[chrisreeves]

NOTE: This question sat in the video processor forum for a few days with no replies, so I moved it. It's relevant to many people with expensive projectors, and there seems to be more forum activity over here.

This thought occurred to me recently as I've been learning about color correction in film/video production.

Grading films/videos is preferably done on RAW image files, or at least Pro-res (a compressed lossy format which is designed to retain a lot of information for image manipulation in post production).

What is not recommended is attempting to grade images which are encoded in formats meant for final desplay - because these formats are highly compressed and contain much less information than the original capture formats did.

So - is it also possible that attempting to apply a LUT to a compressed 8 bit Blu Ray signal will degrade it as well?

And thus, would it be preferable to always calibrate colors, gamma and grayscale as closely as possible in the projector's CMS itself - so that you are not altering (by transcoding) the compressed 8 bit video signal itself?

Or am I misunderstanding the way Lumagens process the video signal - do they avoid transcoding the signal itself, and merely change the values of chrominence and luminence on the Lumagen's outputs?

 

[chrisreeves]

One non-scientific data point which may or may not be applicable to my question is some comparisons I did on some test shoots.

I own the Blackmagic Pocket Cinema Camera - which can record in lossless 12 bit Cinema DNG RAW format, or 10 bit compressed Apple ProRes 4:2:2. Note that the Prores, while lossy and compressed, is still designed as a capture format to be graded later.

I shot an interview subject in RAW, and then again in the same set-up in Prores.

In post I applied a high quality commercial LUT by Filmconvert to both files, then exported the graded files to Quicktime format.

Low and behold, on my 8 bit display monitor the 8 bit clips which were originally graded from 12 bit RAW files looked noticeably sharper than the ones graded from 10 bit Prores 4:2:2. Don't get me wrong - the clips originating in Prores still looked great - but side by side there was an undeniable difference noticed by myself and a couple other people who examined the clips.

So - thus my Lumagen question.

 

[stanger89]

It's always best to do it as late in the chain as possible, if you can do your CMS/gamma calibration in your display that's the best place to do it.

The problem is, lots of projectors can't be calibrated that way. They either lack the functionality entirely, or they have broken functionality, or maybe it's just prohibitively difficult to do it in the display. In these cases I suppose you could say it's the lesser of two evils, uncalibrated image, or calibrated with quantization errors.

However, consider that the Lumagens do their processing at 10 (or is it 12, I can't remember) bits, and you can set them to output 10-12 bit color, so that should minimize or eliminate any quantization errors. Though that's 10-12bit YPbPr, I wonder if that's better or worse than an extra YPbPr to RGB conversion?

 

[stanger89]

Low and behold, on my 8 bit display monitor the 8 bit clips which were originally graded from 12 bit RAW files looked noticeably sharper than the ones graded from 10 bit Prores 4:2:2. Don't get me wrong - the clips originating in Prores still looked great - but side by side there was an undeniable difference noticed by myself and a couple other people who examined the clips.

This is really a different issue, here you're seeing the difference between and extra step of lossy compression vs not. Lumagen, actually all, video processors only process raw video, so they won't introduce any of the losses you see when you run through a lossy codec. Unless you've got it setup wrong, something like a Lumagen shouldn't do anything other than add some quantization errors, this is of course ignoring scaling/sharpening/denoise functions which are a whole different story.

 

[DavidHir]

Chris,

I would also try posting this in the display calibration forum. One of the pro calibrators might be able to offer advice in there too.