CST Vs Input Color Space

Could someone please help enlighten me regarding the difference of a CST vs. right-clicking a clip and changing "Input Color Space"?


I have a Prores 422 clip shot on Alexa Mini.
Color Management is set to "Davinci YRGB Color Managed" and "Davinci Wide Gamut"
Output color space is set to "Rec.709 Gamma 2.4"

If I make a CST Node setting Input Color Space/Gamma to ARRI Alexa/Arri LogC the image looks crushed. See photo.

If I remove the CST and instead right-click and select Input Color Space > Arri LogC, the image opens up.

If I want to grade it for Rec.709, Gamma 2.4, what is the correct approach?

Input Color Space is the correct approach.

Thank you, Jim. That also makes sense from just looking at the image after the change.

Could you, or anyone, explain why CST with the same settings looks so different?

I thought a CST did the same thing as changing the Input Color Space, but obviously something very different happens. I guess I'm just not clever enough to know what the difference is or when to use one over the other.

For CST to work as expected, image data before CST must match what you set the input to. My guess is that you already have an implicit transform there, due to color management, so it is modified. What is your clip interpreted as by default?

Thanks Hendrik,

I know it's shot on a Alexa Mini - and it's Prores 422HQ. I'm not sure how I can see what it's interpreted as other than that? Color Science is set to Davinci YRGB Color Managed.

When you have Color Management enabled, your footage will always be interpreted as something.

If you haven't configured the Input Color Space, Resolve attempts to set it based on the metadata. Normally with Log footage, this ends up being interpreted as Rec.709.

When you're adding a CST to a Color Managed clip, you're doing a Color Space / Gamma conversion on that footage that has already been "interpreted" and normalized to the timeline (in your case DaVinci Wide Gamut / Intermediate). So the interpreted source is wrong and then the CST is further compounding that wrong.

Thank you for a detailed answer Asim.

So you would normally only apply CST to non-Color Managed clips?

The CST can be used anywhere. You just need to be aware of what the settings are at your point in the pipeline.

For example, you can set input color space to bypass and use a CST to transform that footage. But this is just extra effort.

The CST can be used anywhere. You just need to be aware of what the settings are at your point in the pipeline.

For example, you can set input color space to bypass and use a CST to transform that footage. But this is just extra effort.

Got it - thank you very much for the explanation.

Just did some testing and wanted to add some info.

Found the weirdest behavior. It seems that the CST doesn't understand Color Managed.
The output space and gamma set to timeline is actually taking the output space of managed and not the timeline space. This is why everything becomes crushed.

If you'd want to use CSTs for whatever reason over setting the Input Color Space at clip level you'd have to manually set the CST's output to DaVinci Wide Gamut / DaVinci Intermediate and not Use Timeline.

In general I wouldn't recommend this workflow in Color Managed unless you really want to visually see what's going on. But at that point you might as well do a non-managed custom setup.

Interesting input.

Having to manually override the "Use Timeline settings" sounds like a bug, but I don't know if there's another reason for that.