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Washed out x264 and 8x8 DCT flag

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Michael Shick
Washed out x264 and 8x8 DCT flag
on Apr 24, 2009 at 6:08:18 pm

Like many out there, I've suffered through the apparent Apple/Quicktime H.264 wash-out "bug". I've tried time after time to work out a method for exporting from FCP that won't result in gamma shifts in video encoded for the web, never to much success.

Today I started playing with the x264encoder.component for Quicktime. It gives much finer control over the encode and container settings than the default H.264 codec. I think it's really fantastic.

Getting to the point: I found that at the default settings it would give me great, gamma accurate output. However, using the "Tuned" settings, I got exactly the same wash out as Apple H.264. I went through the settings one-by-one, and determined that the FLAG2_8x8DCT setting was singularly responsible.

Now, I'm trying to figure out what this is, why it might be important, and why it leads to the wash out. I was hoping somebody with more knowledge of codecs might have some ideas. Is it also conceivable that this is what the Apple codec does as well, and, if so, with this knowledge could the root of this problem be sussed out?

Thanks,
Michael


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Daniel Low
Re: Washed out x264 and 8x8 DCT flag
on Apr 24, 2009 at 10:29:40 pm

Turn this setting off. Enabling it will make the file not compatible with Quicktime, it forces the profile from 'main' to 'high'

Make sure that your frame size is equally divisible by 8, or ideally 16 on both axis.

This is no relation to the QT H.264 gamma issue/bug



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Michael Shick
Re: Washed out x264 and 8x8 DCT flag
on Apr 25, 2009 at 1:33:48 pm

Yeah, actually I played around some more and think I figured out what was going on. I believe the 8x8 flag was forcing an inappropriate nclc, and that was the real culprit. I found that the default 2-2-2 would always give a washed out look.

I think the gamma bug comes into play, in this case, because my source is HDV, which uses an SD color space, but has HD dimensions. I think Quicktime, in whatever way it is hooked up with FCP, is always forcing an HD color space on my material. I was able to resolve all my problems, and get really excellent output, using x264, and forcing the SD color space. YMMV, but I'm really happy to have a working solution.


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Daniel Low
Re: Washed out x264 and 8x8 DCT flag
on Apr 25, 2009 at 6:40:04 pm

[Michael Shick] "I think the gamma bug comes into play, in this case, because my source is HDV, which uses an SD color space, but has HD dimensions."

What do you mean by colour space exactly? Are you referring colour subsampling, bit depth, or?

[Michael Shick] "I think Quicktime, in whatever way it is hooked up with FCP, is always forcing an HD color space on my material"

Quicktime is not 'hooked up with FCP'. FCP is QuickTime. FCP is entirely built upon QuickTime, it would not exist without it.





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Michael Shick
Re: Washed out x264 and 8x8 DCT flag
on Apr 27, 2009 at 5:21:03 pm

Not sampling, but rather, the color space as defined by ITU Rec. 601. This gets defined in Quicktime containers by the "nclc" flag, 6-1-6 (what that stands for is beyond me).

Comparatively, the HD color space is ITU Rec. 709. Professional codecs will use this. It uses nclc flag 1-1-1 in the Quicktime container.

I realize that FCP is substantially QT, but there's no denying that opening the exact same export in a FCP viewer gives me different gamma than opening it in QT, with or without the FCP compatibility setting. FCP must be better about ignoring/interpreting other color space data within the file.

So, I'm wondering if this bug, which most people complain about in the context of seemingly less professional video, is the result of weird pro-sumer formats that are using old color spaces (straight compatibility with SD TVs?), but at HD-resolutions

As to 8x8, it's working well for me in QT, and the FFMPEG-based encoder my video host uses, but if I have to worry about compatibility I might remove it. Is there anything else I should be concerned about in terms of compatibility?



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Daniel Low
Re: Washed out x264 and 8x8 DCT flag
on Apr 27, 2009 at 10:42:48 pm

I very seriously doubt whether you could tell the difference between Rec. 601 and Rec. 709, even with the best monitor available and everything else taken into consideration.

Compare blue and red lines here: http://home.comcast.net/%7Etlhuffman/gamuts_white.jpg

That said, HDV is Rec. 709, not Rec. 601 as you state - those recommendations apply not only to colour space but also to many other elements of the picture.

Rec. 601 came about before there was specification for display gamut.

More: http://forums.creativecow.net/thread/162/866143

It's also highly unlikely that you have your monitoring perfectly calibrated, no disrespect here, but even those who proclaim to be expert at calibration can't get two high end monitors to match.

And there's an argument that much professional monitoring equipment has not been updated to Rec. 709 specs.

You do however raise some very interesting points relating to the way FCP displays an image next to plain QT player. I'd always trust the picture in FCP over what QT player displays.

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Michael Shick
Re: Washed out x264 and 8x8 DCT flag
on Apr 27, 2009 at 11:08:38 pm

Ah, well, the Wikipedia entry on HDV must have bad info (no surprise). I think some early HDV cameras used 601, maybe that's the issue.

My monitor is definitely not 100% calibrated, but what I'm talking about is a pretty huge difference in the appearance of the video on a variety of monitors and computers (this is all work for the web).

Oddly though, I'm noticing the same wash out with DVCPRO-HD content. Which defeats my earlier theory...

This is really puzzling. Maybe Quicktime always interprets the 6-1-6 nclc as a 2.2 gamma, and displays the video accordingly (just like FCP does), whereas it plays around when displaying in 1-1-1?



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Daniel Low
Re: Washed out x264 and 8x8 DCT flag
on Apr 27, 2009 at 11:14:37 pm

[Michael Shick] "This is really puzzling. Maybe Quicktime always interprets the 6-1-6 nclc as a 2.2 gamma, and displays the video accordingly (just like FCP does), whereas it plays around when displaying in 1-1-1? "

You might find this thread interesting (not a solution):

http://osdir.com/ml/quicktime-api/2009-04/msg00004.html

I trust these issues will all be sorted with Quicktime X, in Snow Leopard.





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