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+--- Thread: Does posterize black and white use same value ranges each time? (/Thread-Does-posterize-black-and-white-use-same-value-ranges-each-time)
Does posterize black and white use same value ranges each time? - topher - 05-05-2022
Hi,
I am trying to print the equivalent of a value swatch for graphite and charcoal drawings that I do and I am still fairly new to Gimp.
Ultimately when I posterize a black and white image with say 7 levels I am trying to get a print out of the 7 grey values used in the posterized image so I can match to graphite and charcoal values of my pencils and tools.
My question(s) around this:
- apart from indexing the image, creating a colour palette and then using gradients to print the colour palette, is there an easier way to be able to print out the grey values used?
- if I have multiple black and white images all using 7 layers when I posterize the image, will all images automatically have the same 7 values or will they be different slightly for each image?
Really appreciate any help on this.
Cheers,
Topher
RE: Does posterize black and white use same value ranges each time? - rich2005 - 05-05-2022
As with most graphics, it depends. You can check in Gimp Colors -> Info where there is a color cube analysis (also worth looking at the histogram there)
Second question first:
Quote:- if I have multiple black and white images all using 7 layers when I posterize the image, will all images automatically have the same 7 values or will they be different slightly for each image?
I am assuming that the pencil drawings are scanned. Scan in color RGB or Grayscale ? It does make a difference.
This various pencils on art paper scanned both ways.
The grayscale scan has 226 colours, the RGB scan 62649 Comparison: https://i.imgur.com/TdOtbbn.jpg
Using Gimp posterize and 7 levels, Grayscale gives 7 colours while the RGB scan does not look that different but there are 112 colors, there are fringes somewhere on the image.
You can get just 7 colours going Image -> Mode -> Greyscale then posterize. If you want to go back to RGB the 7 remain. https://i.imgur.com/nEwux3y.jpg
Quote:- apart from indexing the image, creating a colour palette and then using gradients to print the colour palette, is there an easier way to be able to print out the grey values used?
An easier way ? I do not know one. If you go the grayscale route, then the 7 value posterized palette is equally divided and is #ffffff #d5d5d5 #aaaaaa #808080 #555555 #2b2b2b #000000
RE: Does posterize black and white use same value ranges each time? - Ofnuts - 05-05-2022
Experimentally:
- Create an image with a grayscale gradient
- Make two duplicates, make one darker, one lighter, by moving the middle values in curves (so image sill contains pure black and pure white)
- Posterize all three to 8 levels
[attachment=7881]
As you can see from the histograms of the results the levels are exactly the same in all three. Similar tests but make darker/lighter version by changing the darker/lighter values so that the results no longer contain pure white or pure black:
[attachment=7882]
The Levels are the same in all three, even if some levels are not used in the picture...
RE: Does posterize black and white use same value ranges each time? - rich2005 - 05-05-2022
An easy way ????
Not Gimp, This using ImageMagick command and a grayscale 7 level posterized image.
Code:
magick gray.png -colors 7 -define histogram:unique-colors=true -format "%c" histogram:info:Outputs this
1289: (0,0,0) #000000 gray(0)
20400: (43,43,43) #2B2B2B gray(43)
29548: (85,85,85) #555555 gray(85)
50923: (128,128,128) #808080 gray(128)
55710: (170,170,170) #AAAAAA gray(170)
200560: (213,213,213) #D5D5D5 gray(213)
2358370: (255,255,255) #FFFFFF gray(255)