commit e5a9c68d0529562195807f1251329a0f24827892
Author: Brion Vibber <brion@pobox.com>
Date:   Sat Nov 5 17:57:34 2022 -0700

    initial commit

diff --git a/dither4.js b/dither4.js
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index 0000000..9cff868
--- /dev/null
+++ b/dither4.js
@@ -0,0 +1,505 @@
+class RGB {
+    constructor(r, g, b) {
+        this.r = r;
+        this.g = g;
+        this.b = b;
+    }
+
+    static fromHex(val) {
+        let r = val & 0xff;
+        let g = (val >> 8) & 0xff;
+        let b = (val >> 16) & 0xff;
+        return new RGB(r,g,b);
+    }
+
+    add(other) {
+        return new RGB(
+            this.r + other.r,
+            this.g + other.g,
+            this.b + other.b
+        );
+    }
+
+    difference(other) {
+        return new RGB(
+            this.r - other.r,
+            this.g - other.g,
+            this.b - other.b
+        );
+    }
+
+    magnitude() {
+        return Math.sqrt(
+            this.r * this.r +
+            this.g * this.g +
+            this.b * this.b
+        );
+    }
+    
+    distance(other) {
+        return this.difference(other).magnitude();
+    }
+}
+
+// snarfed from https://lospec.com/palette-list/atari-8-bit-family-gtia
+// which was calculated with Retrospecs App's Atari 800 emulator
+let palette256 = [
+    0x000000,
+    0x111111,
+    0x222222,
+    0x333333,
+    0x444444,
+    0x555555,
+    0x666666,
+    0x777777,
+    0x888888,
+    0x999999,
+    0xaaaaaa,
+    0xbbbbbb,
+    0xcccccc,
+    0xdddddd,
+    0xeeeeee,
+    0xffffff,
+    0x190700,
+    0x2a1800,
+    0x3b2900,
+    0x4c3a00,
+    0x5d4b00,
+    0x6e5c00,
+    0x7f6d00,
+    0x907e09,
+    0xa18f1a,
+    0xb3a02b,
+    0xc3b13c,
+    0xd4c24d,
+    0xe5d35e,
+    0xf7e46f,
+    0xfff582,
+    0xffff96,
+    0x310000,
+    0x3f0000,
+    0x531700,
+    0x642800,
+    0x753900,
+    0x864a00,
+    0x975b0a,
+    0xa86c1b,
+    0xb97d2c,
+    0xca8e3d,
+    0xdb9f4e,
+    0xecb05f,
+    0xfdc170,
+    0xffd285,
+    0xffe39c,
+    0xfff4b2,
+    0x420404,
+    0x4f0000,
+    0x600800,
+    0x711900,
+    0x822a0d,
+    0x933b1e,
+    0xa44c2f,
+    0xb55d40,
+    0xc66e51,
+    0xd77f62,
+    0xe89073,
+    0xf9a183,
+    0xffb298,
+    0xffc3ae,
+    0xffd4c4,
+    0xffe5da,
+    0x410103,
+    0x50000f,
+    0x61001b,
+    0x720f2b,
+    0x83203c,
+    0x94314d,
+    0xa5425e,
+    0xb6536f,
+    0xc76480,
+    0xd87591,
+    0xe986a2,
+    0xfa97b3,
+    0xffa8c8,
+    0xffb9de,
+    0xffcaef,
+    0xfbdcf6,
+    0x330035,
+    0x440041,
+    0x55004c,
+    0x660c5c,
+    0x771d6d,
+    0x882e7e,
+    0x993f8f,
+    0xaa50a0,
+    0xbb61b1,
+    0xcc72c2,
+    0xdd83d3,
+    0xee94e4,
+    0xffa5e4,
+    0xffb6e9,
+    0xffc7ee,
+    0xffd8f3,
+    0x1d005c,
+    0x2e0068,
+    0x400074,
+    0x511084,
+    0x622195,
+    0x7332a6,
+    0x8443b7,
+    0x9554c8,
+    0xa665d9,
+    0xb776ea,
+    0xc887eb,
+    0xd998eb,
+    0xe9a9ec,
+    0xfbbaeb,
+    0xffcbef,
+    0xffdff9,
+    0x020071,
+    0x13007d,
+    0x240b8c,
+    0x351c9d,
+    0x462dae,
+    0x573ebf,
+    0x684fd0,
+    0x7960e1,
+    0x8a71f2,
+    0x9b82f7,
+    0xac93f7,
