deltae.py

Analyze dE in text files and images

Popular go-to page for checking dE in images is deltae.picturae.com ( https://deltae.picturae.com/ ; later d.p.c). Sadly, while it is working it was not updated in several years. While new features are not always welcome it is working with old reference values for X-Rite Colorchecker as for before November 2014:

https://www.xrite.com/pl-pl/service-support/new_color_specifications_for_colorchecker_sg_and_classic_charts

Difference between values for Colorchecker before and after November 2014 is 1.09 dE so while it is not distinguishable for human eye it matters when trying to get 4 stars on FADGI scale which require total dE of less than 3.

Another issue with d.p.c is Lab values of patches. Tested on the same file in Adobe Photoshop and in deltae.py script patches have very similar values. Values I got from d.p.c site are different. These errors accumulate and in images in difficult lightning conditions final dE differs up to 4 between d.p.c and deltae.py.

Usage

usage: deltae.py [-h] [--checker [{cc24,halfcc,nanocc,halfnanocc,gtdl,gt20,gt10,gt05}]] [--orientation [{S,W,N,E}]] [--deltae [{2k,76}]] [--color COLOR] [--coordinates COORDINATES]
                 testfile

Test color data

positional arguments:
  testfile              File to test

optional arguments:
  -h, --help            show this help message and exit
  --checker [{cc24,halfcc,nanocc,halfnanocc,gtdl,gt20,gt10,gt05}], -c [{cc24,halfcc,nanocc,halfnanocc,gtdl,gt20,gt10,gt05}]
                        Name of checker, supported values: - c24 (classic and mini) - default, - halfcc (lower half of them - grays and BGRYMC), - nanocc (nano version of classic CC), -
                        halfnanocc (lower half - grays and BGRYMC), - gtdl (GoldenThread Device Level), - gt20 (GoldenThread Big), - gt10 (GoldenThread Regular), - gt05 (GoldenThread
                        Small)
  --orientation [{S,W,N,E}], -o [{S,W,N,E}]
                        Orientation of checker: possible values are S, W, N, E (default S) S - in case of CC family greys are at the bottom, in case of GT you can read text normally, W -
                        greys are on the left, N - greys are on the top, E - greys are on the left
  --deltae [{2k,76}], -d [{2k,76}]
                        DeltaE difference according to: - 2k (default) deltaE 2000 (ass. with FADGI) - 76 deltaE 1976 (ass. with Metamorfoze)
  --color COLOR         L*a*b* data in file a la CTAGS
  --coordinates COORDINATES, -x COORDINATES
                        File with coordinates of fields in percentages of file (must be in tune with color data)

Text files

Program recognizes csv and txt as text files. Expects file in format used by d.p.c.: first 4 lines set-up (ignored by program), CSV header with 7 fields and later patch fields:

cc.jpg
Collected targets/patches

Targettype: CC
Patch,R,G,B,L,a,b
A1,112.3,69.3,51.2,33.53,24.12,22
...
F4,112.3,69.3,51.2,33.53,24.12,22

Now program doesn't do format checking so it will exit immediately.

Image files

deltae.py [image]

Program recognizes JPG, TIFF, PNG files. By default deltae.py verifies against cc24 geometry and values.

Supported checkers are:

• ColorChecker Classic and Mini (+ half, with only 2 rows - greys and BGRYCM)
• ColorChecker Nano (+ half, with only 2 rows - greys and BGRYCM)
• GoldenThread Object Level Big (2.0)
• GoldenThread Object Level Regular (1.0)
• GoldenThread Object Level Mini (0.5)
• GoldenThread Device Level

To prepare image for analysis user must have:

1. Rotate so grey row is at the bottom of file.

2. Crop down files quite precisely:

   • ColorCheckers Classic and Mini: down to crop marks in the corners
   • in case of half versions cut at the + in the middle
   • ColorChecker Nano: crop along physical edges of checker
   • in case of half version cut down along patches with minimal border
   • GoldenThread Big (gt20) and Regular (gt10) crop along outside of yellow border
   • GoldenThread Mini (gt05) and GoldenThread DeviceLevel (gtdl) crop down to black border