Re: [Avid-L2] Re: DNxHD QuckTime FCP Import Question

One more attempt at my understanding of the principles, using 8-bit values
for reference.

If you look at a shot from a video camera and watch it on a waveform
monitor, you will see the blacks at (or slightly over) 16, but the whites
may well exceed 235. That's what I see when I look at shots from DigiBeta
rigs, P2, DV/DVCAM. These cams all use some of the available headroom.

If you export from Avid, and you tell it to export to RGB 0-255, it will
stretch the 16-235 over 0-255. 16 becomes 0, 235 becomes 255 and all values
in between are divided over the spectrum. All values over 235 or under 16
are clipped or crushed to 235 or 16.

If you export from Avid, and you tell it to export to 709/601 16-235, it
will leave the levels alone. Meaning that 16 remains 16, 235 remains 235,
and all levels in between it will remain the same as well. Any values under
16 or over 235 will remain at their original values as well. It does not cut
away anything below 16 or over 235.

So if you take this original video shot with white levels that are over 235,
and you export to RGB, you are clipping the white info over 235. That's
typically white details in clouds, skies, highlights.

So exporting to RGB 0-255 is typically what you do it you have perfectly
legal 16-235 values that you need to display on a computer monitor. If you
have raw or ungraded material, that will undergo further treatment, you
should maintain levels.

Also note the way the Avid codec works in QuickTime Pro. If you use QT Pro
to encode into Avid codec, the encoder will ask you to specify if the
_source_ file is 601/709 or RGB. If you select 601/709 it will NOT ALTER the
levels (even if they are 0-255) and encode. If you select RGB, it will remap
0-255 to 0-235.

Some more info:

http://www.kwancentral.com/blog/post/2008/07/Avid-Editor-Quicktime-Work-Flow
-For-Proper-Brightness-Levels.aspx

So you can also "cheat" your way through Avid if you like. You can import an
RGB 0-255 graphic, tell the Avid that it is 601/709. The Avid will leave the
levels alone, so you will have an import that on the scope will show as
completely illegal (blacks at 0, whites up to 255). If you then export that
graphic and tell the Avid to export at 709/601, the Avid will still leave
those levels untouched (it assumes that whatever is IN the system is
601/709). So if you do that, you have not have performed any conversion at
any point. You will have cheated the Avid into working in 0-255 all the way
through (although your monitoring inside and outside the Avid is of course
way off, since that is likely set to 601/709).

However, if your source is already 16-235 and your destination will also be
16-235, you should never expand to 0-255 at any point in the pipeline.

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