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illegal luma levels on render

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Alan Fisher
illegal luma levels on render
on Jun 1, 2010 at 4:55:32 pm

After color-correcting and legalizing our levels clip by clip in FCP, watching external scopes, we render and discover illegal luma levels on our waveform. Doesn't seem to matter which filters we use or in what combination or order, some clips' highlights render higher than they're set when previewing same frame. (Even the "broadcast safe" filter appears to clip to a safe level until it's rendered.) Though I have seen this before, for our previous season of shows this was not a regular problem. Not sure what versions of everything we were running then, but now we can't get around it. Help!

More details:
Happens with our G5 and MacPro systems, both with AJA io hardware;
We're editing SD footage, captured 10 bit uncompressed via SDI,
rendering in 10 bit high-precision YUV... FCP 6.0.6, QT 7.6.4, OS 10.5.8

Any thoughts?

Thanks,
Alan


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Chris Borjis
Re: illegal luma levels on render
on Jun 1, 2010 at 5:12:42 pm


I noticed this before as well.

how low are you taking your white levels down?

usually I take mine down to at least 95%. Rendered, they stay below 100.




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Jeremy Garchow
Re: illegal luma levels on render
on Jun 1, 2010 at 5:20:44 pm

Make sure your cc and broadcast safe filters are at the bottom of the filter stack so they get rendered last.

Jeremy


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Gregory Butner
Re: illegal luma levels on render
on Jun 1, 2010 at 11:06:38 pm

Are you using any composite modes like multiply? Sometimes the broadcast safe filter doesn't work when there is more than one layer being combined. If this is the case then export that particular section that is going out of luma safe as a quicktime and then apply the broadcast safe filter to the rendered video.



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Alan Fisher
Re: illegal luma levels on render
on Jun 1, 2010 at 11:22:15 pm

I'm afraid we're not doing any compositing, just cuts and dissolves, and we're definitely doing the color correction and/or broadcast safe applying as the last filters...it just doesn't show the adjusted levels after a render, as if it was never applied. Frustrating!

Searching around I've seen some references to this being a bug, maybe particularly with 10 bit footage, but so far we haven't gotten any of the workarounds (nesting and applying the filter, or exporting and re-importing) to help. Would Color work where Final Cut is failing us? We'll try bringing the whites down to 95 until we can convince it to work right....

I appreciate the ideas,
Alan


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Michael Gissing
Re: illegal luma levels on render
on Jun 2, 2010 at 12:56:13 am


It has been reported here many times that broadcast safe is not safe after render. There are a few way to fix this.

I used to use RGB limiter rather than broadcast safe. I now use a legaliser that was part of The Grading Sweet plugins. Download the demo and the legaliser is free

https://thefilmbakery.com/store/index.php?main_page=index&cPath=1&zenid=b7d...


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Jeremy Garchow
Re: illegal luma levels on render
on Jun 2, 2010 at 1:27:49 am

And if you know Color, by all means use it. It has superior quality than built
in fcp plugs.


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Mark Raudonis
Re: illegal luma levels on render
on Jun 2, 2010 at 4:11:54 am

After dealing with this "10 per cent difference" upon rendering, we bought hardware legalizers. Nothing like a hardware "clamp" to guarantee you'll be legal.

At a certain point we stopped trying to trouble shoot, acknowledged that it's a problem, and moved on to the solution (hardward legalizers). Life's too short!

Mark



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Rafael Amador
Re: illegal luma levels on render
on Jun 2, 2010 at 5:21:16 am

I really don't understand how can fails a filter which only mission is to clamp Y' when its reach a certain value.
We have to notice that where in the Broadcast Safe filter says "Chroma", It should say "Composite".
We need to keep legal the Luma (>100%) and the composite signal (>133% and depending on the system specs).

We have to understand that we need legal four different things:
- Luma
- Chroma Amplitude
- Composite signal (Luma + Chroma)
- RGB Gamut

The Brodcast safe filter can control Luma, Composite and RGB, but have no way to control Chroma.

While the Luma and the Chroma may be legal, the resulting Composite signal can be illegal.
Or while our Composite signal is legal, we may still have RGB values out of specs.

Have been pointed here the RGB Limiter. The RGB limit filter is of not use to control Luma: a 100% Red (256) is is absolutely illegal, while the value of Y' is perfectly legal: 30%

In FC we can monitor Luma, Composite (limited) and RGB, but no way to monitor Chroma.
In Colors we miss the possibility to monitor the Composite signal.
Both applications are limited on monitoring, so you can not get a full picture.

Anyway, I wonder if some people are using properly their WFMs. Are they monitoring proper Luma or instead are monitoring the Composite signal.
Rafael



http://www.nagavideo.com


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Neil Ryan
Re: illegal luma levels on render
on Jun 20, 2012 at 2:37:29 am

Hi Mark,
What hardware legaliser did you get or do you recommend?

Neil.


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Steve MacMillan
Re: illegal luma levels on render
on Jun 11, 2010 at 9:03:13 am

I just started my own post on this.

It has been my recent experience, that rendering breaks the Broadcast Safe filter with ProResHQ. If you export a clip as 10bit uncompressed, and then drag it back into the ProResHQ timeline, add the Broadcast Safe filter and render, it works as expected.

STeve


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