I have a hypothesis to explain my original observations but need the group's expertise as to whether this explanation is, in fact, plausible.
Let's assume that the sRGB Granger chart has been properly generated and that its embedded sRGB profile correctly matches the colour space used to generate the image. Also recall that there is no control for the sensitivity of Soft Proofing gamut warnings. I assume then that even minor (perhaps barely perceptible) colour mismatches will be flagged with Destination gamut warnings.
I understand Lightroom performs all its internal calculations using the (very wide gamut) Pro Photo colour space. Thus both the specified soft proofing sRGB profile and the embedded image sRGB profile would have to be converted to Pro Photo.
I suspect that small differences in the sRGB profiles (or possibly LR is using a slightly different conversion algorithm in each case) could account for the observed gamut warnings. To verify this explanation, one would have to be able to measure the deviation between the flagged image colours and the corresponding reference colours used by Soft Proofing.
Comments?
- David
I've used the ExifTool to extract the following information directly from
Granger Chart sRGB IEC 61966-2.1.jpg (Copyright 2011© Douglas Janson) and from an image exported from Lightroom.
Granger sRGB ProfileDevice Manufacturer : IEC
Device Model : sRGB
Device Attributes : Reflective, Glossy, Positive, Color
Rendering Intent : Media-Relative Colorimetric
Connection Space Illuminant : 0.9642 1 0.82491
Profile Creator : HP
Profile ID : 0
Profile Copyright : Copyright (c) 1998 Hewlett-Packard Company
Profile Description : sRGB IEC61966-2.1
Media White Point : 0.95045 1 1.08905
Media Black Point : 0 0 0
Red Matrix Column : 0.43607 0.22249 0.01392
Green Matrix Column : 0.38515 0.71687 0.09708
Blue Matrix Column : 0.14307 0.06061 0.7141
Device Mfg Desc : IEC
http://www.iec.chDevice Model Desc : IEC 61966-2.1 Default RGB colour space - sRGB
Lightroom sRGB ProfileDevice Manufacturer : IEC
Device Model : sRGB
Device Attributes : Reflective, Glossy, Positive, Color
Rendering Intent : Perceptual
Connection Space Illuminant : 0.9642 1 0.82491
Profile Creator : HP
Profile ID : 0
Profile Copyright : Copyright (c) 1998 Hewlett-Packard Company
Profile Description : sRGB IEC61966-2.1
Media White Point : 0.95045 1 1.08905
Media Black Point : 0 0 0
Red Matrix Column : 0.43607 0.22249 0.01392
Green Matrix Column : 0.38515 0.71687 0.09708
Blue Matrix Column : 0.14307 0.06061 0.7141
Device Mfg Desc : IEC
http://www.iec.chDevice Model Desc : IEC 61966-2.1 Default RGB colour space - sRGB
Update 1: I tried to remove the Destination Gamut Warnings by using the Primary Saturation sliders. Contrary to my hypothesis, the gamut warnings are highlighting colours that Lightroom believes are well outside the gamut that Soft Proofing is testing for (see attached screenshot).
Update 2: Photoshop Feedback user
LRuserXY noted that turning the GPU on or off affects the gamut warnings displayed. I confirmed this for the Granger chart. The Destination Gamut Warning area is larger when the GPU is enabled. This behaviour may be confirmation that colour characterization is a highly sensitive to calculation errors.
Update 3: When I extracted LR's sRGB profile from an JPEG exported RAW file and viewed the Exif data, I noticed a difference in the Rendering Intent (see above).
Q: Could this difference possibly impact the display gamut warnings?
A: No, gamut warnings are
independent of the Rendering Intent.
See
Introduction to Rending Intents and
Understanding Rendering Intents for more information.
Update 4: Using an ICC profile viewer, the SRGB and embedded Granger Chart gamuts seem to be identical (see attached plot).
Update 5: Final nail in the coffin => sRGB Granger Chart soft proofed with its own (embedded) colour profile still displays identical gamut warnings (see
Gamut Warning for Granger Chart Soft Proofed with own Colour Profile). I am convinced that observed behaviour is due to a deficiency within Lightroom.
- David