ACES in depth ?

[MartoStyle]

Hello im happy that you you embedded ACES! I have some questions about how it is made and what are the principles of working. Is it just some filmic tonemaping function which handles better with over exposure. Please if it possible give us some tutorial just for ACES. I saw this tutorial for Vray and everything is explained there. First  ACES support more colors and when you turn it on you reach more deeper color both in VRay and Corona BUT after that to work propertly you have to convert basic colors and textures you are using for ACES  after that you reach not more vibrant and saturated colors but the opposite more natural and super realistic colors. In the Corona i dont see that natural look when turn on ACES just more somethin like more darker maby i am wrong i dont know but i think its not doing its thing propertly. Is it everythin automatic for corona for example converting  basic colors and all textures and does physcal materials  support it natively. Please give more information how it works or if it is possible whole video just for it.  Thanks in advance!

[Nejc Kilar]

Howdy! I'll try to explain things in a way that hopefully will make the most sense. I'll generalize a bit too because color spaces are complex :)

The ACES OT operator in Corona takes care of the tone mapping part according to the ACES specs. To put it in layman terms, it'll tone map the image in that filmic ACES style and so the highlights will look more "natural" and the darker parts will get a nice treatment as well. You'll also notice that depending on the intensity of the colors and the exposure you're using that the colors will behave much more like they would if you were taking a photo with your camera. That's what the ACES OT operator does.

The ACES tone mapping part is just one bit of what ACES does as a whole though. Primarily it is used to offer a color space (or a set of) that artists in a production can easily transition in and out of. Without it it's harder to keep everyone on the same page because converting between all the different color spaces that are out there is... Tough. This whole ordeal is not implemented in Corona at the moment. From my understanding that would be the OCIO part mainly.

The ACES color space can also be used for rendering calculations. Some renderers use the somewhat color limited sRGB color space to calculate things under the hood. Other renderers, like Corona, use wider ones. There's pros and cons to each but generally speaking the current trend is that more is better and since ACES offers a really wide gamut of colors it is typically what gets implemented as the color space in which renderers make it calculations these days. As mentioned though, Corona always used a wider color space so it's pretty much ACES like from the start. Are there wider color spaces than ACES? Yep. But ACES is just kind of becoming the one everyone is getting used to working with lately plus its wide enough as is. TLDR though, Corona is using a wider color space by default and while it isn't exactly like ACES it's more similar to ACES than to sRGB. And again, from my understanding there's pros and cons to sRGB and wider stuff.

So the above is again, a bit generalized but hopefully it helps understand things a bit better :)

Oh and dully noted on the tutorial part. We're giving this one a +1. Thank you!

[MartoStyle]

Howdy! I'll try to explain things in a way that hopefully will make the most sense. I'll generalize a bit too because color spaces are complex :)

The ACES OT operator in Corona takes care of the tone mapping part according to the ACES specs. To put it in layman terms, it'll tone map the image in that filmic ACES style and so the highlights will look more "natural" and the darker parts will get a nice treatment as well. You'll also notice that depending on the intensity of the colors and the exposure you're using that the colors will behave much more like they would if you were taking a photo with your camera. That's what the ACES OT operator does.

The ACES tone mapping part is just one bit of what ACES does as a whole though. Primarily it is used to offer a color space (or a set of) that artists in a production can easily transition in and out of. Without it it's harder to keep everyone on the same page because converting between all the different color spaces that are out there is... Tough. This whole ordeal is not implemented in Corona at the moment. From my understanding that would be the OCIO part mainly.

The ACES color space can also be used for rendering calculations. Some renderers use the somewhat color limited sRGB color space to calculate things under the hood. Other renderers, like Corona, use wider ones. There's pros and cons to each but generally speaking the current trend is that more is better and since ACES offers a really wide gamut of colors it is typically what gets implemented as the color space in which renderers make it calculations these days. As mentioned though, Corona always used a wider color space so it's pretty much ACES like from the start. Are there wider color spaces than ACES? Yep. But ACES is just kind of becoming the one everyone is getting used to working with lately plus its wide enough as is. TLDR though, Corona is using a wider color space by default and while it isn't exactly like ACES it's more similar to ACES than to sRGB. And again, from my understanding there's pros and cons to sRGB and wider stuff.

So the above is again, a bit generalized but hopefully it helps understand things a bit better :)

Oh and dully noted on the tutorial part. We're giving this one a +1. Thank you!

Thank you man! I understand it Coronoa uses wider color range from the start thats why it was looking better than all other engines evrica. I understand that it is near ACES than sRGB wihch is cool but it is not the same right now i noticed that somethink like SDR than HDR . But in this case ACES TO  is just a tonemap not a real ACES  like in Vray with its real calculations unrer the bonet with OCIO. Thank you for the information!

[alexyork]

Bumping this.

We have discovered that using ACES destroys most of the dynamic range in EXR renders (16-bit half float or 32 float, same result). So when you load the EXR into for example PS and reduce the exposure, you don't recover any data at all. It just goes to a grey/black mush. As soon as I disable ACES and re-save the EXR I get full dynamic range again.

Is this working as intended? If so this is really problematic because being able to recover dynamic range in EXRs is super useful sometimes, for example when basically your whole render is perfect but one area is a bit blown out, you can just load in a second version of the EXR, lower the exposure a bit, and merge that bit back in.

Is there not a way to use ACES while retaining the usual amount of dynamic range?

[Nejc Kilar]

Howdy!

Yep, you got it right - ACES OT will clamp the image as it is a tone mapping operator much like highlight compression or filmic mapping is. Any tone mapping operation will clamp the image because that is basically what tone mapping is in this context - it remaps the pixel values to something that looks decent on a regular display.

I suppose if you wanted to keep the ACES OT look without baking it into the .EXRs you'd probably go the OCIO route in your post-production software of choice while saving the EXRs without ACES OT enabled.

[alexyork]

Thanks Nejc. Is there no technical way at all of retaining at least some depth when using ACES? EXRs seem to have the potential to carry enormous dynamic range, so when rendering with ACES which is now on by default you're just losing almost all of that benefit.

[Juraj]

The OT meaning Output Transform (HDR-> SDR) it's by design not meant to retain any dynamic range. ACES workflow requires all HDR post-production to be done inbetween input and output transforms to work correctly.
On other hand, Corona had implement nice trick for LUTs which by design also clamp to pure 1-255 value in SDR, but they extend the clamped value to infinity, or not sure how else to describe it :- ). But it doesn't cut off all the dynamic range as it would otherwise. LUTs are also meant purely as final output transform.
No matter what trick would be possible to implement, any color transformation on the extended range would be quite incorrect though.

Until Photoshop adopts full HDR workflow (and it will, since beta of it already exists in AdobeCameraRaw plugin), where it will implement output transforms and it will be possible to just import unclamped HDR file, it's best to either avoid ACES OT, or adopt ACES OT only as preview, and re-create it as output transform in editor that can do so, like Fusion.