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Hello I'm a newbie here.....I can't find much on this direct subject so hope it is ok to post a question. My problem may be more associated with not knowing how to search for what I am looking for....and if so I apologies in advance.

Anyway, I am trying to create an optical mask from a photo...of the sort you would use to make a silk screen for printing t shirts for example and I need the image to be sharp areas of solid black and white. Because I started with a colour image, back ground areas that I need to be pure white are in varying shades of grey....the shade of grey being created by the low black pixel density.

I'm using GIMP and I wonder if anyone can show me how to perform a lithographic change to my image such that light grey (low pixel density) areas are converted to solid white and dark grey (high pixel density) areas are converted to solid black. It sounds like something that the threshold tool could achieve except that that works on a pixel by pixel basis and appears to be intended to convert coloured pixels to black or white...if I use it on my image it has no effect whatsoever (turns black pixels black and white pixels white making no difference to the overall image!).

Many thanks

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    Hi and welcome to GDSE! Could you please post an image for us to look at (both at 100% and zoomed out). It's hard to understand exactly what the problem is. If threshold makes no difference it sounds like your image is already completely black and white. What you refer to as gray areas must be some kind of noise pattern or halftone pattern which consists of black and white pixels. If that is the case you need to manually paint the areas you want black or white. You could also backtrack a little and work on your original image before converting to black and white.
    – Wolff
    Commented Dec 7, 2020 at 20:55
  • Are you talking about creating a half-tone? GIMP has Filters > Distorts > Newsprint. See example
    – Billy Kerr
    Commented Dec 8, 2020 at 1:27

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You have something like this, I presume:

enter image description here

I guess you want to undo the existing half-toning and apply strict black-white tresholding. You probably know that it would be much easier if you had the original photo, but I guess it's either unavailable or the half-toning is the original.

You can get something maybe usable with filters. At first increase the image pixel dimensions so that each dot is several pixels wide. In the next image the smallest dots are 4 pixels wide.

enter image description here

It doesn't harm to use blurry scaling. The original half-toning had 1 pixel dots. It's scaled to 400% with linear resampling

Restore the greyscale. Steep low-pass filtering would be ideal, but without extensions there's available only blurs. In the next image there's applied Filters > Blur > Median Blur

enter image description here

Make it BW by applying Color > Treshold:

enter image description here

Cubic resampling to bigger size and smoothing with Gaussian Blur would give less torn edges but also fade more details. G'MIC extension package has better filters for the job, but I skip it.

Basic GIMP has filter Artistic > Oilify which can make edges smoother:

enter image description here

Smoother doesn't mean more accurate, but maybe looks better. BTW. Oilify alone couldn't do the job.

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  • Hello everybody and thanks for your suggestions so far... I'm away for the next 24hrs or so but will add an image asap. My image is an artwork for a world war II aircraft camouflage pattern, it is a painting or graphic design rather than a photo but it is in colour. There is little detail as such and clear demarkation between the areas of interest and so I thought it would be easy for software such as gimp to be able to differentiate between between high and low pixel densities (>x pixels per sq in=black, <x pixels per sq in = white).
    – Damien Walker
    Commented Dec 8, 2020 at 22:33
  • My intended mask creation process was: 1) Convert to black and white (giving high and low pixel densities) 2) Create optical mask by filling the holes in the high density areas and removing the speccles from the low density areas. Doing this long hand would be laborious....it is a luftwaffe mottle camouflage pattern which was basically dabbed onto the airframe in the field by a squaddie with a brush, it is not regular or predictable and it should be a trivial process for computer assuming someone has created a filter to do it! I really appreciate your help...picture asap
    – Damien Walker
    Commented Dec 8, 2020 at 22:33
  • user287001....that looks spot on! I was going to post some images but your guess is correct, your images are very representative of what I am working with: random 2-tone grey scale blotches! I haven't been able to try your method yet, but I will do and will report back. Many thanks, much appreciated.
    – Damien Walker
    Commented Dec 10, 2020 at 0:16
  • sorted! Many thanks and particularly to user287001 - your method works very well. All very much appreciated, thank you. Best regards.
    – Damien Walker
    Commented Dec 11, 2020 at 20:55

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