What Value is the Filmic color-space Using to calculate Maximum Scene sRGB Intensities?
Another set of questions follow:
How is this value retrieved from Blender's 3D scene?
Is this a global calculation, or is it based on camera rays received?
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If your question is which scene referred value would become the display referred (1.0, 1.0, 1.0) triplet after the Filmic View Transform has done its job, the answer (currently) is:
(2 to the power of 6.5) * 0.18
which in numbers is
16.291740238....
The 0.18 is the scene referred value that Filmic deliberately has chosen as the 'middle grey' reference value. The 2 to the power of 6.5 part basically comes from the maximum value being defined as 6.5 stops over middle grey. A table of sample values is provided in the readme of Troy Sobotka's filmic-blender GitHub repository.
The value is taken from the resulting pixel values after the rendering stage. In photographic terms, that would be the amount of energy that lands on the film stock or digital sensor. So in essence, camera rays received.
Be aware that these values are deliberate choices by Troy Sobotka, and are bound to change with future iterations of Filmic. Also it's worth noting that any scene referred value above the given 16.291740238 will also give display referred (1.0, 1.0, 1.0) in sRGB color space.
User remco together with Troy himself pointed out to me:
That 0.18 (or 18%) is the gray value that is (or was) considered to correspond to the gray that the human eye sees as halfway between black and white, and used on e.g. exposition targets (Kodak grey card). Thus, it's not a completely arbitrary choice.
It is important for me to point out that with deliberate i was never attempting to say arbitrary. Designing a View Transform like filmic-blender is both a matter of color science and craftsmanship. Thus there is some level of freedom for the one designing it to define the boundaries. The high clip value which the initial question was asking for is such a choice. It could technically be at a different energy level, but in a rendering context the choice made has proven to serve a high number of use cases well. The scene referred 0.18 middle grey value however is a convention which I missed in the initial answer, so thanks for the comments.
answered Mar 20, 2018 at 14:40
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2$\begingroup$ That 0.18 (or 18%) is the gray value that is (or was) considered to correspond to the gray that the human eye sees as halfway between black and white, and used on e.g. exposition targets (Kodak grey card). Thus, it's not a completely arbitrary choice. $\endgroup$– remcoCommented Mar 20, 2018 at 15:04
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$\begingroup$ Agree with you @remco, it is indeed a convention. It has carried over to CGI / photographic manipulation in the digital era. There are some tricky elements coming up with Filmic Mark II that involve varying ideas around a diffuse white, as well as wider gamut. That is, middle grey ideas may shift when viewing on HDR capable displays, given the standards are moving away from a notion of a fixed white perfectly diffuse code value. $\endgroup$– troy_sCommented Mar 20, 2018 at 20:42
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$\begingroup$ @remco I've updated my answer to include your comment, thanks $\endgroup$ Commented Mar 21, 2018 at 8:03
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$\begingroup$ @troy_s I've updated my answer to hopefully better explain the details, and added a link to your repository, as you provide the high clip value there nicely formatted in a table. $\endgroup$ Commented Mar 21, 2018 at 8:04