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what's really going on in ProRes? (or 10bit 4:2:2: uncompressed)?

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josh litle
what's really going on in ProRes? (or 10bit 4:2:2: uncompressed)?
on May 19, 2010 at 3:44:43 pm

So most of us like ProRes for its quality, specs, and size. But I'm wondering how much I can trust it as color-critical format and here's why. I recently had a film color corrected in Scratch and rendered out to Quicktime Apple Uncompressed 10-bit 4:2:2 codec. I then made a ProRes HQ export of this movie. Guess what? They don't look the same. There are definite and obvious color and gamma shifts between the uncompressed 10-bit 4:2:2 and the ProRes-compressed 10-bit 4:2:2.

Now, I would expect that the ProRes might exhibit some compression artificating (like noise, or funky edges) relative to the uncompressed codec. But since both codecs have the same bit depth (10 bit) and use the same 4:2:2 colorspace, should they not maintain reasonably similar color/luma/gamma values, etc in conversion?

I took it a little further and tried some tests. Basically creating bars in both uncompressed and ProRes formats, then exporting them to a standard TIFF format (uncompressed) and comparing them in Photoshop. Still big, obvious differences. The reason I export them to TIFF is it occured to me that Quicktime might be applying different display settings or LUTs to ProRes and Uncompressed (e.g. perhaps some default automatic gamma/color stuff on playback). So by converting to a uniform format (TIFF) I hoped to eliminate Quicktime display trickery.

I also noticed that when I export ProRes from QT, there is an option for "gamma correction" - "auto" or "none". I tried both flavors and they still produce results that are wild different from the uncompressed 4:2:2 codec.

So what's the deal? Does 4:2:2 mean different things to different people? I thought it was industry standard color space based on component video. And I would presume that "10-bit" means linear "10-bit" in both Apple Uncompressed and Apple ProRes codecs? Can anyone explain what's going on behind the scenes? Thanks, Josh

josh
filmmaker


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Jeremy Garchow
Re: what's really going on in ProRes? (or 10bit 4:2:2: uncompressed)?
on May 19, 2010 at 4:50:59 pm

[josh litle] "There are definite and obvious color and gamma shifts between the uncompressed 10-bit 4:2:2 and the ProRes-compressed 10-bit 4:2:2. "

Wild guess here, but instead of using software or hardware that can handle this process, you simply exported out of Quicktime or used Quicktime Conversion?

[josh litle] "Basically creating bars in both uncompressed and ProRes formats, then exporting them to a standard TIFF format (uncompressed) and comparing them in Photoshop."

You are getting way ahead of yourself, again, what is your process for this?

Jeremy


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Sascha Engel
Re: what's really going on in ProRes? (or 10bit 4:2:2: uncompressed)?
on May 19, 2010 at 5:02:21 pm

I experienced a similar phenomena: I exported a finished video clip - one in Uncompressed 10bit 4:2:2, in Uncompressed BlackMagic and in Appple ProRes HQ and all three of them looked very different.
I ended up choosing the Blackmagic codec for looking the most crispy, strongest blacks and best contrast.
But why? I still have NO clue!
Hope somebody can enlighten us!
:-)



Sascha


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Jeremy Garchow
Re: what's really going on in ProRes? (or 10bit 4:2:2: uncompressed)?
on May 19, 2010 at 6:29:59 pm

[Sascha Engel] "I exported a finished video clip - one in Uncompressed 10bit 4:2:2, in Uncompressed BlackMagic and in Appple ProRes HQ and all three of them looked very different. "

And how did you export? Quicktime Conversion?


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Sascha Engel
Re: what's really going on in ProRes? (or 10bit 4:2:2: uncompressed)?
on May 19, 2010 at 9:12:17 pm

I exported it from FCP as follow:

Edit -> Export -> Quicktime Movie (not Conversion!) -> then I adjusted the settings there to the Codecs I mentioned above.


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cowcowcow
Jeremy Garchow
Re: Project created in 59.94, need 29.97
on May 19, 2010 at 11:07:03 pm

[Sascha Engel] "then I adjusted the settings there to the Codecs I mentioned above. "

Yeah, that's probably your problem.

I'd use Compressor to transcode with frame controls on (to ensure 10bit transcode).

Or

Make a new sequence with the appropriate codec and render, then export a movie @ current settings.

You have to do this the 'right' way, or FCP/QT won't handle it properly.

Jeremy


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Sascha Engel
Re: Project created in 59.94, need 29.97
on May 20, 2010 at 6:54:03 pm

Thanx. New thing learned. Will try it next time!


