Remove colors to keep only a few

[Paulr]

Hello,

I'm fairly new to Gimp, and have a project where I'd like to isolate some colors in a photo.
It is to hopefully reveal a hidden message.
I'm thinking I can remove some colors (maybe convert them to a specific color like white) so what's left are all colors except what I picked.
Thanks.
Paul

[rich2005]

Quote:...I'd like to isolate some colors in a photo.
It is to hopefully reveal a hidden message. I'm thinking I can remove some colors (maybe convert them to a specific color like white) so what's left are all colors except what I picked.

Is the 'photo' an indexed image? Typically a gif or a png maybe a bmp, top of the Gimp window will tell you. If it is it is worth trying;
Colors -> Map -> Set Color map
Choose an limited palette, Used volcano here. screenshot https://i.imgur.com/XvcxwBL.jpg
With a bit of luck any 'hidden text' will show. https://i.imgur.com/G5g1jlR.jpg

Originally, all to do with editing an indexed image colormap. Look for a suspicious entry in the colormap. https://i.imgur.com/sZ3KFgd.jpg Assign a new color. https://i.imgur.com/YJ2virv.jpg

On the other hand, If a jpg, tif or other RGB image then the number of colors (apart from grayscale) will be in the thousands and probably some other method than changed colours used to hide the text. Sometimes fast-fourier-transform (FFT) is used.

[Paulr]

[attachment=3198 Wrote:rich2005 pid='14888' dateline='1569397750'][attachment=3198][attachment=3198]

Quote:...I'd like to isolate some colors in a photo.
It is to hopefully reveal a hidden message. I'm thinking I can remove some colors (maybe convert them to a specific color like white) so what's left are all colors except what I picked.

Is the 'photo' an indexed image? Typically a gif or a png maybe a bmp, top of the Gimp window will tell you. If it is it is worth trying;
Colors -> Map -> Set Color map
Choose an limited palette,  Used volcano here.   screenshot https://i.imgur.com/XvcxwBL.jpg
With a bit of luck any 'hidden text' will show. https://i.imgur.com/G5g1jlR.jpg

Originally, all to do with editing an indexed image colormap. Look for a suspicious entry in the colormap.  https://i.imgur.com/sZ3KFgd.jpg    Assign a new color. https://i.imgur.com/YJ2virv.jpg

On the other hand, If a jpg, tif or other RGB image then the number of colors (apart from grayscale) will be in the thousands and probably some other method than changed colours used to hide the text. Sometimes fast-fourier-transform (FFT) is used.

Hello Rich,
Thanks for the answer. It's a JPEG taken with my digital camera. And it has a lot of different colors in it.
I guess i could select all pixels of a certain color, but then I'd need to select many times to remove all the different shades of a certain color.
Would there be a 'color range selector' tool?
Here's a snippet of the image. How can i isolate or only show the darkest shades of the groove?
The rest of the stone can be white.

[rich2005]

Makes me wonder what sort of message?

Things to try out.
1. Posterize Colors -> Posterize You get more colours than the setting but it will reduce colours from 21000 (your clip) to 22 screenshot https://i.imgur.com/VC08d7Y.jpg
2. Convert to an indexed image Image -> Mode -> Indexed with a small number of colours (16 shown) . Edit the resulting colormap. https://i.imgur.com/B4w1ZnE.jpg
3. Tools -> GEGL Operation -> Color Warp Not a filter I know too much about apart from it is there. You can color pick or set both from and to colours. https://i.imgur.com/xDtoYpa.jpg

[Ofnuts]

Very unlikely that you can find a hidden message just by looking at specific colors. This is so dependent on the lighting used when you took the picture (direction, color temperature).

If you wan tot see grooves clearly, you to light the object from the size so that light rays are almost parallel to the surface. In the picture above the light is head on (and possibly a flash), exactly the opposite of what should be done.

[rich2005]

I still do not get the hidden message bit

Maybe a completely different approach. This the image bump-mapped using a lightly blurred desaturated copy as the map.
https://i.imgur.com/stkAil6.jpg
Might show a bit of detail

[Paulr]

Hello Rich,

Thanks for your input.
The owner of the stone is very shy about revealing what sort of message is possibly engraved (or chiselled) in this stone. I don't know much about it.
I've tried the 3 color tools you mentioned without any great result (they worked technically but nothing was 'exposed' better).
I'll play with the bump map, as it appears interesting. Similar to the 3D scans I did before on the stone.

Offnuts: I tried 3D laser scanning (I'm a lot more familiar with this technology than image-editing) and even with 6 million points spaced at 0.01mm -0.1mm average, did not see much. The pits in the stone create a lot of noise and the 3D model looks like the usual orange peel so common with good quality scanners (they pick up too much detail).
And these point clouds are fairly large to work with (3D meshing et al.)
So I tried the optical approach. But as you mention, I may be able to see more shadows by applying what you said.
My photos were taken on a sunny day, in the back of a SUV.

Thanks.

---

Rich,

I tried bump mapping quickly, using the same image as map, and this gave me a fairly decent result.
I'm not a 'power-user'-far from it.
So I'll do as you said, use a B/W copy of the image as map.
The 'help' is pretty good on this topic.

I am impressed with this filter.

Thanks.

[denzjos]

If nothing is helping you, maybe something to try out : I just want to mention that a bought a few years ago a plugin for a graphic program writter by CEH Berger. It is concerning forensic photo color deconvolution. One can try it on following site : https://4n6site.com/improc/decoplugin/webapp.htm