Thanks, @Thanatomanic.
There are clear differences between the results, as if human editors had decided on different relative importances of picture elements.
I am suspicious of the OP “the higher numbers mean a better similarity” if identical images give a similarity of zero.
My usual comparisons are with ImageMagick, the RMSE (root mean squared error) metric. Results are between 0.0 for identical images, and 1.0 for images that are as different as possible. As a rule of thumb, when the result is less than 0.01 and the differences are evenly distributed, I can’t see any visual difference.
With that metric, the RMSE difference from the means, using your supplied JPEGs are:
The Digikam image is 1283 pixels wide, so I have cropped the final 3 columns off.
scene 1:
Biggest difference (Digikam) 0.145474
Smallest difference (Lightroom) 0.058388
scene 5:
Biggest difference (LightZone) 0.123831
Smallest difference (OOC JPEG) 0.140308
Oops. With this measurement, the scene 5 OOC is more different to the mean than the LZ is different to the mean, contradicting your results. But there isn’t much between them; they are both radically different to the mean.
We can blur the images with “-blur 0x3”. This is a Gaussian blur with sigma=3 pixels, so the radius is approximately 9 pixels. Results are then:
scene 1:
Biggest difference (Digikam) 0.145067
Smallest difference (Lightroom) 0.0580373
The blur hasn’t affected the scene 1 results much, because it is quite a “smooth” image (little high-frequency detail).
scene 5:
Biggest difference (LightZone) 0.0580373
Smallest difference (OOC JPEG) 0.0792639
The blur has affected the results because the image has loads of high-frequency detail, but hasn’t changed the relative order.
Of course, we should use 16-bit TIFFs, not 8-bit (lossy?) JPEGs, but I expect that doesn’t make much difference.
For transparency, here is my Windows BAT script:
%IMG7%magick ^
scene1_mean.jpeg ^
( scene1_DK.jpeg -crop 1920x1280+0+0 +repage ) ^
-metric RMSE ^
-format "%%[distortion]\n" ^
-compare ^
info:
%IMG7%magick ^
scene1_mean.jpeg ^
scene1_LR.jpeg ^
-metric RMSE ^
-format "%%[distortion]\n" ^
-compare ^
info:
%IMG7%magick ^
scene5_mean.jpeg ^
scene5_LZ.jpeg ^
-metric RMSE ^
-format "%%[distortion]\n" ^
-compare ^
info:
%IMG7%magick ^
scene5_mean.jpeg ^
scene5_OOC.jpeg ^
-metric RMSE ^
-format "%%[distortion]\n" ^
-compare ^
info:
%IMG7%magick ^
scene1_mean.jpeg ^
( scene1_DK.jpeg -crop 1920x1280+0+0 +repage ) ^
-blur 0x3 ^
-metric RMSE ^
-format "%%[distortion]\n" ^
-compare ^
info:
%IMG7%magick ^
scene1_mean.jpeg ^
scene1_LR.jpeg ^
-blur 0x3 ^
-metric RMSE ^
-format "%%[distortion]\n" ^
-compare ^
info:
%IMG7%magick ^
scene5_mean.jpeg ^
scene5_LZ.jpeg ^
-blur 0x3 ^
-metric RMSE ^
-format "%%[distortion]\n" ^
-compare ^
info:
%IMG7%magick ^
scene5_mean.jpeg ^
scene5_OOC.jpeg ^
-blur 0x3 ^
-metric RMSE ^
-format "%%[distortion]\n" ^
-compare ^
info: