Image Redaction Tool

function processImage(originalImg, redactionAreas = "", redactionColor = "black") {
    // 1. Create a canvas element
    const canvas = document.createElement('canvas');

    // 2. Set the canvas dimensions to match the originalImg
    // Assumes originalImg is a loaded JavaScript Image object (e.g., from new Image() or <img> tag custom_image_data_attributes.onload has fired)
    canvas.width = originalImg.naturalWidth;
    canvas.height = originalImg.naturalHeight;

    // Handle cases where the image might not be loaded or has no dimensions
    if (canvas.width === 0 || canvas.height === 0) {
        console.warn("Original image has zero dimensions. Make sure the image is loaded and valid before processing.");
        // Return an empty canvas or null. Null might be better to indicate failure.
        // Or, return a 0x0 canvas, but it won't be useful.
        const errorCanvas = document.createElement('canvas');
        errorCanvas.width = 0;
        errorCanvas.height = 0;
        // Optionally, you could draw some error text on a small fixed-size canvas if that's desired.
        // For now, returning the 0x0 canvas to fulfill "must return a single canvas"
        return errorCanvas;
    }

    // 3. Get the 2D rendering context
    const ctx = canvas.getContext('2d');
    if (!ctx) {
        console.error("Could not get 2D rendering context for the image redaction tool.");
        // If context cannot be obtained, we cannot proceed.
        // Return the canvas as is (it will be blank and have dimensions).
        return canvas;
    }

    // 4. Draw the originalImg onto the canvas
    try {
        ctx.drawImage(originalImg, 0, 0, canvas.width, canvas.height);
    } catch (e) {
        console.error("Error drawing the original image onto the canvas. This might be due to CORS restrictions if the image is from a different origin.", e);
        // If drawing the original image fails, we can still draw redaction marks on a blank canvas,
        // or return the canvas as is (blank), or return null.
        // For this implementation, we'll proceed to draw redaction marks on what might be a blank canvas.
        // Alternatively, to signal a more critical failure:
        // return canvas; // Returns a blank canvas of correct size
    }

    // 5. Parse the redactionAreas string and apply redactions
    const trimmedRedactionAreas = redactionAreas.trim();
    if (trimmedRedactionAreas !== "") {
        const areas = trimmedRedactionAreas.split(';');
        areas.forEach(areaStrInput => {
            const areaStr = areaStrInput.trim();
            if (areaStr === "") { // Skip empty segments caused by "foo;;bar"
                return;
            }

            const parts = areaStr.split(',').map(s => {
                const num = Number(s.trim());
                return isNaN(num) ? NaN : num; // Ensure non-numeric strings become NaN
            });

            if (parts.length === 4 && parts.every(p => !isNaN(p))) {
                const [x, y, w, h] = parts;

                // Validate width and height (must be positive)
                if (w <= 0 || h <= 0) {
                    console.warn(`Invalid redaction area dimensions: "${areaStr}". Width and height must be positive. Skipping this area.`);
                    return; // Skip this invalid area
                }

                // Set the fill style to redactionColor
                // The browser handles invalid color strings gracefully (usually defaults to black or ignores the change)
                ctx.fillStyle = redactionColor;

                // Draw a filled rectangle
                ctx.fillRect(x, y, w, h);
            } else {
                console.warn(`Invalid redaction area format or non-numeric values: "${areaStr}". Expected "x,y,w,h" with numeric values. Skipping this area.`);
            }
        });
    }

    // 7. Return the canvas with the redacted image
    return canvas;
}