Troy … lots to unpack here.
I agree … the primary workflow in ICC starts with an output referred image. The goal is to reproduce that image, as closely as possible, on a different display device (e.g. RGB monitor) or medium (e.g. printed piece of paper) than the original was encoded for.
I’m not sure what this means.
I’m also not sure exactly what you mean here, but I assume you’re taking issue with the fact the Camera RGB values are generally transformed into estimates of scene colorimetry even if that colorimetry is expressed as RGB vs. XYZ or LMS. I’m not sure if you have an issue with that, or if it’s just a statement.
Rendering, by definition, is the process of deviating from the scene’s colorimetry. The transform converts scene referred images to output referred images. All output referred images have an associated display device or reproduction medium … that’s what makes them output referred.
From ISO 22028 Section 4.4
A colour-rendering transform is used to transform a scene-referred image to an output-referred image. Colour-rendering transforms embody the tone and colour reproduction aims of an imaging system, relating the corresponding scene colourimetry to the desired picture colourimetry.
It should be noted that colour-rendering transforms are usually substantially different from identity transforms and accomplish several important purposes including compensating for differences in the input and output viewing conditions, applying tone and gamut mapping to account for the dynamic range and colour gamut of the output medium, and applying colour adjustments to account for preferences of human observers.
NOTE Colour-rendering transforms are typically proprietary and irreversible.
1D lookup tables are often one step in rendering transforms. It is well known they produce hue and saturation deviations from the scene colorimetry on the output device. 1D lookup tables have been used for decades as an intentional, albeit blunt, mechanism to account for the “several important purposes” outlined in the ISO definition of rendering above. It’s not a perfect mechanism, isn’t capable of the fine tuning one might want, but the deviations from scene colorimetry aren’t unintentional.
Ok, so you want to define the output colorimetry of a given rendering transform, on a specific device, to be hero colorimetry?
Yes, ICC starts with an output referred image, intended to be displayed on a particular device or reproduced on a particular medium. That output referred image is transformed to a new output referred image intended to be displayed on a different device or reproduced on a different medium. The ICC “rendering intents” will dictate how gamut mapping between the devices is achieved.
ICC doesn’t do conversion of scene referred images to output referred images.
A major issue for some types of images is that in ICC there’s a “reference medium” and “reference gamut” built into the ICC conversion from the first output referred image to the second. That reference medium as a dynamic range of 288:1 and a black level of 0.3%. So, if you have an output referred image intended to be displayed on a device with a dynamic range of say 100,000:1 and want to use and ICC profile to convert it to an output referred image for another HDR device (maybe one capable of a very low black level), you can’t use ICC without playing tricks. The result will be lifted blacks and a compressed dynamic range.
Output referred = medium.
The issue with the PCS reference medium is that there’s a source medium associated with the first output referred image, a destination medium associated with the output referred image you’re trying to create, and in the middle there’s a medium associated with the PCS that’s often smaller than the ones on the ends.
One has a choice when converting between output referred image encodings. Say you have an SDR image you want to reproduce on a HDR device.
You have the choice to either :
A. make the output colorimetry match and look exactly the same on both devices
B. stretch the SDR output referred colorimetry across the HDR output device’s dynamic range
C. use the scene referred image, if available, to re-render a new output referred image taking advantage of the HDR device’s specific capabilities.
No choice is right or wrong, but it’s a choice that needs to be made. Only A. will “match”, but you’re also not taking any advantage of the HDR device’s unique capabilities. Your audience literally won’t even know the displays are different if shown side-by-side.
Assuming the first SDR devices gamut volume encloses the second, the same choices available here … just less extreme.
If it doesn’t you need to gamut map between the 2 if you want them to “match”. I put match in quotes because the devices are different, so they will never strictly match.
I think you’re saying your preference is to reproduce scene colorimetry on an output device. This is a choice one can make.