+    0xbda4f7,
+    0xceb5f7,
+    0xdfc6f7,
+    0xf0d7f7,
+    0xffe8f8,
+    0x000068,
+    0x000a7c,
+    0x081b90,
+    0x192ca1,
+    0x2a3db2,
+    0x3b4ec3,
+    0x4c5fd4,
+    0x5d70e5,
+    0x6e81f6,
+    0x7f92ff,
+    0x90a3ff,
+    0xa1b4ff,
+    0xb2c5ff,
+    0xc3d6ff,
+    0xd4e7ff,
+    0xe5f8ff,
+    0x000a4d,
+    0x001b63,
+    0x002c79,
+    0x023d8f,
+    0x134ea0,
+    0x245fb1,
+    0x3570c2,
+    0x4681d3,
+    0x5792e4,
+    0x68a3f5,
+    0x79b4ff,
+    0x8ac5ff,
+    0x9bd6ff,
+    0xace7ff,
+    0xbdf8ff,
+    0xceffff,
+    0x001a26,
+    0x002b3c,
+    0x003c52,
+    0x004d68,
+    0x065e7c,
+    0x176f8d,
+    0x28809e,
+    0x3991af,
+    0x4aa2c0,
+    0x5bb3d1,
+    0x6cc4e2,
+    0x7dd5f3,
+    0x8ee6ff,
+    0x9ff7ff,
+    0xb0ffff,
+    0xc1ffff,
+    0x01250a,
+    0x023610,
+    0x004622,
+    0x005738,
+    0x05684d,
+    0x16795e,
+    0x278a6f,
+    0x389b80,
+    0x49ac91,
+    0x5abda2,
+    0x6bceb3,
+    0x7cdfc4,
+    0x8df0d5,
+    0x9effe5,
+    0xaffff1,
+    0xc0fffd,
+    0x04260d,
+    0x043811,
+    0x054713,
+    0x005a1b,
+    0x106b1b,
+    0x217c2c,
+    0x328d3d,
+    0x439e4e,
+    0x54af5f,
+    0x65c070,
+    0x76d181,
+    0x87e292,
+    0x98f3a3,
+    0xa9ffb3,
+    0xbaffbf,
+    0xcbffcb,
+    0x00230a,
+    0x003510,
+    0x044613,
+    0x155613,
+    0x266713,
+    0x377813,
+    0x488914,
+    0x599a25,
+    0x6aab36,
+    0x7bbc47,
+    0x8ccd58,
+    0x9dde69,
+    0xaeef7a,
+    0xbfff8b,
+    0xd0ff97,
+    0xe1ffa3,
+    0x001707,
+    0x0e2808,
+    0x1f3908,
+    0x304a08,
+    0x415b08,
+    0x526c08,
+    0x637d08,
+    0x748e0d,
+    0x859f1e,
+    0x96b02f,
+    0xa7c140,
+    0xb8d251,
+    0xc9e362,
+    0xdaf473,
+    0xebff82,
+    0xfcff8e,
+    0x1b0701,
+    0x2c1801,
+    0x3c2900,
+    0x4d3b00,
+    0x5f4c00,
+    0x705e00,
+    0x816f00,
+    0x938009,
+    0xa4921a,
+    0xb2a02b,
+    0xc7b43d,
+    0xd8c64e,
+    0xead760,
+    0xf6e46f,
+    0xfffa84,
+    0xffff99,
+].map((hex) => RGB.fromHex(hex));
+
+function decimate(input, palette, n) {
+    // to brute-force, the possible palettes are:
+    // 255 * 254 * 253 = 16,386,810
+    //
+    // we could brute force it but that's a lot :D
+    // but can do some bisection :D
+    //
+    // need a fitness metric.
+    // each pixel in the dithered line gives a distance
+    // sum/average them? median? maximum?
+    // summing evens out the ups/downs from dithering
+    // but doesn't distinguish between two close and two distant options
+    // consider median, 90th-percentile, and max of abs(distance)
+    // consider doing the distance for each channel?
+
+    let line = input.slice();
+
+    // Apply dithering with given palette
+    let dither = (palette) => {
+        let fitness = 0;
+        let error = {
+            right: new RGB(0, 0, 0),
+            red: new Float64Array(line.length),
+            green: new Float64Array(line.length),
+            blue: new Float64Array(line.length),
+        }
+
+        let output = new Int32Array(line.length);
+        let popularity = new Int32Array(palette.length);
+
+        // Try dithering with this palette.
