Re: Colorimetric Accuracy in the Field
Re: Colorimetric Accuracy in the Field
- Subject: Re: Colorimetric Accuracy in the Field
- From: Rich Wagner <email@hidden>
- Date: Wed, 05 Jun 2013 08:06:38 -0700
On Tue, 04 Jun 2013 13:18:48 -0400 Iliah Borg wrote:
>>> 3. Use dcpTool. (http://dcptool.sourceforge.net/Introduction.html) This command-line driven software can decompile binary format DNG profiles into XMP, compile XMP files into new compiled DNG profiles, make a profile "invariate" (an invariate profile won’t cause changes in tint when you make adjustments to exposure settings), and untwist a profile. (An invariate profile won’t change color when you adjust exposure settings, but still has hue twists embedded within it. Untwisting a profile removes all hue twists completely. See http://dcptool.sourceforge.net/Hue Twists.html)
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> With dcpTool you can reduce the "DNG profile" (DNG Camera Profile, DCP) to a simple matrix (ColorMatrix), however in ACR/LR white balance is still applied using ColorMatrix interpolation or extrapolation. Unless ad hock dcp is calculated for the light in the scene the result of interpolation/extrapolation is unpredictable in terms of colour.
Iliah,
Of course, DNG profiles can be created from a single illuminant with both the ColorChecker Passport software and the DNG Profile Editor, where both matrices end up identical.
dcpTool can do a lot more than just reduce a DNG profile to a simple color matrix.
TheHueSatDelta tables in DNG profiles have third dimension, value, that in the past was almost always set to 1, but that Adobe now uses to implement the “hue twists” in their new generation profiles. There is also a LookTable, which is exactly the same as a HueSatDelta Table, but is intended to be applied later in the processing pipe, “after any exposure compensation and/or fill light stages, but before any tone curve stage” that can also apply hue twists.
dcpTool can make a profile Invariate. Making a profile ivariate shifts any hue twists to before the basic exposure adjustments stage, so they are applied immediately. Therefore an invariate profile won’t cause changes in tint when you make adjustments to exposure settings. This is a way to modify the standard Adobe profiles if you want to keep the twists, but just prevent colors from changing when you adjust Carmera Raw or Lightroom’s exposure controls. Making a profile ivariate moves any LookTable to the HueSatDelta table. Specifically, if a LookTable exists, and no HueSatDelta tables exist, dcpTool simply copies the LookTable to the HueSatDelta tables, and the removes the original LookTable. If both a LookTable and HueSatDelta tables exist, the tables are mathematically combined, and the combination becomes the new HueSatDelta table. Again, the original LookTable is then removed. If no LookTable exists, then the profile is already invariate.
dcpTool can also Untwist a profile entirely. Untwisting a profile removes all hue twists completely. An untwisted profile will always keep colors true, regardless of original in-camera exposure, or changes to exposure controls in Camera Raw or Lightroom. Untwisting a profile removes all intensity (V in HSV) dependencies in both the LookTable and any HueSatDelta tables. Effectively, the twist at a single point in the twist table is chosen, and applied to all intensities. The twist chosen is that corresponding to a skin tone that is accurately exposed, thus preserving the profile's skin tone.
I'm particularly looking for feedback from people who have been using this tool, as appropriately modified DNG profiles would appear to effectively deal with many of the complaints about LR/ACR and colorimetric accuracy.
Thanks,
--Rich Wagner
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