Re: combining a File-Based LUT Transform and a Display Transform?

Jeremy Selan <jeremy...@...>

On Wed, Feb 1, 2012 at 10:29 AM, Brendan Bolles <bre...@...> wrote:
On Jan 30, 2012, at 12:19 PM, Jeremy Selan wrote:

> The downside is that in this approach you can have a color cast to the 'grayscale' output of 'r,g,b,l' modes (if the viewing transform doesnt map gray to gray), but this is not typically a concern in practice. (and is often desired anyways in some cases)

This is the thing that would seem the weirdest to me - looking at the red channel and seeing it have a color cast.  I'm not sure why that would ever be "desired".  The film look in the spi-vfx config makes a cast.  I'm pretty sure no other program does that.

Depends on the intent of the color cast.

Say you're using a film emulation lut where the bright neutrals get mapped to a warm color, and the dark neutrals get mapped to a cool color. Having your 'luma' mode to this too may look a bit odd.

But say your output device is a DCI P3 projector, where equal code values map to a very greenish white point.   Your output 3D lut may instead be mapping all values equally to a more pleasing white point, in which case if we '1d-ified' the luma mode all of a sudden you'd jump between the color and luma modes and the image would all of a sudden become greenish.

Consider a grayscale image, where you're toggling between luma mode and color mode.  Would the artist expect the image to change or not?  In the current implementation, they would appear identical.

At the same time, I agree that when you want to look at just the red channel, you probably want to see the image data pre-LUT.  Maybe a really smart implementation would do lin->log->red or lin->sRGB->red instead of lin->film->red when just the red channel was being viewed.

In the old SPI library (the internal predecessor to OCIO) when viewing individual channels we dynamically computed a version of the 3D viewing lut that didnt have any crosstalk. (which makes input equal code values always map to output equal code values) but this always felt a bit too magical for my tastes, and also looked weird on the DLP.. (greenish)

-- Jeremy



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