Fitting a polyline to a 3d mesh (contour)

[inoodle]

Continuing the discussion from Smoothing polyline over several frames:

Dear all, dear @zach,

I am attempting to get the contour / outline of a 3d mesh and came across the very interesting discussion in the linked thread. I think the example code @Zach provided is very relevant, but I am not able to apply these techniques to a mesh and get a 2d outline.

My initial idea was to take a frontal projection of a mesh and then somehow take a (2d) outline of that.

Does anyone have any clues on how / whether this can be done somehow?

Many thanks!

Smoothing polyline over several frames

I don’t know if it’s helpful but I’ve got some spline examples here:

https://github.com/ofZach/spline-fitting

I have just added some very messy code which is a port from cinder of their bsplinefit (which orignally comes from wildmagic library).

the code has a section (besides alot of copy and paste) that looks like this:

BSpline3f spline = fitBSpline( temp.getVertices(), 3, temp.getVertices().size() *0.8);

    BSpline3f spline = fitBSpline( temp.getVertices(), 3, temp.getVertices().size() *0.8);

where it fits the bspline to a polyline.

then, I iterate through it for drawing:

    int count =  ofMap(mouseX, 0, ofGetWidth(), 3, 10000, true);
    mesh.setMode(OF_PRIMITIVE_LINE_STRIP);
    for (int i = 0; i < count; i++){
        mesh.addVertex(spline.getPosition( i / (float)count));
    }
    mesh.draw();

I’m messing with the count based on mouse just to show how you can resample the bspline…

memo interpolation also looks really useful / promising:

https://github.com/memo/ofxMSAInterpolator

[amnon]

Ok, thinking out loud here. How about using worldToScreen to get the screen position of each vertex in the mesh. Then use a 2D convex hull algorithm on the list of points to get the outline.

[inoodle]

Thanks! that seems like an interesting approach.

A different idea I had is working in openGL directly and use front / back face culling in some way to achieve the same effect. Something like what they attempt here:

create 3d outline?

However, when I try to use functions like the following, nothing happens at all. Any mesh I draw just shows up as normal… I am not used to programming directly in OpenGL so maybe I am missing something simple?

glEnable(GL_CULL_FACE); glCullFace(GL_FRONT)

//go into wire mode
glPolygonMode(GL_BACK, GL_LINE); //wireframe mode
glEnable(GL_LINE_SMOOTH);
glLineWidth(5); //fat lines

[amnon]

The technique described there only gets you a ‘visual outline’ where the solution I decribed above gets you the actual vertices of the outline. In the thread title you spoke of a polyline, so I assumed you wanted the actual vertices. Or do you just want the visual effect?

[inoodle]

Yes, you are right. Your method gives you the actual vertices - this offers much more flexibility.

At this point I am just experimenting with different approaches, also to test for real time performence etc. The OpenGL method I described above so far is not too impressive, I’ll give your method a try as well