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Alan Okey
Re: what's really going on in ProRes? (or 10bit 4:2:2: uncompressed)?
on May 19, 2010 at 7:50:36 pm

[josh litle] "I recently had a film color corrected in Scratch and rendered out to Quicktime Apple Uncompressed 10-bit 4:2:2 codec. I then made a ProRes HQ export of this movie. Guess what? They don't look the same. There are definite and obvious color and gamma shifts between the uncompressed 10-bit 4:2:2 and the ProRes-compressed 10-bit 4:2:2. "

The most important piece of information that's missing here is how you are monitoring these two files. Are you monitoring the output on a broadcast monitor by doing an A/B between the two different files in FCP, or are you looking at them in Quicktime player on your computer monitor?


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josh litle
Re: what's really going on in ProRes? (or 10bit 4:2:2: uncompressed)?
on May 20, 2010 at 2:10:08 am

Well, yes I did export from Quicktime. So I will attempt some tests following the recommendations above.

Regarding the monitoring question, yes I am monitoring on the desktop. That is why I tried creating RGB Tiff files - so I could compare the processed images in the same format in the same app (Photoshop).

Note to Apple: The FCP / QT / Compressor apps really should all handle this stuff (e.g. codec, bit-depth, colorspace, transcode) identically. As users we should not have to know all of these obscure work-around for simple export to be handled correctly. Grump. I always assumed the QT engine was at the heart of all three apps.

josh
filmmaker


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Alan Okey
Re: what's really going on in ProRes? (or 10bit 4:2:2: uncompressed)?
on May 20, 2010 at 4:03:27 am

[josh litle] "Regarding the monitoring question, yes I am monitoring on the desktop. That is why I tried creating RGB Tiff files - so I could compare the processed images in the same format in the same app (Photoshop). "

Your video files are not RGB. Uncompressed 10-bit and ProRes 422 are both YCrCb codecs, not RGB. By viewing them on a computer monitor in RGB colorspace, you're at the mercy of Quicktime Player's YCrCb to RGB conversion, plus Quicktime's notorious gamma issues. The inconsistencies you see are most likely the result of how the files are being (incorrectly) displayed in RGB colorspace by Quicktime Player, not necessarily a problem with the video codecs themselves.

Keep in mind that the only way to accurately view and evaluate the color of your video files is on a proper broadcast monitor and/or hardware scopes fed by a proper video I/O device like an AJA Kona card. Trying to compare them in Quicktime Player on a computer monitor will not yield accurate results for a multitude of reasons.



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josh litle
Re: what's really going on in ProRes? (or 10bit 4:2:2: uncompressed)?
on May 20, 2010 at 2:27:20 pm

Thanks Alan,

I understand your point about the YCrBr to RGB conversion. I do think that since both files are being converted to RGB using the same app (quicktime player) that presumably they are undergoing the same conversion process (accurate or not); then when viewed as RGB TIFF file in the same App (Photoshop) they theoretically will look the same (if the original Quicktimes were the same). But that's really academic, since it relies on visual comparisons and many different variables in the chain.

I performed a much more accurate comparison that didn't involve display issues or the human eye: I imported both QT's onto seperate layers and composited them using difference mode (in AE), which only displays the parts of the files that are different from one another. If the files are identical this will result in an absolute black image (no difference between the files). However in this case you could clearly see a strong "ghost" image (much more than normal compression artifacts). I also compared the two files inside of FCP using the built in Vectorscope and waveform tools. Again the values were clearly different. Both of these methods should (in theory) bypass any display or color managing by Quicktime, FCP or the operating system. They are (in theory) mathematical operations based on the actual pixel values present in the files.

I still need to try the various export methodologies outlined above. I will report back on that.

josh
filmmaker


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B.J. Ahlen
Re: what's really going on in ProRes? (or 10bit 4:2:2: uncompressed)?
on May 21, 2010 at 3:58:27 am

ProRes 422 colors in After Effects



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Dennis Couzin
Re: what's really going on in ProRes? (or 10bit 4:2:2: uncompressed)?
on Jul 26, 2010 at 5:24:45 am

Josh Litle: "They are (in theory) mathematical operations based on the actual pixel values present in the files."


They are mathematical operations, but unfortunately they are not both based on the actual pixel values present in the files. The uncompressed 10-bit 4:2:2 file contains actual pixel values. (I've viewed and played with the values for uncompressed 8-bit in a binary editor and it's fun.) But the ProRes file does not contain actual pixel values. That file needs to be interpreted (decoded) to produce pixel values. The decoder can play games with little details like clipping, tone compression, even gamma.

In line with similar experiments I've done, I suggest you operate on a grey scale and read the RGB values (in Photoshop) after each step. At least you'll determine the nature of the difference. Then you must determine if it's real or an artifact of your tools.


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