Intra Line Copy for HEVC Screen Content Intra-Picture Prediction

This paper presents an intra line copy (ILC) technique for HEVC screen content coding. It shares the same origin with two other prominent techniques, intra string copy (ISC) and intra block copy (IBC), in applying the notion of string matching to intra-frame coding. This work combines their merits in one scheme with both good compression performance and high regularity. Specifically, it forms a prediction of a coding block by decomposing it into horizontal or vertical lines of pixels and performing line-based predictions based on previously coded data from the current frame. To address the massive amounts of search operations, our fast search algorithm first searches in the horizontal and vertical directions, then checks line vector candidates from spatial and temporal neighbors, and finally references lines having an identical hash value as the prediction line. The resulting line vectors are further predicted adaptively to minimize their coding overhead. Extensive experiments based on SCM-4.0, which includes IBC as an integral component, show that ILC can provide an additional 4%-7% BD-rate savings when the search area extends to the entire frame and 3%-4% improvements with a restricted local search. Compared with ISC, it achieves comparable performance without all its complications from sequential string parsing.