I have a PointCloud called "cloud" centered at (0,0,0) with around 1000 vertices. The vertices' positions are updated using a vertex shader. I now would like to print out each vertex's position to the console every few seconds from the render loop.
If I inspect the point cloud's vertices using
console.log(cloud.geometry.vertices[100])
in the render loop, I always get a vertex with all zeros, which I can see from all the particles zipping around is not true.
I looked at this similar post: How to get the absolute position of a vertex in three.js? and tried
var vector = cloud.geometry.vertices[100].clone();
vector.applyMatrix4( cloud.matrixWorld );
which still gave me a vector of all zeros (for all of the vertices). Using scene.matrixWorld in place of cloud.matrixWorld did not work either.
I also tried using cloud.updateMatrixWorld() and scene.updateMatrixWorld(). Lastly, I tried setting cloud.geometry.vertexNeedsUpdate = true.
So far, all of the advice I've seen has used the above, but none of them work for me. I have a feeling that the array just isn't getting updated with the correct values, but I don't know why that is. Any ideas?
That is because the vertices never change their properties on the cpu, but only on the gpu (vertex shader). This is a one-way ticket, the communication goes cpu -> gpu not the other way around.
If you need to work with the vertex position then you have to do the calculations on the cpu and send the vertex batch everytime something changed back to the gpu.