How to convert CAD file to points on 3D grid? [URGENT]

Hi,

For a project I'm working on, I need to calculate the buoyancy force on a boats hull. The buoyancy of a hull is the volume of the part under the fluid surface multiplied by the density of the fluid. I wrote a little script to calculate that volume and center of that volume (from list of coordinates of subvolumes) to have the magnitude of the force and the point of engagement.

The only thing I need to do now is to convert my CAD model into a set of points (subvolumes) with x,y and z coordinates. My Idea was to do this manually but I need about 2000 points. Is there a way to convert my CAD model to a set of point on a fixed 3D grid? Basically like when you want to make someting in minecraft, an array with al the coordinates of where the blocks would be placed.

Afterthought: I basically want to check if a point on my 3d grid (which will define the 'mesh' size) is within the boundaries of the CAD model and then add it to the list of points.

UPDATE:

I got the alphashape thing working but the shape it produces is off. The concave part of my model is skipped (see pictures) I tried playing with the alpha, region and hole parameters and that kinda fixes the front but messes up the back.

Thanks in advance

Jens

[Christopher McCausland]

Hi Jens,

I'm not sure how much this will help you hence why I haven't included it as an answer. Someone out there will know more and hopefully be able to offer more direction. However there are some inbuilt functions that allow MATLAB to import .STL file information. Obviously you can save any file as an STL file from you CAD package of choice. If you are using SolidEdge or semiens NX make sure you have selected the correct units in the 'properties' menu before saving to ensure the correct size.

Let me know how that goes, I have lots of experiance in CAD and MATLAB but little combining the two.

Christopher

[Jens Sonck]

Thanks @Christopher McCausland, I already imported the file into simscape for the visualisation part but I can't find a way to do what I described. I basically want to check if a point on my 3d grid is within the boundaries of the CAD model and then add it to the list of points. I looked at 'inpolyhedron' but can't find info on how to convert a CAD file to a matlab polyhedron.

[Christopher McCausland]

No worries @Jens Sonck.

Inpolyhedron has been developed by @Sven who can be found on the forum. Although I'm not a fan of dragging people unwillingly into posts I think they are probably the best person to ask. While the code is well written I am not sure I fully understand the input parameters that are asked for either.

Christopher.

[Jens Sonck]

Yeah I have the same problem with the code. I think it does what I want but I don't really know how I get it to do that. I'm not really familiar with the different files. I hope we summoned @Sven by mentioning him here :D

[darova]

What about STL format?

[Jens Sonck]

@darova what about it? Can you elaborate on this? How can I use an STL file to detect if my coordinates are within the boudaries of the STL? Can you give some documentation?

[Precise Simulation]

I think you could import your CAD model into the PDE or FEATool Multiphysics toolboxes, generate a volume mesh, and use the (Delaunay) mesh to check which points are in or outside of the mesh using for example tsearchn "tsearchn(X,TRI,XI) returns the indices t of the triangulation TRI for each point in XI, and NaN for all points outside the convex hull of X".

[Ali Grysah]

• hlo can help me i need matlab 2008 ,,,windows7,32bt

Answer 1

[darova]

Try alphaShape with inShape option

[Jens Sonck]

Seems useful but I need to have the x y values of the shape. I don't have a mathematical model of the model just a CAD file. How do I convert that?

[darova]

YOu need to convert your CAD model into STL format. Use stlread then to import data

[Jens Sonck]

I've imported the stl file with stlread and this gives me a Triangulation. How do I proceed from there? I'm only getting errors trying to get the alphaShape function to work on that.

[darova]

I don't have access to MATLAB now, can't help you with this. Sorry

[Jens Sonck]

No problem, get back to me as soon as you have access. I think you have the final piece of the puzzle. My algorithm is ready and the rest of my model is finished.

[darova]

Please show the error you have

[Jens Sonck]

I'm at work right now. The error was something like the input was wrong, it needed 'double' column vectors. When I get home I'll get the full error, maybe this helps already?

[darova]

I think you have triangulation structure (object) but alphaShape wants alphaShape object (don't know how it should look like)

[Jens Sonck]

Any idea how I can convert the triangulation to an alphaShape?

[Jens Sonck]

Error using alphaShape

Input arguments to alphaShape for points, alpha value, and threshold values, if specified, must be full (nonsparse), real, and double. For Name-Value pairs, the Name should be a character vector.

Here you go @darova

[darova]

You already have data (triangulation), you don't need alphaShape. You should adapt your object to use inShape

[Jens Sonck]

Now I get this error following the tutorial on inShape:

Check for missing argument or incorrect argument data type in call to function 'inShape'.

[Jens Sonck]

@darova I've attached a .zip with the model and the lines that give me the error, hope this clears things up.

[Jens Sonck]

@darova I've got it working but the shape is off. I've updated the original question with pictures.

Answer 2

[darova]

What about this? You can find volume under each triangle using this formula

[Jens Sonck]

So for every tetrahedron:

• Check if it's below the water plane. Is this done with the points? Are the points the centers of the tetrahedron or are those the vertexes?
• Calculate the volume of the tetrahedron. Where do I find the dimensions?

My original Idea was to have a fixed set of points to check and add. Simple calculations because I have to do this calculation multiple times per second during my simulation. I'd rather stick to the first Idea with the alphashape for simplicity sake.

[darova]

• I'd rather stick to the first Idea with the alphashape for simplicity sake.

I understand. Did you try simply to pass points into alphaShape?

[Jens Sonck]

I was just completing the program to show it to you. I slightly altered the cad model to exclude the concave parts. I only need the part that sit's in the water to be accurately presented. This worked:

shp = alphaShape(model.Points(:,1),model.Points(:,2),model.Points(:,3),1000);

I'm now in the process of finding the right meshsize to not kill my pc :D

[darova]

Maybe you don't such precise mesh. alphaShape has function volume for calculation volume

[Jens Sonck]

My algorithm takes 73ms to calculate 77k points so that's adequate for the simulation. I'm now having trouble in simscape with the dynamics.