Image structure

[aka]

Hi,

I am begining with gimp and please need an advice ; take a look to the jointed picture.

My question is : which is the best way to have :

1. the green only in the red shap (NO green out the red shape).
2. the blue only out of the red shape (NO blue in the red shape).

I tried several things, but I think that people with experience can help me a lot !

Thanks,

aka

[rich2005]

A little unclear to me but I think this is your request.

Work with layers Layers -> Mask -> Add layer mask  then you can choose various types white / black..and others
A layer mask will isolate a part of the layer that is visible.

One layer and layer mask for inside a shape. (for blue)
Another layer and layer mask for outside the shape. (for green)

For your question, essentially the same layer mask, inverted colours defining  areas.

The attached .xcf as an example.

[aka]

Thanks, it's exactly what I searched.

It remains a last thing, please : is it possible to edit an existing mask ? I don't find this function...

Best regards !

[rich2005]

(09-08-2021, 05:11 AM)aka Wrote: ....It remains a last thing, please : is it possible to edit an existing mask ? I don't find this function...

A mask is a greyscale image where White = 100% transparent Black = 100% opaque and 'any grey values', vary transparency.
You can make a mask on its own by creating a greyscale  image. 

More customary is to paint it straight into Gimp.
Click in a layer mask to make it active (active shows a white border on the layers icons) 
Then paint in your canvas using White or Black. The X key swaps between foreground and background colours which makes editing quicker.

If you need to see and edit the mask, use  Layer -> Mask -> Show layer Mask make you edit the turn it off.

When finished with the layer mask click in the layer icon (gives white border) for painting.

See if you can make sense of this: https://i.imgur.com/hSsYT6G.mp4