I have compared the accuracy of five different camera input profiles in Raw Therapee (RT). My custom ICC profiles and the Canon 400D DCP profile distributed with RT proved satisfactory. The camera standard (dcraw) profile in RT was inaccurate. I can also confirm that the 24 patch ColorChecker Classic target is adequate for making simple matrix profiles.
The procedure outlined here can be modified for other targets and raw photo software. I used Raw Therapee version 4.08 and Argyll CMS version 1.4.0.
I have previously made custom camera input ICC profiles for RT. Testing accuracy with the same photo as profiled would bias the results in favour of my own profiles. I used a different photo, taken 6 months later. I did use the same target and reference data however, and the results might not be completely unbiased.
For testing profiles I photographed an X-Rite ColorChecker Digital SG (CCSG) target in direct morning sunlight (near D50) with a Canon 400D (Digital Rebel XTi) digital SLR camera. I metered exposure off an 18% grey card. I am yet to evaluate accuracy under other illuminants.
I processed the test raw photo in RT version 4.0.8 with a variety of profiles.
- Working and output profiles were set to ProPhoto, a larger colour space than the CCSG target.
- Custom WB was made on neutral-grey patches.
- Exposure was adjusted with the middle-grey patch H5 (target L* = 50).
- The RGB tone curve was linear.
- No other image processing was performed.
- Results were exported as 16 bit TIFF files.
Exposure adjustments were required when highlight recovery was active. Secondly, the brightness of the test photo was not exactly the same as the brightness of the photo used in profiling.
Next, I used Argyll CMS version 1.4.0 and scanin to read the patch RGB values. I then copied RGB measurements from the .ti3 output text files to a spreadsheet. I converted output RGB to L*a*b* using the equations and RGB to XYZ matrix from Bruce Lindbloom’s site. Finally, I compared delta E 94 (DE94) colour differences between output L*a*b* and the X-Rite reference data. I used graphic arts weighting factors in DE94.
It would have been easier to use Argyll CMS profcheck to calculate colour differences, but profcheck can’t evaluate DCP profiles and the built-in “dcraw” profile in RT.
Five camera profiles were evaluated and one profile (“matrix”) was evaluated in two different scenarios:
- Custom ICC matrix profile, based on an overexposed image (“hot matrix”).
- Alternative custom ICC matrix profile, based on an image exposed as-metered off an 18% grey card (“matrix”).
- Alternative custom ICC matrix profile, as above, with 1-stop highlight recovery processing (“matrix HR”).
- Alternative custom ICC matrix profile, based on the 24 ColorChecker Classic like patches in the CCSG target (“matrix 24”).
- Camera standard profile in RT (“dcraw”).
- Canon 400D DCP profile distributed with RT (“DCP”).
|Exposure adjustments and delta E 94 error summaries for six profile tests. The dcraw profile was most inaccurate.|
Differences between profiles are examined more closely for the 24 classic patches. Results for the dcraw profile illustrate the well known “anaemic red” problem (for Canon cameras at least): red DE94 = 8.5 (patch G04) and moderate red DE94 = 6.3 (patch G03).
Color differences were much reduced using my custom ICC profiles. Both highlight recovery strategies “hot matrix” and “matrix HR” did not increase errors, i.e. linear transforms are well behaved. Errors with the 24 patch matrix profile were quite acceptable, i.e. the 24 patch ColorChecker Classic is adequate for matrix profiles.
Accuracy of the RT Canon 400D DCP profile was similar to my matrix ICC profiles except the yellow patch error was a bit high (DE94 = 4.9, patch H4).
In practise, my custom ICC profiles and the RT Canon 400D DCP profile do show differences in side-by-side comparisons: between profiles mean DE94 = 2.3, peak DE94 = 5.9. DE around 2 is a just noticeable difference.
I have noticed differences in blue skies and the DCP profile seems to have a slight reddish bias. I haven’t done enough comparisons to reach a conclusion on which profile is “best”.