[1220512064]

Display P3 (distinct from cinema display P3, because names are hard ig) is used as a render target color space. ACES (and its internal color spaces) are designed as working spaces.

If you make a color space for a display, the intent is that you can (eventually) get a display which can display all those colors. However, given the shape of the human color gamut, you can't choose three color primaries which form a triangle which precisely contain the human color gamut. With a display color space, you want to pick primaries which live inside the gamut; else you'd be wasting your display on colors that people can't see. For a working space, you want to pick primaries which contain the entire human color gamut, including some colors people can't see (since it can be helpful when rendering to avoid clipping).

Beyond that, ACES isn't just one color space; it's several. ACEScg, for example, uses a linear transfer function, and is useful for rendering applications. A colorist would likely transform ACEScg colors into ACEScc (or something of that ilk) so that the response curves of their coloring tools are closer to what they're used it (i.e. they have a logarithmic response similar to old-fashioned analogue telecine machines).

[throwaway290]

no monitor uses ACES so it always needs to be converted to P3 to even see what you're doing right?

or you are saying if there is some intermediate transform that makes color go beyond P3 it will get clipped? then I understand...

[Uehreka]

Yeah, like, let’s say that in your compositing workflow you increase exposure then decrease brightness. If your working color space is too small, your highlights will clip when you increase exposure, then all land flat at the same level when you decrease brightness. If your working space is bigger than the gamut people can see, but your last step is to tone map into Display P3, you’ll appreciate the non-clipped highlights, even if your eyes could never comprehend what they looked like in the post-exposure-boost-pre-brightness-drop phase of the pipeline.

[adgjlsfhk1]

The key point is that your ray tracing color space and your display color space don't need to be the same thing. Even if your monitor only displays SRGB colors, it still can be useful to have more pure primaries in your rendering system.

[1220512064]

> or you are saying if there is some intermediate transform that makes color go beyond P3 it will get clipped?

Exactly! The conversion between ACES (or any working color space) and the display color space benefits from manual tweaking to preserve artistic intent.