I wonder if you could help me with a conundrum I have been trying to get my head around?
In Lightroom, sometimes a file displayed in the Library module looks different to the same file in the Develop module.
My understanding has until now been that the Library module simply honours the original profile and keeping the file in its original colour space saves the preview as an 8-bit JPEG and displays it via the monitor profile. So say a ProphotoRGB file is saved as an 8-bit AdobeRGB JPEG and is displayed via the monitor profile (say AdobeRGB for wide gamut).
In the Develop module things are handled differently. The original file is passed through the Lightroom internal colour space (ProphotoRGB with a linear gamma rather than 1.8) and then filtered through MelissaRGB (Prophoto with a tweaked sRGB tone curve gamma 2.2) before being displayed via the monitor profile say, AdobeRGB gamma 2.2 for wide gamut.
My understanding has been that these different processing paths through widely differing colour spaces and architectures (and therefore with scope for quantisation errors?) is what can lead to a colour shift between the two modules.
Also, I understand Lightroom does not rebuild the standard or 1:1 previews in the Library after changing develop settings but I am sure it updates them on the fly as I never see a difference when switching from one to the other.
The other probably more commonly quoted reason in the real world is inaccurate prosumer monitors which have been poorly calibrated - if at all. Garbage in, garbage out.
A friend of mine came across this colour shift and ascribes the phenomenon to something else.
He has a high-end monitor (NEC PA272W) which he claims can display colour beyond AdobeRGB (which it probably can in the reds and magentas probably with trade-offs elsewhere) and it is this which he claims explains the phenomenon.
(I have never seen this phenomenon on my Eizo but then again I have not gone looking for it. I have checked a range of images and can’t find it. Library and Develop previews look the same to me.)
Anyway, as proof, he created a Prophoto JPEG containing 6 colour blocks - red, green, blue, magenta, cyan and yellow. Each block has three colour strips with the most saturated colour that can be contained in ProphotoRGB, AdobeRGB and sRGB respectively - using RED RGB 255,0,0 in each strip.
When displayed in Library the ProphotoRGB and AdobeRGB strips match, with sRGB looking darker and undersaturated.
However, when displayed in the Develop module, there are distinct differences between between all three strips especially in red and magenta but also in green, blue and cyan but virtually none in yellow. While I agree with his findings, I am not sure they explain the problem as very few people use high end, wide gamut monitors - most use cooking quality sRGB screens or at best, prosumer grade devices yet still report seeing such a shift.
I can reproduce these findings on my Eizo CG318-4K but have struggled to explain it to my own satisfaction.
This is what I have come up with:
In the Library module, the ProphotoRGB file is saved as an 8-bit AdobeRGB JPEG and displayed via the wide gamut AdobeRGB monitor profile.
The ProphotoRGB contains more colours than Prophoto allows and so can only display AdobeRGB.
The AdobeRGB contains all the AdobeRGB colours and displays the full gamut correctly.
The sRGB contains too few colours for the AdobeRGB space and so displays truncated colour.
This agrees with what I see on screen.
In the Develop module the preview is live, the file following the route > LR RGB > MelissaRGB > monitor AdobeRGB.
The ProphotoRGB contains more colours than the 'expanded’ AdobeRGB of the monitor and so displays somewhat more than standard AdobeRGB.
The AdobeRGB contains fewer colours than the monitor is capable of displaying and so displays AdobeRGB correctly - but less than the full capability of the monitor.
The sRGB contains fewer colours than AdobeRGB and so displays truncated - but even more so than before given the "Adobe-plus" gamut of the monitor.
1. Is this what is happening?
2. Does this explain the colour shift between modules?
3. It seems to me that this test is as extreme as it gets, it’s relevance in real world imaging is pretty marginal and would in any event be masked by similar but opposite shifts for other colours.
So, bottom line, what causes the occasional colour shift between the Library and Develop modules in Lightroom?
1. Quantisation errors/use of different colour spaces/different preview architecture - however you want to put it.
2. Insufficiently accurate monitors/inadequate calibration and profiling.
3. Use of highly accurate, well calibrated, expanded gamut monitors.
One, two or some of all three?
Please correct any nonsense and please forgive the length of this email.