Video encoding professionals should take note of four papers presented at the recent International Symposium on Electronic Imaging. Read on for a detailed assessment.
By Jan Ozer
The Society for Imaging Science and Technology hosts the annual International Symposium on Electronic Imaging, held this year in San Francisco, California, from February 14 to 18. The Symposium has eight tracks across a range of disciplines, where researchers from industry and academia present papers and findings.
A Subjective Study for the Design of Multi-Resolution ABR Video Streams With the VP9 Codec
One common problem facing encoding professionals is identifying when to switch between streams in an adaptive group. This paper, authored by Chao Chen, Sasi Inguva, and Anil Kokaram from YouTube/Google, presented a hybrid objective/subjective technique for identifying the appropriate data rate for switching stream resolutions. Though the experiment focused on the 4K/2K decision point using the VP9 codec, the technique can be used for any decision point and codec.
Adaptive streaming involves a group of encoding configurations at various resolution and data rate pairs. At each data rate in the ladder, the player has to choose the appropriate resolution. Intuitively, it’s the resolution that delivers the highest quality at that data rate, as shown in Figure 1, and Google broke no new ground in sharing this observation.
Figure 1. Theoretically, you want to switch resolutions to maximize quality throughout the encoding ladder.
As mentioned, Google’s focus was on the appropriate data rate to switch between 2K and 4K videos, and the short answer is that it’s between 4 Mbps and 5 Mbps when encoding with the VP9 codec. How Google got there is the interesting part.
Google selected 7966 4K videos uploaded to YouTube, created 2K versions, encoded both the 4K and 2K versions with VP9 at various data rates, and computed their Structural Similarity Index (SSIM) scores. Based upon these scores, the average switching rate between 2K and 4K was 4 Mbps. That is, below 4 Mbps, most 2K clips had a higher SSIM rating, while above 4 Mbps, the 4K videos had a higher SSIM rating.
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