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Importing a DNxHD .mov without gamma shift

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Stuart BruceImporting a DNxHD .mov without gamma shift
by on Mar 25, 2009 at 3:36:14 pm

This question might belong in the "basics" forum?

I have a QuickTime movie, created with Avid on a PC, using the DNxHD codec. When I open it in QuickTime Player on a PC, the colours are fine, but when I import it into Final Cut on a Mac, the colours are a lot brighter- gamma shift, apparently.

I'm looking at the result on a calibrated broadcast monitor through the HD-SDI output of a BlackMagic card, which I believe means I can't blame it on the monitors.

Changing the User Preference for how RGB video is handled gamma-wise doesn't appear to make any difference?

Googling for 'gamma' and terms like 'PC', 'Mac', etc. turns up a lot of frustrated people with gamma shift problems, but I didn't spot a solution.

How can I get this DNxHD into the Mac with the correct gamma?

Thanks

Stuart



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Dave BerezaiRe: Importing a DNxHD .mov without gamma shift
by on Mar 25, 2009 at 5:19:44 pm

Hi Stuart, what kind of broadcast monitor are you using? A CRT? And are you using the one monitor to compare? Ie. Are you pumping out your Avid output to the same monitor of your FCP output and doing the compare there?

Dave Berezai
http://www.editmule.com


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Stuart BruceRe: Importing a DNxHD .mov without gamma shift
by on Mar 25, 2009 at 5:34:00 pm

The Avid is off-site. On-site we have the Final Cut system, and we have one Panasonic calibrated LCD monitor, sadly it's not a CRT or grade 1.

I have ProRes and DNxHD clips of the same thing on the timeline. When I switch from a frame in one clip to the same frame in the other clip, there is a very visible difference between the two. The DNxHD version is brighter and 'milky'.

I've tried exporting DNxHD from Final Cut and it has the same problem as the DNxHD sourced from the Avid.

So it's looking to me, in this instance, that whatever DNxHD file I pull into Final Cut, it's always looking milky?



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Dave BerezaiRe: Importing a DNxHD .mov without gamma shift
by on Mar 25, 2009 at 6:30:33 pm

Hmmm... Well first off The Gamma Shift of your original file (checked on both a PC and Mac) is due to the difference in Gamma level outputs. Most PC's show a Gamma level of somewhere between 2.2 and 2.4. A Mac's default Gamma is 1.8 I believe. Check out this link for furthur reading:

http://www.kenstone.net/fcp_homepage/gamma_mac_pc.html

How do you need to deliver your project? If your delivering for the web (where most people view videos on pc) then I would change your mac gamma settings in the mac OS ColorSync control to the value of a PC so that you can view your file (on your mac desktop) as most would see it. Heres a link on how to do it:

http://version2.andrewkendall.com/pages/learning/osx/mactopcgamma/

Alternatively you could add a gamma correction filter from fcp and adjust it to match the output of your PC (just to see the file as you were seeing it on a PC).

I would strongly suggest checking your file on a CRT as the Gamma output would be very similar to a PC. Its also worth checking your gamma settings on your monitor and video card (if they are adjustable).

As for the Gamma jump between the two files with different codecs in the same timeline I can only assume that the DNxHD codec has a gamma value suitable for PC's and not macs.
Hope this helps.





Dave Berezai
http://www.editmule.com


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Stuart BruceRe: Importing a DNxHD .mov without gamma shift
by on Mar 26, 2009 at 12:27:43 pm

Thanks for all the suggestions, much appreciated.

Ultimately our delivery is going to be on HDCAM, so it's the HD-SDI signal (that our Panasonic monitor is showing) that will need to be right. Since that's a hardware output, none of the ColorSync type monitor settings would make a difference- I think?

The root of the problem seems to be how Final Cut copes with the DNxHD, or rather how it doesn't cope- as well as being very wrong in the colours, Final Cut crashes a lot when I try to get it to use the DNxHD file. I think we're going to have to find an alternative workflow that doesn't use DNxHD.

Thanks again





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Stuart BruceRe: Importing a DNxHD .mov without gamma shift
by on Mar 26, 2009 at 4:34:27 pm

As a very quick follow-up, it seems that with the DNxHD file we've got, using Compressor and going via an uncompressed intermediate file works, while using QuickTime Pro or pulling it straight into Final Cut was causing some bad colour shifts.



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Ben ScottRe: Importing a DNxHD .mov without gamma shift
by on Mar 27, 2009 at 3:31:03 pm

any non native codecs to final cut pro should be transcoded first through compressor

best way to check if the gamma has shifted is take an image with tonal ranges which vary over a gradient e.g. sky

look at the scopes in avid for the dnx file
look at the scopes in final cut pro for the dnx file

no guess work

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Check my podcast at http://cowcast.creativecow.net/final_cut_pro/index.html
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Bjoern AdamskiRe: Importing a DNxHD .mov without gamma shift
by on Apr 3, 2009 at 8:48:57 am

It's the codec. DNxHD reports RGB values to FCP not Y'CbCr. Therefore FCP applies its internal RGB interpretation which causes the gamma shift you see.

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alius satoRe: Importing a DNxHD .mov without gamma shift
by on Apr 19, 2011 at 2:31:20 am

picking up an old thread, but:
avid can export either RGB or 601/709 dnxHD files, there's a choice for that in the export settings dialogue.

however, both types of files get flagged as YUV (601/709) inside the .mov container.

which means that:
- the export is faulty in the case that you've exported RGB, but it's been flagged as YUV.
- how you import to FCP will have to depend on whether you did an RGB or YUV export.


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