10      Yellow  FF0    2            
3       White   FFF    3            11      White   FFF    3            
4       Black   000    0            12      Black   000    0            
5       Red     D00    1            13      Red     D00    1            
6       Green   0E0    2            14      Green   0E0    2            
7       White   FFF    3            15      White   FFF    3           

Note: The entries in the minipalettes for the standard 640-mode color table are set up so black and white appear in the same positions in each palette. This provides pure black and white at full 640 resolution, allowing crisper text display.

The standard color dither together to form these 16 colors:

------------------------------
Pixel                    Dithered
value   Name             components
-------------------------------
0       Black            black, black
1       Dark blue        black, blue
2       Olive            black, yellow
3       Grey1            black, white
4       Red              red, black
5       Purple           red, blue
6       Orange           red, yellow
7       Pink             red, white
8       Green            green, black
9       Aquamarine       green, blue
10      Lime green       green, yellow
11      Light green      green, white
12      Grey2            white, black
13      Cornflower blue  white, blue
14      Light yellow     white, yellow
15      White            white, white

Dithered colors Dithered colors, enlarged

To summarize, 320 mode uses a palette of 16 colors. 640 mode splits it up into 4 mini palettes of 4 colors. The palette used is based on the pixel position. With dithering, you can pretend to have 16 colors (but at a 320 resolution). With the standard desktop colors, black and white are available in all 4 mini palettes so they are available at any pixel for true 640 resolution.

  • I only count a total of 6 unique colors in the 16 color "Minipalette". – Brian H Nov 26 '18 at 1:12
  • 3
    The given set of colors is chosen to allow arbitrary black and white pictures to be displayed with 640x200 resolution, and allow any pictures that can be shown at all to be relocated to any even pixel boundary. The design allows programmers to trade resolution for color in whatever way is most useful. – supercat Nov 26 '18 at 1:37
  • I'm also seeing a 17th colour; a light grey appearing at the intersections between each 2x2 grid of colours – bodgit Nov 26 '18 at 14:48
  • @bodgit that's an optical illusion. See en.wikipedia.org/wiki/Grid_illusion – RETRAC Nov 26 '18 at 19:53

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