ZX Spectrum 262144-color palette via 65 dithers per 8×8px RGBW quad subpixel
zx-spectrum-262144-color-palette-via-65-dithers-per-8x8px-rgbw-quad-subpixel-{= prefix of 24[+1 .Txt] Uri filenames}


DOCUMENTATION

• Simulation: 262144-color palette ('666'-bit or 18 bits = 1-64th of 24-bit true color).

• Application: on the vanilla/standard Sinclair ZX Spectrum computer/displays (16/48/128K), and of course on any system (retro or not) that supports pure RGBWK colors (red, green, blue, white, black).

• Relation: tested shortly after my same kind of easy/small dither-based project that made use of 8×24-pixel striped RGB subpixels (which could simulate an even larger palette with a higher [non-Speccy] display resolution, strictly via 192 dithers per RGB component).

• Conversion: in a spreadsheet: column A (input): R,G,B;R… (lines A1,A2… (0-255 values)) | column B (Red): =Round(A1/4;) | column C (Green): =Round(A2/4;) | column D (Blue): =Round(A3/4;) | Column E (White): =Round(Min(A1:A3)/4;). The input (column A) can be e.g. pasted from an edited Ascii .Ppm file; the output (columns A-E) then copied as 4 Ascii .Pgm data values. The results are not great (even on the non-dithered normalized pre-versions), but it's the simplest method (and mitigating the whitening effect is beyond the scope of this retro demonstration and minimalist routine).

• Dithering: via 65-dither (0-dot + 1-to-64-dot dithers), 2×2-WRBG 8×8-pixel cells. WR (top) and BG (bottom) = diagonal (i.e. non-aligned) W & G components (which produces a checkered texture, hopefully less annoying than horizontal or vertical 'artifacts').

• Disclaimer: please bear in mind that most of my miscalculations are left to your sagacity (your forked/serious projects are quite welcome).


SOURCES, CREDITS, EDITIONS

My acknowledgement to the following people (besides and as usual, a considerable amount of uncited programmers that provided their useful tools or insight):

• Clive [photo]: 39-6021315dac9f55128a0.webp (2400×3563×24bpp size version), from Alamy Stock Photo (image #7457016, ID AC5NK9): “Clive Sinclair British entrepreneur inventor holding his newly developed ZX Spectrum home computer”, by Peter Jordan, Britain, on 1982-12-01 (Sir Clive Marles Sinclair was then aged 42), www.alamy.com/clive-sinclair-british-entrepreneur-inventor-holding-his-newly-developed-image7457016.html

• Speccy [photo]: ZXSpectrum48k.jpg (2168×1593×24bpp before my downsampling via the Lanczos filter), from Wikimedia Commons, by Bill Bertram on 2005-05-29, commons.wikimedia.org/wiki/File:ZXSpectrum48k.jpg

• Ara [photo]: RGB_24bits_palette_sample_image.jpg (150×200×24bpp (downsampled) thumbnail by Ricardo Cancho Niemietz on 2008-02-01 (even 2007), before my own downsampling via the Lanczos filter), commons.wikimedia.org/wiki/File:RGB_24bits_palette_sample_image.jpg, from the original Wikimedia Commons file: “Scarlet Macaw ('Ara macao') at Combe Martin Wildlife and Dinosaur Park, North Devon, England. Taken by Adrian Pingstone in July 2004 and released to the public domain. First upload in En Wikipedia on 16:53”, commons.wikimedia.org/wiki/File:Parrot.red.macaw.1.arp.750pix.jpg

• DBW [3D art]: dbw_glass.png (800×600×24bpp, after/from David B. Wecker on 1987 (Amiga 500) and John H. Lowery on 1989 (Ms-Dos)), a.k.a. DBW-Render glass (balls), by David Kinder on 2007 (Windows x86), davidkinder.co.uk/dbw.html

• Palette [2D map]: 262144colors.png (512×512×24bpp before my (rotated and) downscaled 4096-color sampled version, i.e. 64×64 pixels to fit the width of my collage), from this 262144-hue palette version: “Bitmap2LCD for GLCD”, by Bernie on 2017-04-19, bitmap2lcd.com/blog/tag/262144-colors

• Dithers [pixel art]: by dpla (with misc. contributions):
· dithering #1 (after the single 8×8-pixel legacy patterns freely featured in commons.wikimedia.org/wiki/File:ZX_Spectrum_standard_palette_with_8x8_dithering.png, by dpla on 2015-05-02)
· dithering #2 (after the single 16×16-pixel halftones by dpla on 2018-02-12: dpla.fr/images/pixel-art/minimalism/256-1-bit-16x16-dot-halftones-by-dpla-ani.gif, remade as single 8×8-pixel counterparts with a quartered clockwise progression on 2021-02-08)
· dithering #3 (4 text characters from Sinclair Research back to their ZX81 on 1981-03-05, en.wikipedia.org/wiki/ZX81_character_set, weight edited/animated and grouped in 4-character, Ascii art alike cells by dpla on 2021-02-08)
· dithering #4 (22 single and Ascii art text characters mixed from Sinclair Research's ZX81 on 1981-03-05, en.wikipedia.org/wiki/ZX81_character_set, and ZX Spectrum on 1982-04-23, en.wikipedia.org/wiki/ZX_Spectrum, plus 11 single value-filling gaming 'sprites' or patterns by dpla on 2021-02-09)
· dithering #5 (after the rounded and twice-shorter gradient by dpla on 2017-06-27, dpla.fr/images/pixel-art/minimalism/32-solid-8x8-gradient-in-1-bit-by-dpla.png, remade as strict and 4-mirror diagonals on 2021-02-08)
NB These different types of ordered dithers are just provided as personal suggestions for the occasion (since this limited choice cannot fit any valuable study, either)

• Title [pixel font]: “Spectrum” in all capitals (after the identical ZX81 uppercase character set by Sinclair Research, Britain, on 1981-03-05, commons.wikimedia.org/wiki/File:ZX81_characters_0x00-3F,_0x80-BF.png), at 1:1 size, from the ROM 8×8-pixel character set that the same authors recycled on 1982-04-23, commons.wikimedia.org/wiki/File:ZX_Spectrum_character_set.png


AUTHOR

dpla 2021-04-01 (since 2021-01-25), France, dpla.fr