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ProRes VBR and MPEG CBR conversion problems

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Brian Lynn
ProRes VBR and MPEG CBR conversion problems
on Jul 19, 2015 at 7:33:27 pm

I work on a system that basically requires Constant Bit Rate video files to perform at its best.

When I work I use this workflow:
AfterEffects to PNG sequence
Adobe Media Encoder PNG sequence to MPEG2 CBR.

When I have videos that are identical frame counts I end up with file sizes that are identical. Example: 4 videos, all 1080p@60 1797 frames long, all end up 439,007 as file size.

My client uses this workflow:
AfterEffects to ProRes.
Adobe Media Encoder ProRes to MPEG CBR.

When this happens it is impossible to get files that are the same size.

Even if I get the ProRes files from the client and re-code them into a PNG sequence and then render the PNG sequence to MPEG 2 CBR my files sizes are -still- different!!

Knowing what I think I know about CBR video files, and my sources are all the same (1080p@60 1797 frames long), and using an identical render setting preset for all the video files I -should- end up with video files that are identical size in kb/mb at the end...

I should be encoding for example 1000mb/s for 10 seconds, no variance, and that should produce an extremely predicable file size...

But with the ProRes work flow this fails miserable and I can't figure out how to get it back on track.

Any ideas as to what might be causing the files to be different sizes?

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Chris Wright
Re: ProRes VBR and MPEG CBR conversion problems
on Jul 20, 2015 at 2:45:51 am
Last Edited By Chris Wright on Jul 20, 2015 at 5:08:51 pm

prores has many versions. PNG sequences are usually 8/16 bit 4:4:4, so any encoding would have to match that. If you need the exact same file size, prores is visually lossless but not mathamatically lossless, including its 444 XQ which is confusingly called 444 but only the alpha channel is preserved! the RGB values are log encoded compressed so there's data loss.

Quicktime's PNG video is a very small size, even smaller than quicktime animation, but again, its 8 bit, but at least its RGB is true 444. If you want no generation loss, you'll need 10bit HD or 12bit (dolby vision) 444 encoding.

OpenEXR is pretty good but mostly overkill for file size, but has a small math difference error. The same with PNG seq.

Tiff and PSD sequences can handle 16bit with 0.00% loss if LZW compression is turned off.

FFV1 is a free 10bit 444 video codec but not standard.

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Brian Lynn
Re: ProRes VBR and MPEG CBR conversion problems
on Jul 21, 2015 at 2:16:40 pm
Last Edited By Brian Lynn on Jul 21, 2015 at 2:22:08 pm

I am not worried about generation loss. I am fully aware of the different versions of ProRes and what they are for.

What I am asking about is forcing X number of bits per second.

Assuming all the video files are the same resolution and frame rate, total numbers of frames are equal, and I force CBR at 20mbps each video file should be the exact same number of bits after the render.

If I am doing this myself on my PC I have no issues. I render to uncompressed AVI or an image sequence and then encode to MPEG2 CBR and all the files have identical bit counts.

When I catch ProRes files from clients it doesn't matter what I do: I can -never- get the final bit counts to match! I have tried rendering the ProRes to uncompressed AVI, render to image sequence, and then recode to MPEG2 and nothing seems to help.

When I control the work flow from start to finish I end up like this:
File 1 Size 149,998
File 2 Size 149,998
File 3 Size 149,997
File 4 Size 149,998

All the files are within 1 bit of each other.

When I catch ProRes files and convert them I end up like this:
File 1 Size 150,032
File 2 Size 150,989
File 3 Size 149,983
File 4 Size 149,959

These may seem like small differences but when the software is designed to process X bits/second it can throw things off after a few hours of running.


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Chris Wright
Re: ProRes VBR and MPEG CBR conversion problems
on Jul 21, 2015 at 4:20:01 pm

interesting. see if the pixel resolution size is different. or the audio...
it might be a small bug too or metadata...

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