Steinar Midtskogen

The Thor Video Codec

The Thor video codec is being developed by Cisco with the intention to use only royalty-free coding tools. The video codec is based on the well-known block-based hybrid video coding approach, but with some significant changes compared to existing standards such as AVC/H.264 and HEVC/H.265. These changes include transform coefficient coding, deblocking filter, a separate deringing filter, and interpolated reference frames. Experimental results show 14.5% and 15.3% BDR loss compared to the HEVC/H.265 reference software for low-delay and high-delay configurations respectively. When compared to optimized software implementations of VP9 and x265, Thor provides similar or better compression performance for the same frame rate.

Recent improvements to Thor with emphasis on perceptual coding tools

Thor supports a number of coding tools that have the potential to improve perceptual as well as objective quality. Synthetic reference frames may be used to support high frame rate applications in circumstances where encoders might typically transmit reduced frame rate content. Quantization matrices can be used to give improved visual quality by allocating bits more closely according to perceptual significance. Thor’s Constrained Low Pass Loop Filter provides significant subjective benefit in removing coding artefacts such as ringing. Improved colour fidelity can also be obtained by leveraging luma information. This paper discusses developments in these tools and their objective and subjective performance.