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.