Lumi masking

Lumi masking is a technique used by video compression software, which reduces quality in very bright or very dark areas of the picture, as quality loss in these areas is less likely to be visible. It is also known as "psychovisual enhancements" or "adaptive quantization". The reduction in quality (and therefore bitrate) in certain areas of the picture caused by using lumi masking allows more bits to be allocated to the rest of the video, thus improving overall quality. Lumi masking is not perfect, however, and in some cases the degradation in quality it causes is visible.

Trivia

Lumi is Finnish for snow but in this context it is more likely to mean luma (or luminance) than the Finnish meaning.