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7d H264 footage: ProRes LT or 422 for projection?

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Jason Ferris
7d H264 footage: ProRes LT or 422 for projection?
on Jan 15, 2011 at 6:10:49 pm

I've been looking, but haven't found the exact answer to this question. I'm shooting a doc with my 7D and would like the final output to be good enough for projection.

It's my understanding that the Canon H264 bitrate is 48 mbs. (I'm shooting 24p/1080p). ProRes LT is 102. ProRes 422 is 147. Am I gaining anything if I transcode to ProRes 422, or is ProRes LT (already twice the size of H264) just as good?

Or is my logic about bitrate totally wrong?

Thanks for your input!!
Jason



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Craig Seeman
Re: 7d H264 footage: ProRes LT or 422 for projection?
on Jan 15, 2011 at 6:29:19 pm

[Jason Ferris] "Or is my logic about bitrate totally wrong? "

Not totally wrong. Just incomplete.
Even within a codec, bit rate alone isn't enough to determine how quality is impacted.

GOP based codecs tend to be more efficient than I frame codecs (as in, they can look better at lower bit rates). I Frame codecs, because there's less "estimation" can hold up a bit better in certain post workflows.

Codecs themselves are "formulas" which have impact. H.264 may actually look better than the same bit rate for MPEG2

Even within the same codec, bit rate doesn't tell the whole story. A video camera's AVCHD 24mbps with a good built in encoder can look equal or better than a DSLR's H.264 48mbps encode. An IBP type GOP may is more efficient than an IP type GOP for example. CABAC entropy may be better than CAVLC. High Profile may look better than Main and especially Baseline Profile.

You really need to know the full anatomy of the encode. To use an analogy one could superfecially guess that a 300lb might be stronger than 150lb person. The 300lb person could be an NFL Linebacker or maybe an out of shape couch potato. The 150lb could be the couch potato or maybe an Olympic caliber triathlete.

As to what codec to use. Generally people would go from DSLR's H.264 to ProRes (at least if you're Mac based) for editing because I frame based ProRes would not have GOP based real time decode issues. Additionally the 4:2:2 color space in ProRes will hold up to compositing better.

As for playback for projection. . . These days H.264 is fine for distribution ASSUMING the playback device can handle it. That can depend on data rate, entropy, profile of the encoded file and whether the hardware can decode and play it in real time.

The above contains some sweeping generalizations and there are many exceptions.



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Jason Ferris
Re: 7d H264 footage: ProRes LT or 422 for projection?
on Jan 15, 2011 at 7:09:49 pm

Thanks Craig! This definitely helps, but I also have a specific question about ProRes LT vs ProRes 422. Basically, do I gain anything by transcoding to the larger ProRes 422 when ProRes LT is already twice the bitrate of H264?



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Jeff Greenberg
Re: 7d H264 footage: ProRes LT or 422 for projection?
on Jan 16, 2011 at 3:03:22 am

It's hard to say - it's a better analogy to run the bitrates against uncompressed HD, rather than each other. Running a distribution codec (despite it's use!) compared to an editorial codec doesn't have a straight direct translation.

As Craig pointed out:
It is important to understand that h.264 is a long gop VBR codec.
It's also important to understand that ProRes is an VBR I-frame only codec.

Uncompressed HD is around 880 mb/s
So, the 7d is running around 48mb/s - about a 20:1 compression
ProRes LT is about 100mb/s; about an 8:1
ProRes 422 is about 140mb/s: about a 6:1 compression.
(I didn't look those up - I'm sure they're off - but get us in the ballpark)

You're not going to make a 'huge' gain to go to ProRes 422. A gain? Sure. A gain you can see? Questionable. Additionally, you'll get some forgiveness with projection.

If I intended to do lots of color correction and compositing, yeah, I might choose ProRes422 over LT. If system performance or drive space is any level of an issue? Then LT.

Best,

Jeff G


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Jason Ferris
Re: 7d H264 footage: ProRes LT or 422 for projection?
on Jan 16, 2011 at 3:11:52 am

"If I intended to do lots of color correction and compositing, yeah, I might choose ProRes422 over LT. If system performance or drive space is any level of an issue? Then LT."

Thanks, Jeff. This is what I was looking for: some idea of the relative advantage of each codec. I think I'll buy some new drives and start transcoding at ProRes 422. As you say, it may produce some benefit and will help with color correction and projection.

In the meantime, I'll begin learning more about the other points you and Craig raise.



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Olin Padilla
Re: 7d H264 footage: ProRes LT or 422 for projection?
on Jan 19, 2011 at 2:03:48 am

Craig, I'm wondering how true this is.

If the h.264 is already compressed to 8bit 4:2:0, than is the extra information in 422 even valid? In other words, does the ProRes 422 codec actually recover more information from the h.264 than the LT does, or is the higher bit rate just redundant data? I always assumed the latter.

If 422 is actually producing better results, then why not use HQ or 4444, and where do you draw the line?

I have never heard a logical answer to this, but if you have one please let me know. Thanks.

Jason, what are you planning to project with? If you are eventually planning to print to film, then you may as well buy a nice big drive and not risk it.


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Jeff Greenberg
Re: 7d H264 footage: ProRes LT or 422 for projection?
on Jan 19, 2011 at 1:02:11 pm

Olin - sorry to jump in, but I love this question!


[Olin Padilla] "
If the h.264 is already compressed to 8bit 4:2:0, than is the extra information in 422 even valid? In other words, does the ProRes 422 codec actually recover more information from the h.264 than the LT does, or is the higher bit rate just redundant data? I always assumed the latter."


The extra information? Absolutely valid. It's just interpolated and stored - it's essentially a similar decode to playback in NTSC/PAL on a monitor. Does it provide extra detail? No.

Codecs have a point of diminishing return for their data rate. Throw a high enough data rate at any codec, and essentially you get uncompressed video. I'm making no mention of the possibility of decoding such, mind you.

[Olin Padilla] "If 422 is actually producing better results, then why not use HQ or 4444, and where do you draw the line?
"


Let's say that ProRes LT produces a better picture over ProRes Proxy because while they fundamentally are the same codec (and ideal for FCP), the higher data rate handles the transcoded/interpolated information better. With each step up along Apple's codec, you get a higher data rate, less degradation from the transcode, a large file, and a less damaged picture.

Only you can say that you feel the picture quality is 'good enough'. For some people ProRes LT is good enough - a fair trade between file size and picture quality. Going to 4444 produces less possible degradation, but a gigantic file that stresses drives. If you can't see the difference, does it matter? If you were truly going for the 'best picture' possible, why would you shoot with a h.264 based codec?

So, it's a bit of technical and a bit of opinion - where your choice to 'draw the line' is where you choose to draw it.

Best,

Jeff G

Apple Master Trainer
Avid Cert. Instructor DS/MC
Avid & Color Videos Vasst.com
Compressor Essentials Lynda.com


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Jason Ferris
Re: 7d H264 footage: ProRes LT or 422 for projection?
on Jan 25, 2011 at 3:16:04 pm

Olin and Jeff,

Thanks for your comments. For this project, I'm going to use ProRes 422, but I'll continue to use ProRes LT for all of my web videos.

"If you were truly going for the 'best picture' possible, why would you shoot with a h.264 based codec?"

The answer to that question is simple: because I can't afford a nicer camera!



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