Image Glow Effect Applicator

function processImage(originalImg, glowColor = 'rgba(255, 255, 255, 0.7)', glowRadius = 10) {
    // Sanitize glowRadius: ensure it's a non-negative number.
    // Number() converts various types, including strings like "15", to numbers.
    // If conversion results in NaN (e.g., for "abc") or if it's negative, default to 0.
    let numericGlowRadius = Number(glowRadius);
    if (isNaN(numericGlowRadius) || numericGlowRadius < 0) {
        numericGlowRadius = 0;
    }
    // Use this sanitized, non-negative radius for all calculations.
    const actualGlowRadius = numericGlowRadius;

    const canvas = document.createElement('canvas');
    const ctx = canvas.getContext('2d');

    // Determine canvas dimensions. It needs to be large enough to contain the image
    // plus the glow effect extending outwards from the image.
    // The image will be drawn offset by actualGlowRadius from the top-left of the canvas.
    const canvasWidth = originalImg.naturalWidth + actualGlowRadius * 2;
    const canvasHeight = originalImg.naturalHeight + actualGlowRadius * 2;

    canvas.width = canvasWidth;
    canvas.height = canvasHeight;

    // If glowRadius is positive, set up the shadow properties for the glow effect.
    // The shadowColor property accepts CSS color strings, e.g., 'red', '#FF0000', 'rgba(255,0,0,0.5)'.
    // The shadowBlur property dictates how blurred the shadow is. Larger values mean more blur.
    if (actualGlowRadius > 0) {
        ctx.shadowColor = glowColor;
        ctx.shadowBlur = actualGlowRadius;
        
        // shadowOffsetX and shadowOffsetY control the shadow's position relative to the shape.
        // For a centered glow effect, these should be 0. They are 0 by default,
        // but setting them explicitly clarifies the intent.
        ctx.shadowOffsetX = 0;
        ctx.shadowOffsetY = 0;
    }

    // Draw the original image onto the canvas.
    // It's offset by actualGlowRadius in both x and y directions. This provides
    // space for the glow to appear around the image.
    // The canvas shadow properties (if set) will cause a shadow to be rendered
    // simultaneously with the image. The shadow is based on the alpha channel of the
    // image being drawn (i.e., transparent areas of the image won't cast a shadow).
    ctx.drawImage(
        originalImg,
        actualGlowRadius, // x-coordinate of the top-left corner of the image on the canvas
        actualGlowRadius, // y-coordinate of the top-left corner of the image on the canvas
        originalImg.naturalWidth,
        originalImg.naturalHeight
    );

    // The canvas now contains the image with the glow effect applied.
    return canvas;
}