Lossy digital video encoding works in a non-linear color space. In a non-linear color space, the digital code words are not proportional to the light output. In a linear color space, the digital code words that represent the video signal are proportional to the light output. This requirement arises because a non-linear color space more closely represents how the Human Visual System (HVS) responds to changes in the content and therefore allows more efficient encoding of the signal.
However, the drawback of working in a non-linear color space is that changes to the high frequency content of the picture can change the apparent brightness of the image at lower frequencies. As the compression level of the system increases, these changes to the low frequency content become more and more apparent. If the compression level was constant and the input content did not move, this artifact would not be that important. But as the compression level or the high frequency content changes, the low frequency brightness will change. These changes are seen as unnatural because the changes are not related to other changes in the scene, for example a light source. Furthermore, they are at lower frequencies and easily detected even at further than ideal viewing distances. Therefore, a need exists for an encoding system that allows the non-linear video space to be used for compression while also preventing shifts in the image brightness.