Image Transparent Line Editor

/**
 * Edits an image by making transparent lines across it, either horizontally or vertically.
 *
 * @param {Image} originalImg The original Image object to process. It's assumed this image is fully loaded.
 * @param {number} [skipCount=10] The number of solid lines to draw before drawing one transparent line.
 * @param {string} [direction='horizontal'] The direction of the lines. Accepts 'horizontal' or 'vertical'.
 * @returns {HTMLCanvasElement} A canvas element displaying the modified image with transparent lines.
 */
function processImage(originalImg, skipCount = 10, direction = 'horizontal') {
    // Create a new canvas element to work with.
    const canvas = document.createElement('canvas');
    const ctx = canvas.getContext('2d');

    // Set the canvas dimensions to match the original image.
    // Using naturalWidth/Height is robust as it reflects the intrinsic size of the image.
    canvas.width = originalImg.naturalWidth;
    canvas.height = originalImg.naturalHeight;

    // Draw the original image onto the canvas as the starting point.
    ctx.drawImage(originalImg, 0, 0, canvas.width, canvas.height);

    // Get the pixel data of the entire canvas.
    // A try-catch block handles potential security errors if the image is from a
    // different origin (which taints the canvas).
    let imageData;
    try {
        imageData = ctx.getImageData(0, 0, canvas.width, canvas.height);
    } catch (e) {
        console.error("Could not get image data. The image might be cross-origin.", e);
        // In case of an error, return the canvas with the original, unmodified image.
        return canvas;
    }

    const data = imageData.data; // The flat array of RGBA values for every pixel.
    const width = canvas.width;
    const height = canvas.height;

    // Ensure skipCount is a non-negative integer for predictable calculations.
    const safeSkipCount = Math.max(0, Math.floor(skipCount));
    
    // The pattern repeats every (N skipped lines + 1 transparent line).
    const cycleLength = safeSkipCount + 1;

    // Process the image based on the chosen line direction.
    if (direction === 'horizontal') {
        // Iterate through each row of pixels (y-coordinate).
        for (let y = 0; y < height; y++) {
            // Check if the current row should be made transparent.
            // This happens on the last line of each cycle (e.g., if skipCount=2, cycleLength=3, line 2 is transparent).
            // A row `y` is transparent if `y % cycleLength` equals `safeSkipCount`.
            if (y % cycleLength === safeSkipCount) {
                // If it's a transparent row, iterate through each pixel in that row.
                for (let x = 0; x < width; x++) {
                    // Calculate the index of the alpha component for the pixel at (x, y).
                    // Each pixel uses 4 array elements (R, G, B, A).
                    const alphaIndex = (y * width + x) * 4 + 3;
                    // Set the alpha value to 0 to make the pixel fully transparent.
                    data[alphaIndex] = 0;
                }
            }
        }
    } else if (direction === 'vertical') {
        // Iterate through each column of pixels (x-coordinate).
        for (let x = 0; x < width; x++) {
            // Check if the current column should be made transparent using the same modulo logic.
            if (x % cycleLength === safeSkipCount) {
                // If it's a transparent column, iterate through each pixel in that column.
                for (let y = 0; y < height; y++) {
                    // Calculate the index for the alpha component.
                    const alphaIndex = (y * width + x) * 4 + 3;
                    // Set the alpha value to 0.
                    data[alphaIndex] = 0;
                }
            }
        }
    }

    // After modifying the pixel data array, put it back onto the canvas.
    ctx.putImageData(imageData, 0, 0);

    // Return the canvas element, which now holds the edited image.
    return canvas;
}