Image Truth Tester

async function processImage(originalImg, jpegQuality = "0.90", errorMultiplier = "20") {
    // This function performs Error Level Analysis (ELA) to test if an image has been manipulated.
    // It compares the original image with a re-compressed JPEG version.
    
    const quality = parseFloat(jpegQuality);
    const multiplier = parseFloat(errorMultiplier);

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

    // Create a canvas and draw the original image
    const canvas = document.createElement('canvas');
    canvas.width = width;
    canvas.height = height;
    
    const ctx = canvas.getContext('2d');
    ctx.drawImage(originalImg, 0, 0);
    
    // Get image data of the original image
    const origData = ctx.getImageData(0, 0, width, height);
    
    // Compress the canvas output to a JPEG to introduce characteristic compression artifacts
    const dataURL = canvas.toDataURL('image/jpeg', quality);
    
    // Load the newly compressed JPEG image
    const compressedImg = new Image();
    await new Promise((resolve, reject) => {
        compressedImg.onload = resolve;
        compressedImg.onerror = reject;
        compressedImg.src = dataURL;
    });
    
    // Draw the compressed image to grab its pixel data
    ctx.clearRect(0, 0, width, height);
    ctx.drawImage(compressedImg, 0, 0);
    const compData = ctx.getImageData(0, 0, width, height);
    
    // Create a new canvas to display the Error Level Analysis result
    const outCanvas = document.createElement('canvas');
    outCanvas.width = width;
    outCanvas.height = height;
    const outCtx = outCanvas.getContext('2d');
    const outData = outCtx.createImageData(width, height);
    
    // Calculate the absolute difference between the original and compressed pixels
    for (let i = 0; i < origData.data.length; i += 4) {
        outData.data[i]     = Math.min(255, Math.abs(origData.data[i] - compData.data[i]) * multiplier);     // R
        outData.data[i+1]   = Math.min(255, Math.abs(origData.data[i+1] - compData.data[i+1]) * multiplier); // G
        outData.data[i+2]   = Math.min(255, Math.abs(origData.data[i+2] - compData.data[i+2]) * multiplier); // B
        outData.data[i+3]   = 255; // Alpha
    }
    
    // Write back the ELA data
    outCtx.putImageData(outData, 0, 0);
    
    return outCanvas;
}