Image HSL Hue Adjuster

function processImage(originalImg, hueRotation = 0) {
    /**
     * Adjusts the hue of an image.
     * @param {Image} originalImg - The original javascript Image object.
     * @param {number} hueRotation - The amount to rotate the hue, in degrees (0-360).
     * @returns {HTMLCanvasElement} A canvas element with the hue-adjusted image.
     */

    // Create a canvas element
    const canvas = document.createElement('canvas');
    const ctx = canvas.getContext('2d');

    // Set canvas dimensions to match the original image
    canvas.width = originalImg.naturalWidth;
    canvas.height = originalImg.naturalHeight;

    // Draw the original image onto the canvas
    ctx.drawImage(originalImg, 0, 0);

    // Get the image data from the canvas
    const imageData = ctx.getImageData(0, 0, canvas.width, canvas.height);
    const data = imageData.data;

    // Helper function to convert RGB to HSL
    // Input: r, g, b in [0, 255]
    // Output: h, s, l in [0, 1]
    const rgbToHsl = (r, g, b) => {
        r /= 255;
        g /= 255;
        b /= 255;
        const max = Math.max(r, g, b);
        const min = Math.min(r, g, b);
        let h = 0, s, l = (max + min) / 2;

        if (max === min) {
            h = s = 0; // achromatic
        } else {
            const d = max - min;
            s = l > 0.5 ? d / (2 - max - min) : d / (max + min);
            switch (max) {
                case r:
                    h = (g - b) / d + (g < b ? 6 : 0);
                    break;
                case g:
                    h = (b - r) / d + 2;
                    break;
                case b:
                    h = (r - g) / d + 4;
                    break;
            }
            h /= 6;
        }
        return [h, s, l];
    };

    // Helper function to convert HSL to RGB
    // Input: h, s, l in [0, 1]
    // Output: r, g, b in [0, 255]
    const hslToRgb = (h, s, l) => {
        let r, g, b;
        if (s === 0) {
            r = g = b = l; // achromatic
        } else {
            const hue2rgb = (p, q, t) => {
                if (t < 0) t += 1;
                if (t > 1) t -= 1;
                if (t < 1 / 6) return p + (q - p) * 6 * t;
                if (t < 1 / 2) return q;
                if (t < 2 / 3) return p + (q - p) * (2 / 3 - t) * 6;
                return p;
            };
            const q = l < 0.5 ? l * (1 + s) : l + s - l * s;
            const p = 2 * l - q;
            r = hue2rgb(p, q, h + 1 / 3);
            g = hue2rgb(p, q, h);
            b = hue2rgb(p, q, h - 1 / 3);
        }
        return [Math.round(r * 255), Math.round(g * 255), Math.round(b * 255)];
    };

    // Convert the hue rotation from degrees to a value between 0 and 1
    const hueShift = parseFloat(hueRotation) / 360;

    // Iterate over each pixel
    for (let i = 0; i < data.length; i += 4) {
        // Get the RGB values for the current pixel
        const r = data[i];
        const g = data[i + 1];
        const b = data[i + 2];

        // Convert the pixel's color from RGB to HSL
        let [h, s, l] = rgbToHsl(r, g, b);

        // Adjust the hue
        h += hueShift;
        
        // Ensure hue wraps around correctly (remains within 0 to 1)
        if (h > 1) h -= 1;
        if (h < 0) h += 1;

        // Convert the modified HSL color back to RGB
        const [newR, newG, newB] = hslToRgb(h, s, l);

        // Update the pixel data with the new RGB values
        data[i] = newR;
        data[i + 1] = newG;
        data[i + 2] = newB;
    }

    // Put the modified image data back onto the canvas
    ctx.putImageData(imageData, 0, 0);

    // Return the canvas with the adjusted image
    return canvas;
}