Does posterize black and white use same value ranges each time?

[topher]

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

[rich2005]

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

[Ofnuts]

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
  

   

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:

   

The Levels are the same in all three, even if some levels are not used in the picture...

[rich2005]

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)