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Grading a Log DPX Scan...Anyone Want to Show Me How it's Done?


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#1 Karl Lee

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Posted 21 December 2014 - 04:36 PM

Hi everyone.

 

I recently had some S16 scanned in log color space, and I've been playing around with color grading in SpeedGrade.  This is my first attempt at color correcting a film scan, so I'm not all that experienced.  That said, my approach to color grading has been more of adjusting the levels until something looks good as opposed to using a proper technique or process.

 

If I were to upload a few .dpx frames from my film scan, is there anyone who would be willing to spend a minute or two doing a rough color grade on the stills?  I'd really like to compare someone else's color grading with what I've done, and perhaps see the potential in my film scan in the hands of someone who actually knows what they're doing :)

 

I was a little hesitant to post this request since I don't want to come across as, for the lack of a better phrase, looking for someone to do my work for me.  This is strictly a hobby for me, so I'm just interested in trying to learn through the experience of others.  So, if anyone might be interested, just let me know and I'll try to upload a few scans.

 

Thanks! 


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#2 Jeff L'Heureux

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Posted 21 December 2014 - 07:24 PM

I'm unfamiliar with speedgrade, but I assume like most color grading software it has scopes.  If you're unfamiliar with them, you should look up some tutorials on youtube on how to monitor the scopes for color grading, as it's better than simply eyeballing it.  If you understand what you see on the scopes you can tell when your shadows, midtones, and highlights are in the proper range, then play around with actually grading the image.

 

DaVinci Resolve Lite is free software if you're working with a resolution of 1080p or below.

 

Here's a link to an explanation of the scopes in Resolve.  This would apply to any color grading program that has them.  I personally pay most attention to the waveform monitor.

 


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#3 Perry Paolantonio

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Posted 22 December 2014 - 07:32 AM

Actually, resolve lite handles up to UHD now.
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#4 Karl Lee

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Posted 23 December 2014 - 02:25 PM

Thanks for posting the video, Jeff.  Initially I was just adjusting the contrast and saturation levels in SpeedGrade, but I've been getting much better results using the scopes for reference (particularly the RGB parade) and making adjustments to offset, gamma, and gain. 

 

I've been searching for more video tutorials for SpeedGrade and Resolve, but unfortunately most of what I'm finding involves color correcting and making adjustments to video captured in Rec.709.  Ideally, something that focuses on color grading a log film transfer (or even video shot in log) would be the most helpful, so I'll just have to keep looking.


Edited by Karl Lee, 23 December 2014 - 02:25 PM.

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#5 Dennis Couzin

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Posted 25 December 2014 - 09:04 PM

Allow me to offer some scientific opinions which may or may not correspond to current practices.  When making a video intended for TV or other BT.709 standardized display, it should be graded on a BT.709 display.  No colors outside the BT.709 gamut, no super-whites or sub-blacks, should be included in the final color judgement.  The grader might want to peek at these, in order to see what's available "in reserve" in the image, but should then turn the untapped reserves off.

When making a video intended for DCP release, it should be graded on a DCI P3 compliant display.  The P3 gamut is significantly larger than BT.709.  The CIE xy diagram on which gamuts are often illustrated greatly distorts chromaticity differences.  The CIE u'v' diagram at this link is less distorting.  It shows that very many hues have significantly higher saturation in P3 than in BT.709.  The grader can decide whether and how the DCP can make use of the extra colors by seeing them displayed.

(On this diagram.  NEC PA301W happens to be my monitor.  It can display the BT.709 gamut nicely but not all of P3.  Kodak 2393 is Vision Premier Color Print Film.  It's dominance over P3 is sobering.   No tri-color (additive) display can completely enclose this print film's gamut.)

If a video is intended both for BT.709 and DCP display, you can do two separate gradings.  Or you can dare a compromise grading.  Or you can do a BT.709 grading and restrict the DCP to that.

***

The question must be asked, which cameras are suitable for BT.709, P3, or whatever display?  This has nothing to do with log vs. power-gamma encoding.  It has very little to do with the dynamic range of the camera.  It depends almost entirely on the spectral sensitivities of the camera's R,G,B sensors.  Camera makers never publish those sensitivities, which is silly because a decently equipped optical lab can easily measure them.  There are groups at RIT (Rochester) and at the University of Tokyo busy measuring cameras.  They or others will get to the Alexa, et al., eventually.  From whom are the camera manufacturers hiding their spectral sensitivities?  Not from competing manufacturers, who all have the optical labs.  

Color profiling matrices and the application of 2D and 3D LUTs can only go so far toward making a camera with inappropriate spectral sensitivities into one suitable for a large gamut display.  On the other hand, a cheap camera could, in theory, allow beautiful color reproduction on the "full" gamut.  Even if a camera immediately applies BT.709 precorrection to the RGB channels and then transforms to BT.709 Y'CbCr, that Y'CbCr can contain color information over the full gamut. 

There is a game of second-guessing ongoing between advanced camera manufacturers and advanced color graders.  Arri, for example, says that the RGB records from its Alexa, after linearization, drive certain virtual primaries.  The virtual primaries form a gamut and the implication is that color reproduction is hunky dory on (the real part of) that gamut.  This is false.  Those three virtual primaries correspond to no three spectral sensitivities.  Arri is only saying that assuming those three primaries gives the best overall color reproduction with the camera's spectral sensitivities.  But what Arri finds best overall need not be what the grader finds best overall.  Hence all manner of 3D LUTs must be invented for the grading, and the suitable gamut will not be anything like Arri's implied gamut.

The ACES concept, which boils down to evaluating camera spectral sensitivities by how near they can come to reproducing the original scene colors, is naive.  It is a rewarming of the "Luther Condition" first published in 1927.  Color reproduction is for a limited gamut, and veridicality is anyhow not the measure, since movies are seen in dark theaters, etc.

I don't know the answer to the question in bold.  Such questions will become simpler when digital camera manufacturers publish their spectral sensitivities (the way film manufacturers always have). 


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#6 Karl Lee

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Posted 28 December 2014 - 04:35 PM

I've been playing around in SpeedGrade and trying to color grade a few scenes in my S16 film scan.  I've created an album with a few before and after images showing both the original flat log scan (top image) and my attempt at color correcting the scenes (bottom image).  Could someone take a look and offer some creative criticism?

 

In all three of these scenes, I used overall offset to adjust the black level, overall gain to adjust the maximum IRE, then adjusted overall gamma and saturation levels to what I thought looked best.  While I think my corrections certainly look better than the original log scans, I'm still not overly-impressed with my work so far.  In my opinion, the blue sky in the LV sign and fountain shots seems to have somewhat of a greenish tint...almost with a sort of color reversal-ish film look.  I've tried playing with the offset, gamma, and gain tints/hues, but adjusting those creates an overall color shift that makes the overall image look worse. 

 

My goal with this film scan was to have something to play around with and try my hand at color grading, but as I've discovered, decent color grading is definitely a skill and something that's easier said than done.  Next time, I'll probably stick with having the colorist perform the grading and leave it to someone who knows what they're doing :)   


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