https://dallasgirl79.tumblr.com/post/621050362120617984/shadow-map-tutorial-in-this-brief-tutorial-i-will
In this brief tutorial, I will explain how to make a shadow map that prevents these black lines/pixels and subsequently will fix the issue @grimcookies discussed about the crunchiness that happens with an incorrectly made shadow map. This is happening because the shadow map is probably a PNG – it must be in DDS file format with a proper alpha layer.