That
might be a quick and dirty check, provided you can recreate said lighting conditions at will and be confident of their reliability, including the framing of the Color Checker card a highly unlikely circumstance.
There are at least two methods of comparing two allegedly identical digital images:
c
Comparing allegedly identical images in Photoshop
(The time-honored Photoshop
"Difference" Test should be reviewed. If there should be a very small difference in a given pixel, the otherwise 255,255,255 pixel would then appear as 255,254,255 or 254,255,255 or something similar, which you would still see as black on your monitor, regardless of magnification.)
Two better, more definitive testing methods would be (a) one suggested by Bruce Fraser himself (below), and (b) a new (to me) method suggested by someone in the Color Managament and Photoshop Windows forums, which follows:
(NOTE: only the methodology is of interest and pertinent, not the questionable context in which it has brought up and used.)
* 1) Open the two images to be compared in Photoshop
* 2) Move one image as a layer over the other one
* 3) select "Difference" as blending mode in the layers palette
* 4) now the whole image should appear seemingly black on the monitor
[So far this is the traditional, "time honored" method.]
* 5) select the magic wand tool with these settings: Tolerance: 0/ Anti-alias: no/ Contiguous: no/ Sample All Layers: yes
* 6) click somewhere into the formerly gray area
[This refers to an image of a Color-Checker type of card that had wide gray border around it. The test, therefore, requires a pure gray image in the image, something highly unlikely to change, in order for the magic wand to select all pure-black images (255,255,255). Such a border can easily be created around an image by increasing the canvas size and filling the newly created space with pure gray (128,128,128). ]
Explanation: you just selected all completely black pixels (0,0,0) i.e. all pixels that are identical in both layers.
* 7) you should see "marching ants" forming rectangular patterns
* 8_) invert the selection (Shift Command I)
Explanation: the selection now covers all the other pixels, i.e. all pixels which are different between both layers.
* 9) create a new empty layer and select it in the layers palette
* 10) set the foreground color to white
* 11) fill the selection with white (Alt+backspace on Windows, accordingly on Mac)
* 12) set the blending modes of all layers back to normal
Explanation: you now see all identical pixels in their respective color and all different pixels in white.
This method is a lot more sensitive than the traditional one which stops at step #4 above.
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[more below]
Another method suggested by Bruce Fraser:
A better way of comparing images with identical pixel dimensions is to use Apply Image > Subtract with an offset of 128.
Difference only shows pixels that are lighter in the source than in the target (or maybe it's the other way aroundI forget) where
Subtract with Offset 128 shows differences in both directions.
Pixels that are identical in both images come in as RGB 128 gray, those that are different come in at a value that exactly reflects how different they are.
It also makes it much easier to spot subtle differences
c [EDITED formatting]
