How to use "Color Space Transform" for Log?

I have come across a technique for normalizing LOG footage to Rec 709 from the Blackmagic URSA 4.6K camera in Davinci using the Color Space Transform from OpenFX tab of Color panel.

However, I have seen two different ways of using the Color Space Transform Settings once you add it to a node.

In one tutorial they were set in this way:
Input Color Space: Blackmagic Design 4.6K Film v3
Output Color Space: Rec.709

In another tutorial they were set this way:
Input Gamma: BMD 4.6K Film Log
Output Gamma: Rec. 709

Which is correct, using Gamma or Color Space options?

What kind of Log footage? There are at least 40 or 50 that I'm aware of out there. The specific transform (or method for normalizing) would hinge on the type of source material.

If it's from a Blackmagic camera, there's nothing to stop you from using the LUTs Resolve provides, or you can just decode to Rec709 (or BMD color space) and BMD Film Gamma, then just use a simple curve to manipulate the look to where you want it. There's some flexibility here.

If you really want to see what's happening, shoot a camera color chart with known values and then look at the scopes. They'll tell you when you're getting close and/or when the lum and color values are in the ballpark, assuming normal lighting.

I am using the Film mode with ProRes from the BM URSA 4.6K mini.

Which is correct, using Gamma or Color Space options?

You would usually use both.

As for tutorials, I'd always consult the manual first (generally speaking since you are new, not just about this topic).

@Tom Early

Hello Tom,

Do you mean you would select all these options together for the Color Space Transform settings?:

Input Color Space: Blackmagic Design 4.6K Film v3
Output Color Space: Rec.709

Input Gamma: BMD 4.6K Film Log
Output Gamma: Rec. 709

If you do just pure gamma correction then it's not really what you want in your case as your footage has wide gamut as well. Use color space transform so this way you convert your footage into Rec.709 space for delivery. You also set gamma there so all done in 1 place. You need both.

Hello Andrew,

So, to be clear, should I set both of these options, rather than just one of them for a base correction, then fine tune manually?:
Input Color Space: Blackmagic Design 4.6K Film v3
Output Color Space: Rec.709

Input Gamma: BMD 4.6K Film Log
Output Gamma: Rec. 709

I see different tutorials only using one or the other, not both.

Does Input Color Space equate to gamut? Is this then setting the limit for the colour range, while the Gamma parameter is adjusting the luminance?

Yes, both.
Yes- your original recording can be way outside Rec.709, so it needs to be properly converted to target gamut (same as gamma).

Thanks Andrew.

So, I understand that the Output Color Space setting is converting the LOG colour space to the suit HD broadcast (Rec 709). From my brief research, it does this by remapping very specific values (such as color shades, brightnesses etc.) using mathematics.

I am unclear on what the Gamma setting is doing exactly. Is it just for convenience to make a base correction to the luminance? If I did not set this, could I simply manually adjust the luminance curve to make it look “right”? Or is a complex remapping also being applied by this setting to make it correct for HD broadcast, making it essential to also set?

Yes. You can skip gamma if you prefer to grade from LOG look. It's more difficult, but in some way gives you total control and control over all decisions.
Provided gamma correction from log is "perfect" only if footage was shot properly with precise exposure etc. In many cases it can give you very bad start point as it's fixed math based on perfect source material.
One of the way is to have node before the color space conversion effect as then you can "fix" source before conversion happens. Then 2nd node after it for tweaks.

Output Color Space as gamut is telling what area in imaginary 3D space of all colors your particular color space encloses. Luminance is not part of gamut, because increasing luminance does not change the chromaticity of color.

Gamma is actually transfer function, it says how luminance values are mapped to data values. Log is also gamma in this sense, but instead of power function it uses log function to map linear luminance to stored data values. Log transfer function itself does not say anything about the gamut, and it is not a color space. Color space is a combination of gamut and transfer function and you must know both to use your footage properly.

Thanks for clarifying that Andrew.

If all you want to do is apply the same CST to clip after clip, it makes more sense to use Resolve Color Management. Once it's set up, all the transforms are automatic, and you don't have to worry about node order.

@John Paines

Thanks, I'll look in to that.

Why can’t you just set the timeline settings to Rec709 Gamma 2.x in the project page? Anything you put on the timeline is then automatically converted to 709 when outputted. What’s the advantage of using the CST open effect?


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Why can’t you just set the timeline settings to Rec709 Gamma 2.x in the project page?

You can. I find that a much simpler way to work.

Why can’t you just set the timeline settings to Rec709 Gamma 2.x in the project page? Anything you put on the timeline is then automatically converted to 709 when outputted. What’s the advantage of using the CST open effect?

What the the OP was doing in this old thread isn't all that clear two years after the fact, but when using a display-referred color management method like Resolve YRGB, setting the timeline to Rec. 709 will not normalize log footage.

Why can’t you just set the timeline settings to Rec709 Gamma 2.x in the project page? Anything you put on the timeline is then automatically converted to 709 when outputted. What’s the advantage of using the CST open effect?

What the the OP was doing in this old thread isn't all that clear two years after the fact, but when using a display-referred color management method like Resolve YRGB, setting the timeline to Rec. 709 will not normalize log footage.

I’m using the ‘Resolve colour managed’ with DaVinci wide gamut in the Colour Management page in the project settings and setting the timeline to Rec709. This does appear to change the look of my log footage as soon as I place my footage on the timeline. Are you saying using this method is not the same as using the CST open effect? If so, what’s the difference?

Yes, scene-referred color management, as with RCM or ACEs, normalizes the log footage. No further transforms are required. Why anyone would want to use a CST in an RCM or ACE workflow, I don't know.

Actually, the two methods differ because the Color Managed (scene referred) workflow assigns all the input, timeline and output almost automatically. They are assigned in the Color Settings page. The YRGB method (display referred)makes it completely manual. One can grade from log; or, one can apply the proper transform to set up the base grade. One must manually make all the transforms for input, timeline and output. One way to check if you "understand" the manual workflow is to get a frame grab from the managed pipeline; then compare it to the YRGB method with CST's. The nuances are related to the tone and saturation mapping methods. If you do the comparison, compare images that you have not adjusted.

I credit Joey D'Anna over on MixingLight.com as coming up with the idea of "roll your own color management" a couple of years ago, when the CST nodes began. For the last year, when I get in camera Raw files, I'll traditionally use the manufacturer's color science to debayer and process the material, but then use a CST node as the first node in the tree, and I'll either turn it to Rec709 or some other useful setting. For me, it works fine with Red, but with BMD, I'm better off with Film Gamma once I finish, then I use a contrast curve to normalize it. It's a very flexible way of working, and I prefer it to color-managing the entire session.

The power of Resolve is that it gives you multiple ways to accomplish the same (or similar) results. I think any method is fine as long as it's fast, it works, the client is happy, and the pictures look good.

It's all not worth the hassle anyway as brain fools you every day:




Those boxes (main area inside) are the same color- check in any color picker (or look through small whole so it blocks surrounding background)