Filtering and dithering as pre-processing before encoding

Abstract

Techniques and tools are presented for controlling artifacts such as banding artifacts, ringing artifacts and film scan artifacts in video. For example, before encoding, a pre-processor performs combined filtering and dithering on video such that the weight of dithering at a location depends on the results of filtering at the location. For the combined filtering and dithering, the pre-processor can determine a lowpass signal and highpass residual, weight dithering based on local characteristics of the highpass residual, and then combine the lowpass signal with the weighted dithering. Or, to determine the relative weight, the pre-processor can use a filter whose normalization factor varies depending on how many sample values around a location are within a threshold of similarity to a current sample value at the location. The filtering and dithering can use different strengths for luma and chroma channels.