Does recompression of h.264 files happen when working with them directly?

Community Beginner ,
Jul 31, 2020

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I have several cameras (360 cameras such as the Insta360 One X, Evo, and Vuze XR) that store their files using h.264. During the stitiching process I have the option of creating an intermediary file that uses ProRes 422. I can also create the intermediary file using h.264. The ProRes files are of course huge but they do speed up workflow quite a bit.

 

Some people advocate always going to a ProRes 422 intermediary file because they claim that if you work with the h.264 files, apply some adjustments and effects, and then export with h.264 you will be recompressing the video, ie. compressing an already compressed video and adding more compression artifacts and degrading the quality. But does this really happen? Doesn't Premiere have to decode the h.264 video, rebuild all the frames, and apply all the adjustments and effects, and then just compresses the results? Doesn't the quality end up being the same or does something else happens that degrades the quality? It seems like all the ProRes intermediary file does is save time. Can someone clarify?

 

Thanks,

Christian

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Does recompression of h.264 files happen when working with them directly?

Community Beginner ,
Jul 31, 2020

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I have several cameras (360 cameras such as the Insta360 One X, Evo, and Vuze XR) that store their files using h.264. During the stitiching process I have the option of creating an intermediary file that uses ProRes 422. I can also create the intermediary file using h.264. The ProRes files are of course huge but they do speed up workflow quite a bit.

 

Some people advocate always going to a ProRes 422 intermediary file because they claim that if you work with the h.264 files, apply some adjustments and effects, and then export with h.264 you will be recompressing the video, ie. compressing an already compressed video and adding more compression artifacts and degrading the quality. But does this really happen? Doesn't Premiere have to decode the h.264 video, rebuild all the frames, and apply all the adjustments and effects, and then just compresses the results? Doesn't the quality end up being the same or does something else happens that degrades the quality? It seems like all the ProRes intermediary file does is save time. Can someone clarify?

 

Thanks,

Christian

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Editing, Export

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Jul 31, 2020 0
Adobe Community Professional ,
Jul 31, 2020

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I think so.  If you understand how mpeg video works, only for example, the first and 15th frame include all the information and the 2nd thru 14th frame only have the differences...  then premiere has to decode and the re-encode all the frames.  errors are bound to occur.  But with Prores files each frame is discreet... and afaik, there shouldn't be any loss when you edit it.   But the important question here is will you see the difference?    I have no idea.  I'm supervising an online streaming film festival honoring the works of a 90 year old documentary filmmaker.  Most of our sources are problematic to say the least.  Usually dvd's that were not transferred with a lot of quality control.   Generally I just stay with h264, but with next weeks show, we're combining 2 films, 1 1080 HD and the other from a problematic standard DVD.  I've decided to downrez the 1080 film to 720 and uprez the SD file which also needs to scaled up a bit more to remove "head switching."  If you're interested look up "head switching - welcome to my hell...  I decided to take the m2v file demuxed from the dvd and transcode to Prores422 to maintain what limited quality there was.  Again, will anyone see the difference?  Probably more than you want or need to know...   So generally I can't see the downside to using prores 422 as an intermediate format but bdik (bigger file sizes and longer transcoding times).     Can't hurt to run some tests.  

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Jul 31, 2020 0
Adobe Community Professional ,
Jul 31, 2020

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The compression schema for H.264 is different than say for the full intra-frame codecs like Cineform, ProRes, and DNxHD/R.

 

One of the differences is it starts applying what I guess you could call "rounding factors" quicker. In other words, closely similar pixels are simply recorded as the same to save space. Such as side-by-side pixels with values of 128/127/129 and 128/127/127 which may become two 128/128/128 pixels. That sort of thing.

 

Depending on how much it's re-compressed, you may get a generation or two or even three without "visually notable" macroblocking and banding besides a lowered amount of detail.

 

You can test this easily enough. Create a sequence of the media, without any changes/effects applied. Export/re-import.

 

Place the exported clip on the sequence on V2, and then go to the Effects Control Panel, Opacity, Blending mode, and select "difference". Any pixel that is different between the two clips will show, all parts of the image that are identical will be black. I show how to set this up ...

 

Neil

 

BlendingMode Difference.PNG

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Jul 31, 2020 1
Chumpboy LATEST
Community Beginner ,
Aug 01, 2020

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Thanks! I will definitely give this a try.

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Aug 01, 2020 0
Advocate ,
Jul 31, 2020

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good question.

Codec stands for CO(compress ) DEC(decompress). So when you 'see' h264 on editor it is decompressing every single frame so you can SEE it. That is a function of your CPU ( though some people are trying to make it go through GPU as well, it's really all CPU now ). It means your mice in computer running on the wheel to power your CPU are running REALLY fast.

When you 'render' stuff it converts that long GOP h264 to I frame only. If you proxy to prores or cineform etc. then it is also a 'codec' but less lossy and your computer mice are more happy for edit purpose.

Even RAW is compressed to some extent.

But what h264 does ( to make file size so small ) is it basically THROWS OUT pixels that are similar in characteristics and puts a statement in it's place ... like " pixels from section of frame XYZ are similar and are equal to XYZ ".... putting a simple math statement in place of all the pixels...so it throws out the pixels and describes it with math instead ( as a whole area of pixels ).  That's why it's LOSSY.

And yes, when you work it and then export it, to h264, it is once again throwing out stuff and is LOSSY again.

You won't notice the difference because you don't work on the high level of quality that is required for high fidelity images ( like broadcast and movies in the theatre ). Otherwise you would never use h264 for anything.

 

I personally don't think it matters for most people so unless you are going to work professionally in a post editing house I wouldn't worry about it.

 

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Jul 31, 2020 1
Community Beginner ,
Aug 01, 2020

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Thanks! This really helps explains things.

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Aug 01, 2020 0