1. A method of decoding a bitstream, comprising: receiving a bitstream at a decoder; decoding said bitstream with said decoder into color values and metadata items indicating information about adaptive post-processing operations to be performed by said decoder; performing, with said decoder, one or more high dynamic range (HDR) adaptation operations on said color values based on said metadata items; and performing, with said decoder, one or more fixed post-processing operations to reconstruct an HDR video from said color values, wherein said one or more HDR adaptation operations convert said color values into a format expected by said one or more fixed post-processing operations, wherein one of said fixed post-processing operations is an inverse color space conversion operation and one of said HDR adaptation operations is a color space adaptation operation that converts said color values from an input color space to an output color space expected by said inverse color space conversion operation, wherein said color space adaptation operation first converts at least some components of said color values from said input color space into an intermediate linear color space and then from said intermediate linear color space into said output color space, wherein said input color space is a YCbCr color space and said output color space is a perceptually quantized (PQ) YCbCr color space, and said decoder: converts the Y luma component from said YCbCr color space to a PQ Y luma component for said PQ YCbCr color space using a luma conversion operation, converts the Y luma component, the Cb chroma component, and the Cr chroma component from said YCbCr color space to an intermediate PQ blue component using a blue conversion operation, converts the Y luma component, the Cb chroma component, and the Cr chroma component from said YCbCr color space to an intermediate PQ red component using a red conversion operation, converts the PQ Y luma component, the intermediate PQ blue component, and the intermediate PQ red component into a PQ Cb chroma component using a Cb conversion operation, and converts the PQ Y luma component, the intermediate PQ blue component, and the intermediate PQ red component into a PQ Cr chroma component using a Cr conversion operation, wherein said luma conversion operation, said blue conversion operation, said red conversion operation, said Cb conversion operation, and said Cr conversion operation are defined with mapping functions or lookup tables.