C# Bitmap multiply colour blend

Multiply.cs

public static Bitmap Multiply(this Bitmap bitmap, byte r, byte g, byte b, PixelFormat format = PixelFormat.Format32bppArgb) {
    var size = new Rectangle(0,0, bitmap.Width, bitmap.Height);
    var bitmapData = bitmap.LockBits(size, ImageLockMode.ReadOnly, format);

    var buffer = new byte[bitmapData.Stride * bitmapData.Height];
    Marshal.Copy(bitmapData.Scan0, buffer, 0, buffer.Length);
    bitmap.UnlockBits(bitmapData);

    byte Calc(byte c1, byte c2) {
        var cr = c1 / 255d * c2 / 255d * 255d;
        return (byte) (cr > 255 ? 255 : cr);
    }

    for (var i = 0; i < buffer.Length ; i += 4) {
        buffer[i] = Calc(buffer[i], b);
        buffer[i + 1] = Calc(buffer[i + 1], g);
        buffer[i + 2] = Calc(buffer[i + 2], r);
    }

    var result = new Bitmap(bitmap.Width, bitmap.Height);
    var resultData = result.LockBits(size, ImageLockMode.WriteOnly, format);

    Marshal.Copy(buffer, 0, resultData.Scan0, buffer.Length);
    result.UnlockBits(resultData);

    return result;
}