This is a quick little demo and explanation of a setup I created for doing a particle aura. Particles are emitted based on the object’s normal direction from the camera.
This is a screenshot of the VOP network that is doing the magic. Camera position is imported (into the particles’ object space). Normalized vectors are determined from each object point to the camera. Then the dot product of the object normals and direction vector are taken. If the dot product == 0, then the normals are perpendicular to the camera. Otherwise they are pointing at the camera. The range varies between -1 and 1, which is then made useful for remapping by taking the absolute value and reversing the grad values so the user interface makes a little more sense.
The “emit” point attribute is promoted to a prim attribute after the vop, allowing it to be used as a particle birth attribute.