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Calibrating white and black points
#1
Hello,

I need to "calibrate" digitized images of test prints with the white and black patches of a color card, like the Spydercheckr24 that I have. 
My test prints are alternative photography 256 step wedges, i.e. little squares of degrading tones from the darkest to paper white that I print by hand using an old photographic process.

To better explain my "calibration" needs I can do the parallel with Photoshop. There I open my image containing the test print and the color card at its side (see attached image). I know the Lab (or RGB) values of the patches in my card. Using the Curves command in Photoshop I can customize the pipette values for the white, black and gray points. So I write in the Lab value for the black point and then click on the black patch of the color card in my image. Therefore the black point of the image adapts to the newly customized level. And so I do for the white and gray points inside the Curves command. At the end my image is "calibrated" as per the correct values of luminosity, the darkest and the white colors are as they are supposed to be.

In GIMP I have found that the Levels command has the white and black pipette but unfortunately they seem not to be customizable so they make the white patch to become your white (255,255,255 RGB) and your black patch to become (0,0,0 RGB) and my final test print is out of its real range of tones since the color card has not those values.

My question: is there a way I can perform a color value "calibration" using GIMP as explained above when using Photoshop? 

Thank you in advance for your help!
Paolo

P.S. attached a sample of my scan. I use the gray side of the color card but if of any help (automatic calibration sw) I can use the colored side.


Attached Files Thumbnail(s)
   
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#2
The color picker indicate what values in the input correspond to white or black (blue), but you can use the handles in the output to set the output values for said points (red). In the screen shot below I set output of the black square (input=50) to 50 (so it is kept) and set the output of the white square (input=236) to 200. 

   
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#3
(05-25-2021, 11:39 AM)Ofnuts Wrote: The color picker indicate what values in the input correspond to white or black (blue), but you can use the handles in the output to set the output values for said points (red). In the screen shot below I set output of the black square (input=50) to 50 (so it is kept) and set the output of the white square (input=236) to 200. 

Thank you!!
Such a simple but genius solution, I need to learn more about those technical things, I always wondered how to use properly the output slider other than just manually shorten the extension range of an image.
You solved my dilemma, now I can progress recording my video course about digital negatives for alternative photography. 
If I may I will credit you into my final thanks, should I write a real name?
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#4
(05-25-2021, 12:03 PM)psaccheri@gmail.com Wrote:
(05-25-2021, 11:39 AM)Ofnuts Wrote: The color picker indicate what values in the input correspond to white or black (blue), but you can use the handles in the output to set the output values for said points (red). In the screen shot below I set output of the black square (input=50) to 50 (so it is kept) and set the output of the white square (input=236) to 200. 

Thank you!!
Such a simple but genius solution, I need to learn more about those technical things, I always wondered how to use properly the output slider other than just manually shorten the extension range of an image.
You solved my dilemma, now I can progress recording my video course about digital negatives for alternative photography. 
If I may I will credit you into my final thanks, should I write a real name?

On the Internet, Ofnuts is my real name Smile
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#5
Hello
I write some more details to get some more help on the behaviour of GIMP.

I scan test prints using a Canon Lide 400 scanner where I set output as TIF, 8 bit depth, no automatic editing, Adobe RGB as color matching.
I used GIMP and Photoshop to edit the same scanned image (a step tablet with different shades of color in each step).
Inside GIMP the scan strangely shows up as automatic sRGB so I convert it to Adobe RGB, (I set in Photoshop the same color space).
I use my Spydercheckr 24 Color Card, where I use Adobe RGB values of its patches to calibrate the image into GIMP using the Levels adjustment and the color pickers. Since in GIMP the Output Levels has only one value space for writing the highlight and one value space for the shadows, I use the average value of the three RGB values for each color patch, white and black.
In Photoshop I use the Curves command, with the color pickers (very similar to the Levels in GIMP) and there I can write the three RGB values for each patch, white and black.

I process the scanned and calibrated image through a software that gives me a correction curve, reading the different tones in the different steps.
I get two different curves from the software, the curve of the image edited with Photoshop is higher than the curve of the image edited with GIMP (the image edited with GIMP is lighter than the one edited with Photoshop, so it needs less correction).
Since the only difference is the software that I use and the calibration inside it, I wonder how GIMP makes that calibration compared to Photoshop. And if there is a different way I can calibrate using a color card inside GIMP.
If anybody can help me.
Thank you

Attached a superimposed image of the two curves for easier visualization and discussion.
   
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#6
The fact that Gimp and Adobe don't agree on the colorspace is suspicious. Gimp uses sRGB unless told otherwise.

In Curves and Levels you can set the response on each color channel (Channel selector at top left under the Presets).

Did you try Gimp's color management (a whole raft of options in Edit ➤ Preferences ➤ Color management)
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#7
Thank you for the Channel tip for selecting the three RGB values, I supposed they worked only separately, one or another.

I possibly haven't written clearly about the color space problem.
I scan in TIF format telling in the scanner to use Adobe RGB space.
When I open the image in GIMP I see the file in automatic sRGB (and now I changed the settings to ask what to do so I can say convert to Adobe RGB).
When I open the image in Photoshop it says the image has no color profile embedded and asks what to do so I say assign Adobe RGB (my standard color profile).

So the point is possibly how to have the scanner to embed the color profile properly, because if I say to the scanner to scan with no color management the image I get is visibly different (duller) than the one scanned with sRGB or Adobe RGB settings (I can't distinguish the two by eye), so the scanner is doing something inside the file but writing it in some place that the editing software can't properly read (?). I have written to Canon assistance for the issue.

I attach here an image of all my settings of my Canon LiDE 400 scanner. 
In the Color Settings tab of the Preferences I have chosen Adobe RGB (left unchecked the "Monitor" box...).
In the Scan tab of the Preferences (not visible in the attached image) there is an option to raise the color depth from 8 to 16 bit but unfortunately GIMP seems to change the Levels highest values from 255 (8 bit deth) to 100 (16 bit depth) and I don't know what values to write in the boxes for my color card if I have 100 value(Lab?), so I leave the color depth to 8 which is enough for the work I have to do.


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#8
I have done new tests and found that writing the RGB values of my color card in the separate channels of the output slider in the Levels adjustment as you suggested gives me a correct calibration, my software shows approximately the same curve from GIMP and Photoshop. Hurray!

Hopefully I will solve the color space not embedded by my scanner the way the editing software expects it.

I hope GIMP will let us operate with 255 value even with images of more than 8 bit color depth.
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