If I have a mesh, how do I find out to which block instance it belongs? Is it possible?

alexian007 · February 6, 2019, 1:02pm

Hi !

So, I am in the process of making an exporter plugin for Rhino to some other format using the C++ API, and have now been facing the issue of correctly exporting the Block instance objects.

Currently I am first maintaining an array of meshes, by using the RhinoMeshObjects() function, that saves the meshes into an array called allMeshes.

So, my main concern here is how would I find out if a mesh in the allMeshes belongs to a block instance object, so I can manipulate the mesh’s vertices with the transform of the block object.

How can I achieve that?

nathanletwory · February 6, 2019, 1:18pm

If you are working on an exporter for v6 and are interested primarily in mesh geometry from objects you should look into using the RhRdk::Realtime::ChangeQueue to get out mesh data and with that useful identifiers for mapping between objects and mesh instances - this will take into account block instances.

When you get a mesh ID that is used in multiple instances you’ll be able to see that.

https://developer.rhino3d.com/api/cpp/class_rh_rdk_1_1_realtime_1_1_change_queue.html

Although the ChangeQueue was initially conceived for integration with the display system it works perfectly for exporter scenarios as well.

You essentially will derive from RhRdk::Realtime::ChangeQueue, instantiate one with the doc you want to export, call CreateWorld and Flush. With the correct Apply* functions implemented you’ll get all geometry preprocessed, including all blocks, nested and what not, materials etc.

If this sounds like something you’d be able to use I can type more text to clarify how to do that

alexian007 · February 7, 2019, 4:48am

Hey @nathanletwory, RhRdk::Realtime::ChangeQueue does sound like a viable solution to my problems . But, my code is almost complete and I do not think that I could afford the time to change the RhinoMeshObjects() way I am using right now.
Nevertheless, I would like to know more about RhRdk::Realtime::ChangeQueue to work on it in the future. Thanks!

nathanletwory · February 7, 2019, 6:19am

@alexian007 I’m not pretending I know how you have set up your code, but I’m pretty sure a move to the ChangeQueue won’t be that big a change. Anyway, let me type on a simple sample code to show you how it can be used.

alexian007 · February 7, 2019, 6:28am

Oh, okay then. So will implementing the ChangeQueue give me an easy way to :

a) Access an individual mesh i.e a CRhinoMeshObject or something similar, to get the mesh data, attributes.
b) Find out if the mesh is a part of the instance or not.
c) Easily map the mesh and the object/ block instance it belongs to?

I will be able to do all these, right? If so, I will gladly make the change to ChangeQueue.

So, the basic overview of my project is that I use RhinoMeshObjects() to get an array of CRhinoMeshObject meshes.

Then, I use the objectIterator to get all the objects on the document and store it in another array.

And, then I iterate the object array to search the mesh array according the id and then assign the mesh to the object.

(I do so because I am making another object definition for the software I am exporting to, so I am transferring the data of a Rhino object to another data container that contains all its data and the references to the mesh too from the mesh array I obtained earlier.)

So, what I am doing to associate the mesh to the object is by:

if (allMeshes[i].m_parent_object->Attributes().m_uuid == currentObjID)

Where,
allMeshes = the meshes array obtained from RhinoMeshObjects()
currentObjID = the ID of the current object I got by iterating the object array obtained from objectIterator.

So, if implementing a ChangeQueue will give me better organisation and won’t be much time consuming, I would really love to know that, Nathan.

Thank you, and I am awaiting for the sample code!

nathanletwory · February 7, 2019, 10:08am

So, a few moments of typing and a basis for creating an exporter on top of the all-new RhRdk::Realtime::ChangeQueue is now ready at

I’ve tried to document the Apply* functions heavily with how the process works. If you still have questions then obviously feel free to ask them here! I can improve the sample until there is no more doubt about its mechanisms

alexian007 · February 7, 2019, 10:48am

Yaay! Give me some time to look through it, and I’ll shoot all my questions here.
P.S: You ROCK, Nathan !

nathanletwory · February 7, 2019, 12:08pm

If you want to see the ChangeQueue in action here is the Raytraced code using it in C# (C# is a wrapper around the C++ code, more or less functioning the same way):

github.com

mcneel/RhinoCycles/blob/rhino-6.x/Database/ChangeDatabase.cs#L842


			};
			_renderEngine.View = null;
			_cameraDatabase.AddViewChange(cyclesview);
		}


		/// <summary>
		/// Handle mesh changes
		/// </summary>
		/// <param name="deleted"></param>
		/// <param name="added"></param>
		protected override void ApplyMeshChanges(Guid[] deleted, List<CqMesh> added)
		{
			SceneBoundingBox = GetQueueSceneBoundingBox();
			RcCore.OutputDebugString($"ChangeDatabase ApplyMeshChanges, deleted {deleted.Length}\n");


			var totaldeletes = deleted.Length;
			var curdelete = 0;
			foreach (var guid in deleted)
			{
				curdelete++;
				// only delete those that aren't listed in the added list

github.com

mcneel/RhinoCycles/blob/rhino-6.x/Database/ChangeDatabase.cs#L995


					sceneBoundingBoxDirty = true;
					_sbb = value;
				}
			}
		}


		public double ModelAbsoluteTolerance { get; set; }
		public double ModelAngleToleranceRadians { get; set; }
		public Rhino.UnitSystem ModelUnitSystem { get; set; }


		protected override void ApplyMeshInstanceChanges(List<uint> deleted, List<MeshInstance> addedOrChanged)
		{
			SceneBoundingBox = GetQueueSceneBoundingBox();
			// helper list to ensure we don't add same material multiple times.
			var addedmats = new List<uint>();


			RcCore.OutputDebugString($"ApplyMeshInstanceChanges: Received {deleted.Count} mesh instance deletes\n");
			foreach (var dm in deleted)
			{
				RcCore.OutputDebugString($"\ttold to DELETE {dm}\n");
			}

nathanletwory · February 7, 2019, 3:41pm

Btw, I attached a pre-compiled RHP of the sample here:

alexian007 · February 8, 2019, 6:20am

Yeah, I got it thanks! I am still in the process of understanding the changequeue, and seeing it in action is awesome !

nathanletwory · February 8, 2019, 11:51am

Ask when there is anything that is unclear.