A halo is a white glow around objects like trees, buildings, mountains etc. It is usually formed when you tone map an image. Now you might be thinking what tone mapping is. You must have come across this term if you have done HDR photography. Well, tone mapping is the process of reducing the contrast ratio (Brightness/darkness) of a photograph. It is usually done for expanding the dynamic range or bringing out the details in the shadows and highlight regions in a photograph.