+        for (let x = 0; x < line.length; x++) {
+            let rgb = line[x];
+            rgb = rgb.difference(error.right);
+
+            // find the closest possible color
+            let shortest = Infinity;
+            let pick = -1;
+
+            for (let i = 0; i < palette.length; i++) {
+                let diff = palette[i].difference(rgb);
+                let dist = diff.magnitude();
+                if (dist < shortest) {
+                    nextError = diff;
+                    shortest = dist;
+                    pick = i;
+                }
+            }
+
+            output[x] = pick;
+            popularity[pick]++;
+
+            error.right.r = nextError.r / 2;
+            error.right.g = nextError.g / 2;
+            error.right.b = nextError.b / 2;
+
+            if (x == 0) {
+                error.red[x - 1] += nextError.r / 8;
+                error.green[x - 1] += nextError.g / 8;
+                error.blue[x - 1] += nextError.b / 8;
+            } else {
+                error.red[x] += nextError.r / 8;
+                error.green[x] += nextError.g / 8;
+                error.blue[x] += nextError.b / 8;
+            }
+
+            error.red[x] += nextError.r / 4;
+            error.green[x] += nextError.g / 4;
+            error.blue[x] += nextError.b / 4;
+
+            error.red[x + 1] += nextError.r / 8;
+            error.green[x + 1] += nextError.g / 8;
+            error.blue[x + 1] += nextError.b / 8;
+
+            //fitness += error.r;
+            //fitness += error.g;
+            //fitness += error.b;
+        }
+        return {
+            output,
+            palette,
+            fitness,
+            popularity,
+            error
+        };
+    };
+
+    let start = Date.now();
+    let decimated = palette.slice();
+    while (decimated.length > n) {
+        let {popularity} = dither(decimated);
+
+        // Try dropping least used color on each iteration
+        let least = Infinity;
+        let pick = -1;
+        for (let i = 1; i < decimated.length; i++) {
+            if (popularity[i] < least) {
+                pick = i;
+                least = popularity[i];
+            }
+        }
+        let old = decimated.length;
+        decimated = decimated.filter((rgb, i) => {
+            if (i == 0) {
+                return true; // keep black always
+            }
+            if (i == pick) {
+                return false;
+            }
+            if (popularity[i] == 0) {
+                return false;
+            }
+            return true;
+        });
+        if (decimated.length >= old) {
+            throw new Error('logic error');
+        }
+    }
+    let delta = Date.now() - start;
+    console.log(`${delta}ms for line`);
+
+    // Palette fits
+    return dither(decimated);
+}
+
+function convert(source, sink) {
+
+    let width = 320;
+    let height = 192;
+    
+    let canvas = sink;
+    let ctx = canvas.getContext('2d');
+
+    // Draw the source image down, then grab it
+    // and re-draw it with custom palette & dither.
+
+    ctx.drawImage(source, 0, 0);
+
+    let imageData = ctx.getImageData(0, 0, width, height);
+    let {data} = imageData;
+    let nextError;
+
+    for (let y = 0; y < height; y++) {
+        let line = new Uint8Array(data.buffer, y * width * 4, width * 4);
+        let input = [];
+
+        // Note we take two pixels because we're using the 160-wide 4-color mode
+        for (let x = 0; x < width; x += 2) {
+            let i = x >> 1;
+            let rgb = new RGB(
+                (line[x * 4 + 0] + line[x * 4 + 4]) / 2,
+                (line[x * 4 + 1] + line[x * 4 + 5]) / 2,
+                (line[x * 4 + 2] + line[x * 4 + 6]) / 2
+            );
+            if (nextError) {
+                rgb.r -= nextError.red[i];
+                rgb.g -= nextError.green[i];
+                rgb.b -= nextError.blue[i];
+            } else {
+                if (y > 0) {
+                    debugger;
+                }
+            }
+            input.push(rgb);
+        }
+
+        let {output, palette, error} = decimate(input, palette256, 4);
+        nextError = error;
+
+        for (let x = 0; x < width; x++) {
+            let rgb = palette[output[x >> 1]];
+            line[x * 4 + 0] = rgb.r;
+            line[x * 4 + 1] = rgb.g;
+            line[x * 4 + 2] = rgb.b;
+            line[x * 4 + 3] = 0xff;
+        }
+    }
+
+    ctx.putImageData(imageData, 0, 0);
+}
+
+function run() {
+    for (let i = 0; i < 7; i++) {
+        let source = document.querySelector('#source' + i);
+        let sink = document.querySelector('#sink' + i);
+
+        let doit = () => convert(source, sink);
+
+        if (source.complete) {
+            doit();
+        } else {
+            source.addEventListener('load', doit);
+        }
+    }
+}
+
+if (document.readyState === 'loading') {
+    addEventListener('DOMContentLoaded', run);
+} else {
+    run();
+}
diff --git a/index.html b/index.html
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+<!DOCTYPE html>
+<html>
+    <head>
+        <meta charset="utf-8">
+        <title>Atari 4-color palette dither generator</title>
+        <style type="text/css">
+            body {
+                background-color: #4c5fd4;
+                color: #e5f8ff;
+                font-family: monospace;
+            }
+            img, canvas {
+                /*width: 640px;
+                height: 384px;*/
+                image-rendering: pixelated;
+            }
+        </style>
+        <script async src=dither4.js></script>
+    </head>
+    <body>
+        <img id=source0 with=320 height=192 src=sample0.jpg>
+        <canvas id=sink0 width=320 height=192></canvas>
+
+        <img id=source1 with=320 height=192 src=sample1.jpg>
+        <canvas id=sink1 width=320 height=192></canvas>
+
+        <img id=source2 with=320 height=192 src=sample2.jpg>
+        <canvas id=sink2 width=320 height=192></canvas>
+
+        <img id=source3 with=320 height=192 src=sample3.jpg>
+        <canvas id=sink3 width=320 height=192></canvas>
+
+        <img id=source4 with=320 height=192 src=sample4.jpg>
+        <canvas id=sink4 width=320 height=192></canvas>
+
+        <img id=source5 with=320 height=192 src=sample5.jpg>
+        <canvas id=sink5 width=320 height=192></canvas>
+
+        <img id=source6 with=320 height=192 src=sample6.jpg>
+        <canvas id=sink6 width=320 height=192></canvas>
+    </body>
+</html>
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