Inverse mapping simplification

Reshaping can be used to better exploit the sample value distribution of video data. When reshaping is used inside the coding loop, forward mapping is applied to prediction signals from intra or inter prediction, and inverse mapping is applied to reconstructed signals. To simplify the inverse mapping operations, in one embodiment, the inverse mapping function is modelled as a piece-wise linear function, and each line segment is defined on an equi-distant interval of a length equal to a power of 2. In this embodiment, the inverse mapping function is specified by the number of pieces, initial offset value and the slope for each piece. In one example, the slope parameters can be signaled directly in the bitstream. In another example, an initial estimate of the slope parameters can be obtained from the forward mapping function, and the difference between the initial estimate and the actual parameters are signaled.