Luminous Landscape Forum
Raw & Post Processing, Printing => Colour Management => Topic started by: alecdann on November 06, 2018, 03:02:39 pm
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I'm trying to understand the path that a RAW image takes into Lightroom/ACR.
Does the image pass through the camera profile (Adobe Color for Nikon D850) into the Profile Conversion Space (L*a*b*) then from the PCS to Lightroom/ACR Prophoto?
Or is Lightroom/ACR Prophoto part of the camera profile?
What role does the PCS play, in other words?
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Lightroom / Adobe Camera Raw (ACR) are processing applications that will render raw data from files produced from Digital Cameras they will utilise their own proprietary default settings and profiles to process the raw data which is different to the processing engine utilised by the camera manufacturer, or other raw processing software.LR / ACR utilise a variant of Prophoto RGB as the colour working space. When you render the file to a image file tiff, jpeg etc you can assign a color space of your choice.
I am not aware what "PCS" is, or what role it plays if any.
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PCS is the Profile Connection Space that sits in the computer operating system's colour management module (CMM). Normally, conversions from one space to another require two profiles and they connect through the CMS.
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Not sure if this helps or not but this is what I was told by Adobe and was posted awhile back on the ColorSync list so it's not a secret ;) :
ACR/LR does perform internal color transforms as needed to carry out its various image processing routines (e.g., noise reduction, vibrance, etc.). However, this is all done internally and has no real connection to the user-specified color space of the rendered output file (e.g., sRGB, Adobe RGB, etc.).
The camera profiles produced by DNG Profile Editor and other software (e.g., X-Rite's ColorChecker Passport) are responsible for transforming the camera-native image values (e.g., the primaries determined by a sensor's color filters) to RIMM. At least a color matrix is required, but other components (such as optional lookup tables and a tone curve) can be included, too. This is largely independent of the other internal color transform described in the previous paragraph.
No, L*a*b* is not the ACR internal working space.
ACR's primary internal representation uses the ProPhoto/RIMM/ROMM RGB primaries. ACR's primary internal representation uses the ProPhoto/RIMM/ROMM RGB primaries. Exposure and brightness are based on the traditional logarithmic base 2 formulation (i.e., in stops). So boosting Exposure to +1, for example, means doubling the linear scene-referred values (i.e., increasing exposure by 1 stop). The white balance math (color temperature and tint) simply associates user-chosen white points (e.g., tungsten) to corresponding camera whites, by means of a profile's color matrix or matrices. More details (for developers) can be found in public DNG SDK on the Adobe site.
There may be some 'kind' of PCS used as there has to be some translation above, but it isn't Lab.
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Does the image pass through the camera profile (Adobe Color for Nikon D850)
"Adobe Color" is not a "camera profile", it is a set of adjustments that ACR/LR apply after "Adobe Standard" DCP (actual
"camera profile") for your specific camera model is applied... at the end of the pipeline.
DNG specifications will tell you how ACR/LR code executes color transforms using data in DCP camera profiles = https://www.adobe.com/content/dam/acom/en/products/photoshop/pdfs/dng_spec_1.4.0.0.pdf
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Possibly some of the confusion is with ICC profiles which do have a PCS, either XYZ or Lab. Adobe's ACR's processing is a completely different as are their camera "profiles."
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Thanks all. I appreciate your helpful responses.
Clearly I had a misconception about the process where raw conversion is concerned. Adobe's Raw Conversion Process is 100% proprietary.
After my original post I went so far as to open the Lightroom package and navigate to the Adobe Standard camera profile for Nikon D850. It is not an "open" format (<snip> > ¶« Nikon D850 , ' —Ïˇˇ' †˝ˇˇ' Rˡˇ' î7 ' ® ' ì¸ˇˇ' E ' µ ' •( ' UÒˇˇ' <snip>).
Some of my confusion doubtless came from Adobe's use of the word profile but that's a reasonable use of the term and my mistake was conflating Adobe's profiles with ICC profiles.
Scanners, of course, require ICC input profiles so that's a different process.
Thanks again for your help.
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DNG specifications will tell you how ACR/LR code executes color transforms using data in DCP camera profiles = https://www.adobe.com/content/dam/acom/en/products/photoshop/pdfs/dng_spec_1.4.0.0.pdf
Yes that would be my recommendation too - Chapter 6 in particular, it's entitled "Mapping Camera Color Space to CIE XYZ Space". It's not the easiest read though!
There's nothing secret about it, it's information that is freely available to anyone and is presumably aimed at software developers who need to process DNG files or RAW files using DCP profiles.
In very much simplified terms,
White Balance/CCT is determined using a camera color matrix
A Forward Matrix is used to convert the raw data to XYZ space which is then converted into linear RGB Prophoto which is the "working space" in ACR/Lightroom.
An optional HSV look-up table is used to fine tune the color
A software tool like EXIFTool GUI can be used to read the color matrix data contained in a DNG or DCP file.
Dave
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Does the image pass through the camera profile (Adobe Color for Nikon D850) into the Profile Conversion Space (L*a*b*) then from the PCS to Lightroom/ACR Prophoto?
Or is Lightroom/ACR Prophoto part of the camera profile?
What role does the PCS play, in other words?
It's a multi-step process outlined in the DNG document, as Dave suggests (step 1 is not necessarily the first one performed, the others need to be in the shown sequence though):
1) White balance the raw data / demosaic
2) Convert to XYZ (PCS) via appropriate linear matrix
3) Apply colorimetric HueSaturationValue corrections (table 1) via ProPhoto RGB and back to PCS (recently desaturated, no longer colorimetric)
4) Apply a subjective look (e.g. Color, Neutral, Landscape, Camera Matching etc.) using HueSaturationValue adjustments (table 2) via ProPhoto RGB and back to PCS.
5) Apply tone curve
6) Convert to final color space (say Adobe RGB)
7) Apply appropriate gamma
The data for steps 2-5 is contained in a dcp profile, as described in the DNG standard. In ACR they used to be in the camera tab but in 2018 Adobe re-branded these 'Raw Profiles' and moved them to the front tab. You can see the effect they have on an image step-by-step here
https://www.strollswithmydog.com/raw-file-conversion-steps/
As of this year Adobe have also added a set of presets that they call 'Creative Profiles' (https://blogs.adobe.com/jkost/2018/04/lightroom-cc.html) (Artistic, Modern, Vintage, etc.). These can be used on non-raw images and are applied after step 7). They appear to have nothing to do with dcp profiles or PCS.
Jack
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Thanks, Dave and Jack. Great resources.