Image To G Major 16 Color Filter Converter

function processImage(originalImg, applyGânicoInvert = "true", use16ColorPalette = "true") {
    const canvas = document.createElement('canvas');
    canvas.width = originalImg.width;
    canvas.height = originalImg.height;
    
    const ctx = canvas.getContext('2d');
    ctx.drawImage(originalImg, 0, 0);
    
    const imgData = ctx.getImageData(0, 0, canvas.width, canvas.height);
    const data = imgData.data;

    // Standard 16-color palette (VGA/EGA) heavily associated with retro 16-color filters
    const palette16 = [
        [0, 0, 0], [170, 0, 0], [0, 170, 0], [170, 85, 0],
        [0, 0, 170], [170, 0, 170], [0, 170, 170], [170, 170, 170],
        [85, 85, 85], [255, 85, 85], [85, 255, 85], [255, 255, 85],
        [85, 85, 255], [255, 85, 255], [85, 255, 255], [255, 255, 255]
    ];

    const doInvert = applyGânicoInvert === "true";
    const doPalette = use16ColorPalette === "true";

    for (let i = 0; i < data.length; i += 4) {
        let r = data[i];
        let g = data[i + 1];
        let b = data[i + 2];

        // 1. Apply "G Major" visual base effect (Inversion of colors)
        if (doInvert) {
            r = 255 - r;
            g = 255 - g;
            b = 255 - b;
        }

        // 2. Map exactly to 16 colors (16 Color Filter capability)
        if (doPalette) {
            let minDist = Infinity;
            let closest = palette16[0];

            for (let j = 0; j < palette16.length; j++) {
                const pr = palette16[j][0];
                const pg = palette16[j][1];
                const pb = palette16[j][2];

                // Fast Euclidean distance calculation in RGB space
                const dist = (r - pr) * (r - pr) + (g - pg) * (g - pg) + (b - pb) * (b - pb);
                if (dist < minDist) {
                    minDist = dist;
                    closest = palette16[j];
                }
            }
            
            r = closest[0];
            g = closest[1];
            b = closest[2];
        }

        // Reassign processed values back to array (Alpha channel remains untouched)
        data[i] = r;
        data[i + 1] = g;
        data[i + 2] = b;
    }

    ctx.putImageData(imgData, 0, 0);
    return canvas;
}