GIMP Version: 2.8.14
Operating System: Linux
OS Version: Ubuntu 18
GIMP Experience: Basic Level

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Hi

I have a file containing a geographical map divided into layers. One of the layers contains rivers. I would like to thicken all waterways by double or triple the current situation. As you can imagine, the rivers have a very irregular appearance with many inlets. Is there a quick technique? If so, which one? Any suggestions?
Thanks

A representative crop of the rivers layer would make it easier to give a good answer.

Select the rivers layer.
Layer > Transparency > Alpha to Selection
Select > Grow (by as many pixels as needed)
Create a new transparent layer below the rivers layer.
Bucket fill the selection with river colour
Select > None

Hello loro33
Have you a simplified 2D map or photo realistic 3D map of satellite imagery?
Is the river simplified with only one color that you can select with or

I've tried your technique. It works fine, but there is a problem. I cannot grow every river of same amount of pixels.

It would be great if I could set growth as a percentage. So 100% or 200%, but I don't see that option.
Staying on your solution, I need to do the growth of 12 px for the thicker segments, 8 px for the medium segments, 4 px for the thinner segments.
So how to do the growth in pieces?

Or other ideas?

Thanks.

The map is two-dimensional.

I built the rivers layer starting from the top view map, using the Color picker and Select by Color technique. I basically extracted all the rivers from the map and put them in a new layer. They are not completely filled, there are some small unfilled strokes, but that would not be a problem either, because in the overall view it is not noticeable, you would have to zoom in to see it.
However, with the Fill technique suggested by Blighty II I see that in addition to being able to grow, the filling that I want occurs.

Unfortunately you are correct, doesn't look like it has percentage there.



Unless someone can make a fix for you or you can change your code to make percentage there, (This is not always possible though).

EDIT:-
If you have the plug in GMIC have a look at Layers - stroke. The threshhold for that is in percentage - if that helps at all

This method may be feasible, maybe not.

Split the rivers layer into 3 layers: thick, medium, thin.

For example:
Use the free select tool to select thin rivers
Edit > Cut (or Ctrl-X)
Edit > Paste (or Ctrl-V)
Floating selection to new layer.
Thin rivers are now on their own layer.

Same for medium rivers.

Apply Select > Grow to each layer as required (12, 8 or 4px)

turn the original river selection to a path. then scale the path to your desired %

I would try this. Can you give me instructions step by step? I cannot follow you now, this is a new concept for me.

Can I start from the rivers layer?

thanks

If you can provide a picture it would be easier to give you a step by step instruction

Exported from rivers layer

As transparent png


As jpeg

Any news?