Here is an example image. I want to have one big face here instead of the many edges - is there a way to delete them using geometry nodes?
In Edit Mode I would use Limited Dissolve for this, but with Geometry Nodes I'm not sure how to do it.
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...as already mentioned in the comment: I would use the Decimate modifier in your case.
But if you really want to do this with Geometry Nodes, then you could possibly go this way (Even though I can't recommend this to you!):
Since you can't dissolve edges in Geometry Nodes, you have no choice but to rebuild the geometry where you want to remove edges.
However, this solution only works if your faces are flat and can be reduced to four vertices. Therefore, this variant is certainly not suitable for all scenarios.
In detail the following happens:
First I split the mesh at the pointed edges, so that I get separated faces (Here I scaled the faces for better visibility):
Then I remove from the faces all edges where two faces with the same orientation meet. Since you cannot dissolve edges in Geometry Nodes, the faces are also removed here:
I then separate the edges thus obtained from the rest of the geometry and convert them to curves. But since the intermediate points still remain, they have to be removed as well, because we need exactly four vertices here:
Then I create a geometry with
Mesh Circle, which I duplicate according to the number of splines. The Mesh Circle has the advantage that here the indices of the individual points are arranged in a circle. This way I can transfer the positions of the original vertices to them and get back the original positions of the faces.
Finally, just glue the mesh together and you should eventually get a usable result. Additionally you would have to transfer the normals of the original surfaces, because they can get mixed up by this maneuver.
Have fun with it!