Can I have access to the Loop and Ring Selection algorithm ?

Dimitris_Derm.

Hi community,
I wonder if there's a direct call for accessing Loop and Ring Selection results (element data) by providing an edge.
I tried the indirect method of actually using the interactive user tool on a temporary project but I constantly run in walls either by crashing C4D or just not getting any results back.

Is there a direct method that utilizes the native C4D Ring/Loop Selection geometry traversing algorithms with a direct input of 2 points or an edge ID ?
If not, is there a direct method from the C++ API ?
If explaining the concept of how to use the interactive tool programmatically in an algorithm is too complicated and there's not a similar example for it, can I at least know if the native C4D algorithm is UV-traversing or Geometry-traversing in order to implement one myself ?

I want to create a parametric generator similar to the SceneNodes Loop/Ring Selection Modifier that is more intuitive in use because the current implementation uses as inputs two point IDs. This isn't practical at all as the two points provided most of the time will not be related (belong to the same edge) at all returning nothing back. My implementation will be using an Edge ID counter to cycle the geometry and have just an Index user attribute using a pre-constructed table of all unique possible selections so the user can cycle all unique Ring and Loop selections. (Useful for effects, multiple selections, Field-driven selections etc)

ferdinand

Hello @Dimitris_Derm.,

Welcome to the Maxon developers forum and its community, it is great to have you with us!

Getting Started

Before creating your next postings, we would recommend making yourself accustomed with our forum and support procedures. You did not do anything wrong, we point all new users to these rules.

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It is strongly recommended to read the first two topics carefully, especially the section Support Procedures: How to Ask Questions.

About your First Question

I am not quite sure how your question is meant, but the loop and ring selection tools are accessible in the API. When you are literally asking for the algorithm, we cannot share that, as declared in our Support Procedures:

We will not reveal non-public functionalities of our APIs.

When you just want to use the tools programmatically, SendModelingCommand is the way to go. We also have a dedicated section in our examples for them.

Cheers,
Ferdinand

Result

Code

"""Demonstrates how to use the loop selection command in the Python API. 

The script creates a cylinder, makes it editable, and then performs a loop selection based on two 
vertices and a polygon index. Finally, it inserts the modified object into the active document and 
updates the scene.
"""
import c4d
import mxutils

doc: c4d.documents.BaseDocument  # The currently active document.
op: c4d.BaseObject | None  # The primary selected object in `doc`. Can be `None`.

def main() -> None:
    """Called by Cinema 4D when the script is being executed.
    """
    # Instantiate a cylinder and insert it into a temporary working document.
    cylinder: c4d.BaseObject = mxutils.CheckType(c4d.BaseObject(c4d.Ocylinder))
    temp: c4d.documents.BaseDocument = mxutils.CheckType(c4d.documents.BaseDocument())
    temp.InsertObject(cylinder)

    # Make the object editable. We could also build the caches and get the cache, but since the 
    # subject is here SMC, we can also use SendModelingCommand and MODELINGCOMMAND_MAKEEDITABLE.
    res: list[c4d.BaseObject] | bool = c4d.utils.SendModelingCommand(
        c4d.MCOMMAND_MAKEEDITABLE, list=[cylinder], mode=c4d.MODELINGCOMMANDMODE_ALL, doc=temp)
    if not res:
        raise RuntimeError("Failed to make object editable.")
    
    # The cylinder is now editable.
    cylinder: c4d.PolygonObject = res[0]

    # Now we are going to do the loop selection. These are the IDs defined in toolloopselection.h:

    #     MDATA_LOOP_SEL_STOP_AT_SELECTIONS = 1100, // BOOL
    #     MDATA_LOOP_SEL_STOP_AT_NON_QUADS  = 1110, // BOOL
    #     MDATA_LOOP_SEL_STOP_AT_POLES      = 1120, // BOOL
    #     MDATA_LOOP_FIRST_VERTEX           = 1130, // LONG
    #     MDATA_LOOP_SECOND_VERTEX          = 1131, // LONG
    #     MDATA_LOOP_POLYGON_INDEX          = 1132, // LONG
    #     MDATA_LOOP_BOTH_SIDES             = 1133, // BOOL
    #     MDATA_LOOP_SWAP_SIDES             = 1134, // BOOL
    #     MDATA_LOOP_SELECTION_TYPE         = 1140, // LONG (must be SELECTION_NEW, SELECTION_ADD or SELECTION_SUB)
    #     MDATA_LOOP_SEL_POLYGON_OBJECT     = 1150, // LINK

    settings: c4d.BaseContainer = c4d.BaseContainer()

    # Define the major input, two vertices and a polygon index. You need all three, regardless of the output mode 
    # you choose in SMC.
    settings[c4d.MDATA_LOOP_FIRST_VERTEX] = 2
    settings[c4d.MDATA_LOOP_SECOND_VERTEX] = 3
    settings[c4d.MDATA_LOOP_POLYGON_INDEX] = 1

    # Define the selection mode, this is more cosmetic, as SELECTION_NEW is the default.
    settings[c4d.MDATA_LOOP_SELECTION_TYPE] = c4d.SELECTION_NEW

    # Carry out the loop selection, targeting edge mode.
    res: list[c4d.BaseObject] | bool = c4d.utils.SendModelingCommand(
        c4d.ID_MODELING_LOOP_TOOL, list=[cylinder], mode=c4d.MODELINGCOMMANDMODE_EDGESELECTION, bc=settings, doc=temp)
    if not res:
        raise RuntimeError("Failed to perform loop selection.")

    # Scene elements can only be inserted once, the Python API removes nodes automatically when you call InsertObject, but 
    # C++ does not and there is no guarantee that the Python API babysits you in all cases where you can insert something, 
    # so it is good practice to ensure scene integrity yourself. (#cylinder is currently part of #temp).
    cylinder.Remove()

    # Insert the object into our scene, enable edge mode, make the object selected, and push an update event.
    doc.InsertObject(cylinder)
    doc.SetMode(c4d.Medges)
    doc.SetActiveObject(cylinder)
    c4d.EventAdd()

if __name__ == '__main__':
    main()

Dimitris_Derm.

Thank you, this make more sense.
I was passing vertices not polygon indexes and was calling MDATA_LOOP_SELECTION_TYPE instead of MDATA_LOOP_SELECTION.