How do I 'wrap' a plane over a sphere with three.js?

Question

I am relatively new to three.js and am trying to position and manipulate a plane object to have the effect of laying over the surface of a sphere object (or any for that matter), so that the plane takes the form of the object surface. The intention is to be able to move the plane on the surface later on.

I position the plane in front of the sphere and index through the plane's vertices casting a ray towards the sphere to detect the intersection with the sphere. I then try to change the z position of said vertices, but it does not achieve the desired result. Can anyone give me some guidance on how to get this working, or indeed suggest another method?

This is how I attempt to change the vertices (with an offset of 1 to be visible 'on' the sphere surface);

planeMesh.geometry.vertices[vertexIndex].z = collisionResults[0].distance - 1;

Making sure to set the following before rendering;

planeMesh.geometry.verticesNeedUpdate = true;
planeMesh.geometry.normalsNeedUpdate = true;

I have a fiddle that shows where I am, here I cast my rays in z and I do not get intersections (collisions) with the sphere, and cannot change the plane in the manner I wish.

http://jsfiddle.net/stokewoggle/vuezL/

You can rotate the camera around the scene with the left and right arrows (in chrome anyway) to see the shape of the plane. I have made the sphere see through as I find it useful to see the plane better.

EDIT: Updated fiddle and corrected description mistake.

Answer 1

Sorry for the delay, but it took me a couple of days to figure this one out. The reason why the collisions were not working was because (like we had suspected) the planeMesh vertices are in local space, which is essentially the same as starting in the center of the sphere and not what you're expecting. At first, I thought a quick-fix would be to apply the worldMatrix like stemkoski did on his github three.js collision example I linked to, but that didn't end up working either because the plane itself is defined in x and y coordinates, up and down, left and right - but no z information (depth) is made locally when you create a flat 2D planeMesh.

What ended up working is manually setting the z component of each vertex of the plane. You had originaly wanted the plane to be at z = 201, so I just moved that code inside the loop that goes through each vertex and I manually set each vertex to z = 201; Now, all the ray start-positions were correct (globally) and having a ray direction of (0,0,-1) resulted in correct collisions.

var localVertex = planeMesh.geometry.vertices[vertexIndex].clone();
localVertex.z = 201;

One more thing was in order to make the plane-wrap absolutely perfect in shape, instead of using (0,0,-1) as each ray direction, I manually calculated each ray direction by subtracting each vertex from the sphere's center position location and normalizing the resulting vector. Now, the collisionResult intersection point will be even better.

var directionVector = new THREE.Vector3();
directionVector.subVectors(sphereMesh.position, localVertex);
directionVector.normalize();
var ray = new THREE.Raycaster(localVertex, directionVector);

Here is a working example: http://jsfiddle.net/FLyaY/1/

As you can see, the planeMesh fits snugly on the sphere, kind of like a patch or a band-aid. :)

Hope this helps. Thanks for posting the question on three.js's github page - I wouldn't have seen it here. At first I thought it was a bug in THREE.Raycaster but in the end it was just user (mine) error. I learned a lot about collision code from working on this problem and I will be using it later down the line in my own 3D game projects. You can check out one of my games at: https://github.com/erichlof/SpacePong3D

Best of luck to you! -Erich

Answer 2

Your ray start position is not good. Probably due to vertex coordinates being local to the plane. You start the raycast from inside the sphere so it never hits anything.

I changed the ray start position like this as a test and get 726 collisions:

var rayStart = new THREE.Vector3(0, 0, 500);
var ray = new THREE.Raycaster(rayStart, new THREE.Vector3(0, 0, -1));

Forked jsfiddle: http://jsfiddle.net/H5YSL/

I think you need to transform the vertex coordinates to world coordinates to get the position correctly. That should be easy to figure out from docs and examples.