To determine this, I used the Moscow University Video Quality Measurement tool. Specifically, Figure 4 shows a PSNR comparison of
the Tears of Steel test file in the results visualization screen. The top graph shows the PSNR
comparisons over the entire 3-minute test duration, while the bottom graph shows the detail from the black area in the top graph.
The results shown in red are from the starting point H.264 file, which was 31,348 KB in size.
The blue line tracks the results of the 80% optimized single file VP9 encode, which was 15,494
in size, for a total reduction of just over 50%. As
you can see, the two lines track closely throughout the graph, though the PSNR value of the VP9
file was 39. 86, about 1% higher than the starting
point of 39. 45. That’s pretty impressive.
To Chunk or Not to Chunk?
Let’s briefly revisit the issue of why chunking
worked better for VP9 than for H.264. My guess
is that the rate control mechanism for x264 is
simply better than that in VP9. That is, when en-
coding with 300% constrained VBR, the x264 en-
coder identified the hard-to-encode and easy-to-
encode sections and doled out the data rate as
efficiently for the entire file as it did when work-
ing with the individual segments. With VP9, the
encoder was more efficient working with the
individual segments, which may indicate that
it wasn’t quite as capable in spreading the data
rate out over the longer file.
Should you try the segmented approach
with your own H.264 encoder? Definitely. Even
though segments of 15 seconds didn’t improve
the results, you might achieve superior results
with 6- or 10-second segments. Clearly, if you’re
working with VP9, encoding in segments should
definitely be incorporated.
As a final note, please take all of these techniques as suggestions rather than as gospel.
Try them on your own encoding platforms, but
test, test, test (and then test some more) before
actually deploying them in your download or
streaming delivery systems.
Jan Ozer ( firstname.lastname@example.org) is a streaming
media producer and consultant, a frequent contributor to industry
magazines and websites on streaming-related topics, and the
author of Video Encoding by the Numbers. He blogs frequently at
Comments? Email us at email@example.com, or check
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Comparing the starting
point H.264 file with a
VP9 file at about 50%
of the data rate