Expanding an image with a brush

Dilute.cs

using SixLabors.ImageSharp;
using SixLabors.ImageSharp.Processing;
using SixLabors.ImageSharp.PixelFormats;
using Image = SixLabors.ImageSharp.Image;
using SixLabors.ImageSharp.Formats.Png;

class Program
{
    static readonly PngEncoder encoder = new()
    {
        ColorType = PngColorType.RgbWithAlpha,
        TransparentColorMode = PngTransparentColorMode.Preserve,
        BitDepth = PngBitDepth.Bit8,
        CompressionLevel = PngCompressionLevel.BestSpeed
    };

    static void Main()
    {
        using var original = Image.Load<Rgba32>("star.png"); // original
        using var circle = Image.Load<Rgba32>("circle.png"); // a circle brush with uneven size (9x9 i.e.)
        ApplyDilation(original, circle).SaveAsPng("star_expanded.png", encoder);
    }

    static Image<Rgba32> ApplyDilation(Image<Rgba32> image, Image<Rgba32> circle)
    {
        var expansion = circle.Width / 2;
        var newImage = image.Clone();
        var width = newImage.Width;
        var height = newImage.Height;

        newImage.Mutate(ctx =>
        {
            for (var y = 0; y < height; y++)
                for (var x = 0; x < width; x++)
                    newImage[x, y] = new Rgba32(255, 255, 255, 0);

            for (var y = expansion; y < height - expansion; y++)
                for (var x = expansion; x < width - expansion; x++)
                {
                    var baseA = image[x, y].A;
                    if (baseA < 64)
                        continue;

                    for (var dy = y - expansion; dy <= y + expansion; dy++)
                        for (var dx = x - expansion; dx <= x + expansion; dx++)
                        {
                            byte a = (byte)(baseA * circle[dx - x + expansion, dy - y + expansion].A / 255);
                            if (a > newImage[dx, dy].A)
                                newImage[dx, dy] = new Rgba32(255, 255, 255, a);
                        }
                }
        });

        return newImage;
    }
}