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Is it normal for lossy video codecs to introduce black/white level clipping into a rendered video?

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Rick Anvican
Is it normal for lossy video codecs to introduce black/white level clipping into a rendered video?
on Feb 27, 2016 at 1:02:33 am

Hi all,
I have experimented with rendering a test project to different lossy video (sRGB 16-235) codecs like Sony DV, Cineform and Mainconcept H264. The project consists of footage having video levels ranging from 10IRE (black) to 80IRE (white) and the project has been clamped to 5IRE and 90IRE with the Sony Broadcast Colors filter.

Upon analyzing the videos rendered by the lossy codecs, they all had clipping in white levels, exceeding 100IRE up to about 110IRE (black levels seem to be more "stable" as they tend to stay in the legal levels). The same project rendered to Lagarith (lossless) does not produce a video with clipping on either black or white levels. On several YouTube videos, I found that both black and white clipping occur even if the video levels are using the correct sRGB 0-100IRE range.

I'm not sure if this is a limitation of the codecs' lossy encoding so that's why I'm asking, can anyone confirm this? Thanks in advance.


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John Rofrano
Re: Is it normal for lossy video codecs to introduce black/white level clipping into a rendered video?
on Feb 27, 2016 at 1:18:45 am

[Rick Anvican] "Upon analyzing the videos rendered by the lossy codecs, they all had clipping in white levels, exceeding 100IRE up to about 110IRE (black levels seem to be more "stable" as they tend to stay in the legal levels). The same project rendered to Lagarith (lossless) does not produce a video with clipping on either black or white levels. "
One explanation might be that you are comparing levels in YUV codecs like CineForm with RGB codecs like Lagarith and the levels aren't measured the same.
[Rick Anvican] "On several YouTube videos, I found that both black and white clipping occur even if the video levels are using the correct sRGB 0-100IRE range."
The YouTube player will blindly expand all video. It expects Studio RGB 16-235 and if you give it full Computer RGB 0-100 you will get the wrong levels. Only give YouTube Studio RGB.

~jr

http://www.johnrofrano.com
http://www.vasstsoftware.com



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Rick Anvican
Re: Is it normal for lossy video codecs to introduce black/white level clipping into a rendered video?
on Feb 27, 2016 at 1:32:10 am

[John Rofrano] "The YouTube player will blindly expand all video. It expects Studio RGB 16-235 and if you give it full Computer RGB 0-100 you will get the wrong levels. Only give YouTube Studio RGB."

Hi John,
I am using Studio RGB based on the assumption that the project will be encoded with a codec expecting Studio RGB levels and where the render is to be delivered (e.g. YouTube). Unless it's a limitation with the lossy encoding and compression, I can't find a reason for the lossy codec to introduce clipping into black/white levels where the levels are perfectly in legal boundaries since lossless codecs don't have that problem.


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John Rofrano
Re: Is it normal for lossy video codecs to introduce black/white level clipping into a rendered video?
on Feb 27, 2016 at 1:56:06 pm

[Rick Anvican] "I can't find a reason for the lossy codec to introduce clipping into black/white levels where the levels are perfectly in legal boundaries since lossless codecs don't have that problem."
So you are saying that you set your levels, render to CineForm, drop it back into an empty Vegas project and the levels charge? I've never seen that happen before.

~jr

http://www.johnrofrano.com
http://www.vasstsoftware.com



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Rick Anvican
Re: Is it normal for lossy video codecs to introduce black/white level clipping into a rendered video?
on Feb 28, 2016 at 12:14:53 am

I used the Sony Secondary Color Corrector filter with the clipping (luma) indicator presets from here and they have pointed out areas where the clipping has occurred. They are very minor in terms of the change in value and affected pixels in most cases occurring at random but the clipping becomes more obvious when the video content approaches maximum white levels. You would have to stare very hard at the waveform monitor, but sometimes it's not obvious when the level is over the limit by a fraction of an IRE unit.
I thought that broadcasters in the analog days would have been very strict about this and the area between probably 80IRE to 100IRE would be reserved as peaking headroom for maximum white. In the end I think it's just a nitpick from me.


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