Image Fomapan 100 Filter Effect Application

function processImage(originalImg, contrast = '1.2', grainAmount = '10') {
    const canvas = document.createElement('canvas');
    const ctx = canvas.getContext('2d');

    const imgWidth = originalImg.naturalWidth || originalImg.width;
    const imgHeight = originalImg.naturalHeight || originalImg.height;

    if (imgWidth === 0 || imgHeight === 0) {
        console.warn("Image has zero dimensions. Returning a 1x1 canvas.");
        // Create a minimal canvas to avoid errors downstream if size is 0
        canvas.width = 1;
        canvas.height = 1;
        // Optionally, fill it with a color or leave it transparent
        // ctx.fillStyle = 'rgba(0,0,0,0)'; // Transparent
        // ctx.fillRect(0,0,1,1);
        return canvas;
    }

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

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

    // Parse parameters, providing defaults if parsing fails or invalid
    let numContrast = parseFloat(contrast);
    if (isNaN(numContrast)) {
        numContrast = 1.2; // Default contrast value
    }

    let numGrainAmount = parseInt(grainAmount, 10);
    if (isNaN(numGrainAmount)) {
        numGrainAmount = 10; // Default grain amount
    }
    numGrainAmount = Math.max(0, numGrainAmount); // Ensure grain amount is not negative

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

    // Process each pixel
    for (let i = 0; i < data.length; i += 4) {
        const r = data[i];
        const g = data[i + 1];
        const b = data[i + 2];
        // Alpha data[i + 3] is preserved

        // 1. Convert to grayscale (luminosity method)
        // Formula: gray = 0.299*R + 0.587*G + 0.114*B
        let gray = 0.299 * r + 0.587 * g + 0.114 * b;

        // 2. Apply Contrast
        // Adjusts contrast around mid-gray (127.5).
        // A contrast factor of 1.0 means no change. Greater than 1.0 increases contrast.
        if (numContrast !== 1.0) { // Apply only if contrast factor is not neutral
            gray = (gray / 255.0 - 0.5) * numContrast + 0.5;
            gray *= 255.0;
        }
        
        // 3. Add Film Grain
        if (numGrainAmount > 0) {
            // Generate random noise. (Math.random() - 0.5) gives range [-0.5, 0.5).
            // So, noise is in range [-numGrainAmount/2, numGrainAmount/2).
            // E.g. if numGrainAmount is 10, noise is between -5 and +5.
            const noise = (Math.random() - 0.5) * numGrainAmount;
            gray += noise;
        }

        // Clamp the grayscale value to be within [0, 255]
        gray = Math.max(0, Math.min(255, gray));

        // Set the new RGB values (all to gray for B&W)
        data[i] = gray;
        data[i + 1] = gray;
        data[i + 2] = gray;
    }

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

    return canvas;
}