Would Proper CST and Rec709 file be the same? I'll explain

Obviously, I'm a novice. If you've been on site like Artlist, you can download a scene with several available formats. Sometimes, they will put a native BMG URSA file that needs to be converted with CST as well as the REC709 file of the same scene.

If you were to take the URSA file and correctly transform it with a CST, would grading the file work the exact same as the Rec709 file that is available? Would one file be more likely to "break" during the color grading process? Maybe what I'm asking is totally wrong, but I assume the Rec709 file is the URSA file, just corrected with the CST. Can anyone tell me how I'm thinking of this incorrectly? Thank you

I'm guessing here, but assuming tge Rec.709 file has the smaller color volume. And would "break" faster.

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I mean you can just download both and try out. Since both Resolve and the camera are from the same company (BMD), there is a chance similar transforms are used. Sure.

However generally speaking, no they are not the same. Both are Rec709, but the way they get there is different (a = CST, DaVinci tonemapping for example. b = unknown, some custom transform).

So they are different by default. But is working with one better than the other? Generally speaking. Yes. The original camera file in its log color space with high bit depth, data rate, and original source resolution is of course the better file to work with. But even if all those parameters are identical apart from the color space, the original file is still better as it ensures that no color data in the blacks and whites are lost.

Will it look the same as the Rec709 file? No, but you're color grading it anyway. So why bother?

And even if both files would look identical (considering the CST), there is still a significant difference. With the URSA clip you are able to decide if you want to grade before or after the CST. You can't do that with the Rec709 file.

would grading the file work the exact same as the Rec709 file that is available?

My thinking is no.

One is available in Log color space, which gives more freedom. So you'd take a different approach than grading a Rec.709 file.

And you definitely wouldn't be doing a Rec.709 conversion before grading.