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.
So if you over do this tone mapping, halos are formed around high contrast regions. In other words, it is formed where the dark pixels meet the white pixels in a photograph. Thus, the best way to avoid halos in a photograph is to apply minimal tone mapping. When you tone map an image, you should always keep an eye on this — You should process the images in such a way that halos are not formed. Home Page -http://skl.sh/2ryUqYO 
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