Bloom

Image based lighting is the process of taking an image, and lightening and darkening areas of the image based of it's content. One highly used effect derived from this is the Bloom effect, which is used to create a dreamy and cloudy atmosphere. It is mostly utilized in outdoor levels.

The first step is to take an image:

Then use a color ramp to convert it to a luminosity map of the image:

The step after that is to blur the luminosity map:

Then, this image should be overlayed over the original image using a mix method (not add or subtract). Here is the final result:

This technique can be easily implemented in shaders and other postprocess methods. Another common technique is HDRR (high dynamic range rendering), which highlights specular areas, and grays out dull colors. This technique is similar to Bloom, except the luminosity map is not blurred.