Question

I cannot figure out how to detect it going off screen, can anybody help? I am using WebGL and Three.js.

Answer 1

You can use Frustum testing, a little like this:

// Create a new Frustum object (for efficiency, do this only once)
var frustum = new THREE.Frustum();
// Helper matrix (for efficiency, do this only once) 
var projScreenMatrix = new THREE.Matrix4();

// Set the matrix from camera matrices (which are updated on each renderer.render() call)
projScreenMatrix.multiply( camera.projectionMatrix, camera.matrixWorldInverse );
// Update the frustum
frustum.setFromMatrix( projScreenMatrix );
// Test for visibility
if ( !frustum.contains( object ) ) {
    // It's off-screen!
}

This is copied from WebGLRenderer sources.