New content type to improve grain retention.
For new VP9 only content type adjust the rate distortion and ARF filter based on the relative spatial variance of the source and reconstruction. In regards to the RD loop the method favors modes where the reconstruction variance is similar to the source variance. However it is currently only applied to regions where the source variance is quite low. For very low variance blocks it applies a further bias against intra coding and large prediction block sizes (the later in particular limit the usefulness of the loop filter). The final part of this change is to lower the strength of the ARF filter for blocks where the source has very low spatial variance, to encourage some low amplitude texture or noise to pass through the filter. This change improves the retention of film grain and fine noise / texture in spatially flat regions, but as expected causes a significant drop in PSNR on many clips. This is to be expected because similar but misaligned noise or texture will give a lower PSNR than a flat noise free reconstruction. However, it is worth noting that most clips show a strong gain in FAST SSIM. The features are enabled on the vpxenc command line by setting --tune-content=film. VPX_ENCODER_ABI_VERSION bumped for this change and cvbr. Change-Id: I26a4e4edfa3dc5cacead82fa701fe7a9118ccd0a
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