Key Frame Distance=2 frames yields file size 32% of All-Intra?

[Angelo Lorenzo]

Well the key is VBR. If your file is low-motion then B and P frames are awarded less bitrate. If you try the test on a more kinetic piece of video, you’ll find varying results.

B and P frames can contain varying degrees of information, it’s entropy encoded; it encodes what portions of the image has changed. All-I is a full discrete image every frame.

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Angelo Lorenzo

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[Ivan Myles]

[Angelo Lorenzo] “All-I is a full discrete image every frame.”

That is why I don’t understand how reducing the number of I-frames by 50% reduces the file size by 68%. Consider one second of 24fps video:

24I = 50Mb
12I + 12B = 16Mb

Even if the B-frames don’t require any space, wouldn’t the remaining 12 I-Frames require 25Mb on average? Bit rate was not a constraint (300Mbps target) so why would the I-frames be compressed like this?

[Angelo Lorenzo]

The minutiae of h.264 is beyond me, but I wouldn’t be surprised if it’s attributed to some overall quality tuning changes once partial frames are turned on. Sadly, when you don’t have total control over all settings, most MPEG codecs apply all kinds of automatic tuning very liberally.

You may be better off posting this question on another forum, or in the compression techniques forum on here on CC.

——————–
Angelo Lorenzo

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[Ivan Myles]

Thanks for the reply, Angelo. It appears the AME H.264 encoder requires significantly more bitrate for All-Intra. QuickTime only uses about 20 Mbps to encode the test file compared to 50 Mbps for AME H.264. Data rates at other key frame distances seem to align reasonably well.

[Angelo Lorenzo]

AME uses MainConcepts’ h.264 encoder, as a point of reference, when selecting mp4 as the wrapper. It uses Quicktime when MOV is the wrapper. Just a bit of FYI so you know it’s not anything Adobe proprietary.

What site did you find the chart on?

——————–
Angelo Lorenzo

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[Ivan Myles]

Yeah, Expression Encoder uses MainConcept, too. It sets keyframe distance down to 0.1 seconds, so All-Intra files can’t be created above 15fps.

I generated the chart based on tests that have been running over night. Another batch of files is currently processing at different H.264 profiles and levels, but results so far are consistent with the chart.

[Ivan Myles]

[Angelo Lorenzo] “I wouldn’t be surprised if it’s attributed to some overall quality tuning changes once partial frames are turned on.”

I think you are right. After running the tests two more times it appears the codec is designed to render I-frames in greater detail when N=1. The following image shows sections of two consecutive frames magnified 400%. Accuracy decreases and blocking increases, even in the I-frames, when N>1.