public GlowEffect (int radius = 6, int brightness = 10, int contrast = 10)
{
if (radius < 1 || radius > 20)
throw new ArgumentOutOfRangeException ("radius");
if (brightness < -100 || brightness > 100)
throw new ArgumentOutOfRangeException ("brightness");
if (contrast < -100 || contrast > 100)
throw new ArgumentOutOfRangeException ("contrast");
this.radius = radius;
this.brightness = brightness;
this.contrast = contrast;
blur_effect = new GaussianBlurEffect (radius);
contrast_effect = new BrightnessContrastEffect (brightness, contrast);
screen_op = new ScreenBlendOp ();
}