05-25-2021, 09:53 AM
(This post was last modified: 05-25-2021, 10:40 AM by psaccheri@gmail.com.)
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.
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.