How to get edge 'island' selection

BretBays

Hello there,

I do not have code for this, I was mostly curious if an idea was at all possible and how to approach it in Cinema's SDK.

Steps to reproduce:

Create Plane.
Make it editable
Switch to Edge mode
Use Path Selection tool set to Simple Edge loop to select a few loops like this:

Is there a way to get each of these loops in python? Like, a Base Select would just give me whether or not an edge is selected, but I basically want to get the continuous edges of each loop. So to know that the first loop is made up of edges 1-10, the second loop is 2-20, etc. The end goal would be to take those loops, and do some stuff with each set of verts for each loop. But I am not sure if this is even possible or not. I had looked at the Neighbor class, but I don't know if that would enable me to do it.

There must be some sort of internal method because if you do some loops and use the edge to joint command, it can differentiate the different loop selections to create joints.

ferdinand

Hey @BretBays,

Thank you for reaching out to us. Yes, that is possible but we cannot write the script for you. We can only help you when you make the first steps yourself. You will find all the necessary building blocks in the modeling example scripts.

Create a plane generator object.
Get its cache to get an editable polygon object. For more complex geometry or in general you could also run MCOMMAND_CURRENTSTATETOOBJECT.
The run SendModellingCommand with ID_MODELING_LOOP_TOOL. Sometimes modelling commands can be a bit bumpy ride, when you want to do more niche things. But at the first glance everything you will need seems to be there:

Cheers,
Ferdinand

BretBays

Hey Ferdinand,

I didn't give a good enough example, my apologies. And no worries, I certainly don't expect the script to be done for me, it's just hard to know if something is possible or not and if I knew the answer, I'd likely have the script myself :). Let me try to explain it better.

Check out this video: https://www.dropbox.com/scl/fi/ce9d71lg0tnv3j9qz1s9v/WeightBlend.mp4?rlkey=4s3qj0s2trhniwezgo7yf4cy3&e=1&dl=0 it shows the workflow for a plugin I am developing that this request was even for. If you watch, I am working with 2 points along an edge loop before click apply. What would be preferred is to instead define all the edge loops to then click apply and it will do the blending along each loop.

So the question is if there are any means or ways to determine unique edge loops from an edge selection that I could then go through each edge loop, convert to verts under the hood and then do what I normally do for the weight blending.

ferdinand

Hey @BretBays,

Thank you for your clarification. I think I understand a bit better what you want, but there are still some question marks for me.

The edge loop and ring loop tool work via point indices (and a bunch of other settings). I.e., you set MDATA_LOOP_FIRST_VERTEX and MDATA_LOOP_SECOND_VERTEX and that determines the edge you want to loop (the screenshot above is for toolloopselection.h, the file which defines the loop tool IDs). As I said above, some of the modelling command options can be bit finnicky, but I would help you crossing that bridge when we are there. But we would have to have a foundation we can work on to get started.

Sorting Selections

But from what I understand, you do not primarily want to select loops, but rather sort existing selections into loops, or more generalized: edge strips you consider to be semantically/topologically connected. For something like shown in your screenshot, this is trivial, but a general solution is not. That you have an existing selection is almost irrelevant, it is only slightly easier to solve than the general problem of sorting all edges of a mesh into strips.

I wrote something like this myself a long time ago (while still at university at not a member of Maxon) and struggled quite a bit then to get it right, because you have then to deal with intersecting strips, self intersections of strips, etc. The problem of yours lies generally in the field of topological sorting, which is pretty wide, the concrete term used for the particular problem is often 'edge extraction in mesh processing'. The popular Python mesh processing library PyVista implements for example edge extraction.

If your concrete problem is solvable, depends entirely on how well you define it. For just an given input set of edges, there exist often many solutions. You have to define things like a maximum angle of change between edges in a strip, how to deal with intersections of strips, if you want continuity of non-manifold edges, etc. And even then, you will often end up with multiple solutions. E.g., in the example below, there are three possible ways to sort these edges into two strips (and even more when you do set this conditions to minimize the number of strips). Which can be deemed undesirable.

Implementing an Edge Selection Tool Yourself

I do not understand all details of your tool, but when edge selections are part of it, you could also 'simply' integrate them. Your tool looks currently more like a dialog/command and you could also do it there, but an actual ToolData would be better.

The general drill would be to add a button/mode with which the user can select edges. Once running, you would use c4d.utils.ViewportSelect to get the points/edges the user is clicking or hovering. Once the user wants to commit to a selection, you get the two relevant hoovered points (i.e., edge) and run ID_MODELING_LOOP_TOOL with it. Because you would inspect the user creating the loops one by one yourself, you would naturally know what is a strip in the final selection.

Here are also some hoops to jump through and there might also be obstacles you cannot overcome which I do not see at first glance either (as actual ToolData tools are not very often implemented in Python and therefore a bit niche), but generally this should be possible and this might be less costly than writing a full blown edge extraction solver.

Sorting Edge Selections Without Intersections

If you want to sort edge selections without intersections (or touching), this is relatively easy to implement, you only have to get the selected edges, convert them into point pairs, and then sort them into strips by finding point pairs where one point matches the end of another pair. The problem of this route is that it is pretty much guaranteed not to suffice in real world scenarios.

Cheers,
Ferdinand