Kevin Wheatley: Over the last week Pekka posted about the differences when using different gamuts for limiting. No concrete conclusions. It's preference. Daniel commented on the blue ball going purple. The path of"constant hue" for near blue tends towards purplish, and collapsing that to Rec.709 goes too purple. We think that's the cause, but don't have a solution. The other discussion is about how much of the various color spaces should we be able to handle? AP1? AP0?
Alex Fry: Daniel's blue is not implausible, but our model tweaks to support implausible blues introduce effects in areas where the CAM should track better.
Kevin Wheatley: CG people would expect us to support AP1.
Alex Fry: Perhaps the part of AP0 that's within the spectral locus. What does outside the locus and AP0 mean? A lot of our hacks are in order to do sensible things with implausible values. But real cameras produce these values. Much of Red Xmas is outside AP0. I'd put that down to a bad IDT. Are we working too hard to make that look good?
Kevin Wheatley: The RGC aims to make things positive in AP1.
Alex Fry: All of the ARRI Bar image is plausible.
Nick Shaw: Are we compromising in range values to handle wide out of range values? Making space to pull them in.
Alex Fry: The yellows where we get the inversion in the original CAM are non physical. But there are a lot of blues that are outside AP1 but in the locus. They are up for debate.
Nick Shaw: Maybe we can dial back on the extreme lengths we go to to try to fit everything in.
Alex Fry: I wouldn't want it to produce errors, but maybe some values could clip like the current RRT. I've been experimenting with a gamut compression that"reaches out" to the same gamut for all targets. I tried with AP1. Currently it is based only on the cusp so looks ok in 2D, but isn't right in 3D. Needs more work.
Kevin Wheatley: How much of the compression is part of the rendering? Then we try to represent that on a real display.
Alex Fry: The output of the chroma compressor is the image we are trying to represent. But we can't look at that.
Kevin Wheatley: What shape is that? And is it reasonable?
[Alex showed a 3D visualization of the output of the chroma compress of the spectral sweep]
Alex Fry: The chroma compression pulls the top in, but the bottom bulges out, which may be causing problems.
Carol Payne: We don't need to clip values outside a given range. We just say you shouldn't expect a"reasonable rendering" for them. But they could be kept for inversion.
Kevin Wheatley: The locus is a fuzzy boundary. And some areas within it may be a waste, like the cyan green boundary, which cameras won't produce.
Alex Fry: Thomas's spectrally rendered Cornell box has values out there.
Kevin Wheatley: If you try to capture too much, you could compromise too much. The AP0 blue is negative so doesn't make any sense. I wouldn't expect to render that"correctly". AP0 was just designed to contain the locus.
Alex Fry: AP1 may be too small. There are a lot of plausible values outside it.
Carol Payne: For the RGC we worked out a max of the common camera gamuts as where we reached out to and mapped to AP1.
Nick Shaw: Some think the RGC is too aggressive because cameras won't produce values in the corners of their encoding gamuts.
Kevin Wheatley: The locus isn't totally relevant because a camera not a person saw these images.
Alex Fry: Is that an IDT question?
Kevin Wheatley: We can use the locus as a guide, but shouldn't hard clip there. Then there is the shape of the target gamut. The shapes vary, so you can't just shrink them down. There has to be a trade off. The spectral locus doesn't align with any of our destinations. You could use AP1 and expand it out a bit to cover purples and the problem yellows. Rec.2020 and AP1 are very similar by design. So if you used AP1, a Rec.2020 device would be the best representation of that.
ACES Output Transforms VWG
Meeting #103, May 24th, 1pm PT