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MPEG2 Bitrate & I-frames for WQXGA video via ffmpeg


pemo
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I have to encode 8 x WQGA (2560x1600) image streams (folders of .png images, containing up to 80,000 images per folder) for projection onto a planetarium dome.

 

We've been having a lot of problems with image stuttering & synch problems via Watchout.

 

The encode format is MPEG 2, encoded using ffmpeg, using the following settings:

 

ffmpeg -threads auto -r 30 -i "....path to files/0/%07d.png" -f vob -vcodec mpeg2video -b:v 30000k -minrate 30000k -maxrate 30000k -g 15 -bf 2 -an -trellis 2  "....path to files/outputfile_Display_0.m2v"

 

The 30000k settings are so to achieve a CBR sustainable by the system.

 

The GOP (-g flag) is 15, which is the maximum GOP size according to this recommended setting I read: http://forum.dataton.com/topic/288-mpeg-settings/?hl=%2Bmpeg+%2Bsettings

 

However, it also recommends only I-frames.

  1. - All I-Frames, sequence headers on every Group Of Pictures (GOP), GOP structure 15 frames or less 

which, I ~presume~ means setting the GOP (-g flag) to 1? Or is there a flag I'm missing? ffmpeg documentation is not so great, but I'll read anything sent my way.

 

This currently blows out the data rate with buffer underflows during encode at 30Mbps.  With -g 15 it is fine, but stutters and causes a strange "pulse" or "heartbeat" artefact where the image goes fuzzy every 0.5 seconds (which correlates with a GOP of 15 and 30fps)

 

Am I missing something - can one have an all I-frame structure with a GOP larger than 1? If so, how?

 

Does anyone have experience with ffmpeg and WQXGA video? I assume we would have to increase our data rate to, say, 50+ Mbps for satisfactory results - but before I go down this path of processing a whole heap of data I thought it wisest to ask :)

 

The movies are being played on an array of 4 Dell Precision R7910 machines running Windows 8, 16GB RAM, using ATI GPUs & 256GB  SSDs - so 2 x WQXGA projector per machine.  I don't have the ATI specs to hand, but they are top-flight GPUs (replacing NVIDIA M6000s which had problems - so I am informed) & should handle high data rates just fine - I'm still figuring out the ceiling rate.

 

One more question: in your experience is there any difference between ffmpeg for Windows & ffmpeg for OS X? I would assume the mpeg2 library is the same under both platforms - but I could well be mistaken.

 

Thanks in advance.

 

 

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This currently blows out the data rate with buffer underflows during encode at 30Mbps.  With -g 15 it is fine, but stutters and causes a strange "pulse" or "heartbeat" artefact where the image goes fuzzy every 0.5 seconds (which correlates with a GOP of 15 and 30fps)

 

 

Yes, the pulsating effect is sometimes an issue with ffmpeg. You can try a sm,aller GOP (with a corresponding increase in bitrate).

 

Am I missing something - can one have an all I-frame structure with a GOP larger than 1? If so, how?

 

 

 

No, a GOP of 1 is an all-I-frames. Generally, that's not required in WATCHOUT.

 

If you're on WO6, there are more options for video playback, such as HAP and Prores. HAP should have no trouble with that resolution, and is very easy on the CPU (albeit heavy on data rate).

 

One more question: in your experience is there any difference between ffmpeg for Windows & ffmpeg for OS X? I would assume the mpeg2 library is the same under both platforms - but I could well be mistaken.

 

They should be identical, given the same versions.

 

Mike

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Thanks Mike - very useful! I'll look into using Prorez & will have a crack at HAP too. My understanding is that we're on WO5, but that may be being updated.

 

We now seem to be achieving satisfactory performance with MPEG2 at 50Mbps and a GOP of 1 - this has eliminated the 'pulse' as one would expect.

 

Peter

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