Remi Achard: What is the difference between v58 and the published CTL?
Nick Shaw: I think Pekka gave Scott some updated parameters that probably make no visible difference to the rendering.
Kevin Wheatley: The key difference is the CTL is a minimal implementation, with only the necessary functions.
Scott Dyer: I used Rémi’s NumPy version to compare against, with the parameters updated. I’ll give you the final parameter values. I refactored a little and renamed some functions. And Alex wanted me to structure the CTL is a particular way. And CTL has some limitations.
Remi Achard: The Python was updated yesterday
Nick Shaw: That was v58. But for clarity, the CTL was derived from v58, with small parameter changes. V59 is Alex’s experimental version with different lookup interpolation.
Kevin Wheatley: I’ve been not copying the CTL, but working on a Blink version based on the same principles. I’m still debugging. I put some default values in the Blink, so it doesn’t rely on the Nuke node. I noticed some values get rounded by float conversions. Do those changes make as difference. LMS matrix rounding can make a visible difference. We should look into that. Should we specify LMS primaries or specify the matrix? This is really an implementation issue. We may be better specifying inverses. I also found an issue with the tone scale where after a certain point it drops back to zero.
Nick Shaw: I remember the Daniele curve folds back beyond a certain peak value.
Kevin Wheatley: I added to the test list a check of a ramp over a certain range.
Remi Achard: I noticed a difference in the threshold for the binary search.
Kevin Wheatley: I think it was 10e-2 at one point, but then I increased the precision. We should add a test with a hue sweep and define the expected results.
Scott Dyer: After the discussion last week I added inverses for all the transforms, and updated reference images. I still expect the CTL may change after developer feedback.
Kevin Wheatley: Mostly what we need to look at today is tests.
Scott Dyer: I’ve not done as much as I hoped on the docs. draftdocs.acescentral.com should reflect the dev branch of the aces-docs repo. I want to get Nick’s documentation live on draftdocs, so people don’t need to run the Docker container locally to view it. This is technical documentation for those who want the detail of the maths. I’m woking on adding Pekka’s chroma compression document.
[Scott showed draftdocs running locally on his machine]
Scott Dyer: I’m editing it a bit for a consistent voice. We’re updating the whole ACES Docs pages to make better use of MKDocs.
Nick Shaw: Should we ask Daniele if he can contribute something on the tone scale maths? The main rendering code is simple, but the steps to derive the parameters are not obvious.
Alex Fry: I should get my repo in line with the CTL and bake LUTs.
Pekka Riikonen: I’ll open a PR for my tweaks and call it v60.
Kevin Wheatley: I’ve added some tests as they occurred to me as a programmer, when writing my minimal Blink. But it would be geed to get some tests for the things we have been trying to solve with our recent tweaks.
Nick Shaw: Most of those were about round tripping.
Pekka Riikonen: The latest one was for the P3 round trip. We need to make sure we include the EOTF in the round trip test.
Nick Shaw: Scott, what dod you do when testing your inverses for limited gamut?
Scott Dyer: They round trip to within the limiting gamut. The white point sim and not 10,000 nit PQ transforms won’t fully round trip either. My tests weren’t super scientific.
Kevin Wheatley: We could do the test and get a metric for how good it is.
Nick Shaw: The easiest ones to test are just the Rec.709 and P3 ones.
Kevin Wheatley: Going to AP0 and back. How big should the cube be? Maybe a sequence of 256 images to test an 8-bit cube.
Alex Fry: Do we provide tests for each step through the transforms?
Kevin Wheatley: Ideally. Things like the gamut mapper are the hard ones to create tests for. In my tone scale test, an input value above a threshold drops to zero. I don’t know why. We should note that the P3 primary is outside the spectral locus, so don’t be surprised by that. We should say neutral input should have near zero M.
Pekka Riikonen: It’s interesting it’s not exactly zero.
ACES Output Transforms VWG
Meeting #152, May 15th, 1pm PT
[Meeting Recording]
Attendees
Meeting Notes