Bloom is a post processing effect that blurs bright parts of the HDR render to achieve a glowing visual effect. This allows objects brighter than the traditional brightest color, RGB 255 255 255, to appear even brighter by affecting neighboring pixels. This effect is usually implemented with a Gaussian Blur after a luminosity threshold step.

It is difficult to apply bloom during the main rendering process because the Gaussian Blur layers require access to neighboring pixel's information, which might not be immediately available. Hence, bloom is most often implemented as a post processing effect, meaning it is applied after the majority of the render is completed. While post processing effects cannot directly read critical data about the scene and material, they can still use the depth buffer and other intermediate rendering passes which provide sufficent information to efficently calculate or approximate many photorealistic effects like bloom.