Image Lee 85B Warming Filter Effect Application

async function processImage(originalImg, intensity = 1.0) {
    const canvas = document.createElement('canvas');
    const ctx = canvas.getContext('2d');

    // Ensure originalImg is loaded, though recommendation is it's an Image object
    // For robust handling, one might check originalImg.complete
    const imgWidth = originalImg.naturalWidth || originalImg.width;
    const imgHeight = originalImg.naturalHeight || originalImg.height;

    if (imgWidth === 0 || imgHeight === 0) {
        // Handle cases where image dimensions are not available (e.g., image not loaded)
        // For this example, we'll return an empty canvas or throw an error.
        // Returning the canvas as is, potentially 0x0.
        console.error("Image dimensions are zero. Is the image loaded?");
        canvas.width = 0;
        canvas.height = 0;
        return canvas;
    }

    canvas.width = imgWidth;
    canvas.height = imgHeight;

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

    // Get image data
    let imageData;
    try {
        imageData = ctx.getImageData(0, 0, canvas.width, canvas.height);
    } catch (e) {
        // This can happen due to tainted canvas if image is cross-origin and CORS isn't set up
        console.error("Could not get ImageData:", e);
        // One might return the canvas with the original image drawn, without filter
        return canvas;
    }
    
    const data = imageData.data;

    // Parse and validate intensity parameter
    let s = parseFloat(intensity);
    // If intensity is not a valid number or out of 0-1 range, default to 1.0 (full effect)
    if (isNaN(s)) {
        s = 1.0;
    }
    s = Math.max(0.0, Math.min(1.0, s));


    // Lee 85B Warming Filter target multipliers (full effect)
    // These values aim to simulate an orange warming filter.
    // An 85B filter converts daylight (e.g., 5500K) to tungsten balance (e.g., 3200K),
    // resulting in a significantly warmer, orange-ish image.
    const rFactor85B = 1.02; // Slight boost to red to emphasize warmth
    const gFactor85B = 0.80; // Reduces green, shifting away from yellow towards orange
    const bFactor85B = 0.40; // Strong reduction in blue for a prominent orange cast

    // Calculate effective multipliers based on intensity
    // current_factor = (1 - s) * 1.0 (original) + s * factor_85B (full filter)
    const effectiveRFactor = (1 - s) + s * rFactor85B;
    const effectiveGFactor = (1 - s) + s * gFactor85B;
    const effectiveBFactor = (1 - s) + s * bFactor85B;

    for (let i = 0; i < data.length; i += 4) {
        const r = data[i];
        const g = data[i + 1];
        const b = data[i + 2];
        // Alpha channel (data[i+3]) remains unchanged

        data[i] = Math.min(255, r * effectiveRFactor);     // Red channel
        data[i + 1] = Math.min(255, g * effectiveGFactor); // Green channel
        data[i + 2] = Math.min(255, b * effectiveBFactor); // Blue channel
    }

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

    return canvas;
}