Rotate instances to face one object with Geometry Nodes

Question

I've created a dense forest of plane image trees using Distribute Points on Faces and Instance on Points nodes.

I'm attempting to billboard them to always face the camera.

Plan A was to use object constraints on the original objects. Failed because the instances would not be constrained individually, just mimicking the original asset's rotation.

My current Plan B is to take the camera's position as a vector, subtract it from the position of the instances and use the Align Euler to Vector node to make each instance directly face the camera.

But two issues are in the way of plan B.

1: I don't know if it's possible to use Geometry Nodes to apply this rotation to all instances individually.

2: I don't know how to access the location of these instances en masse to test if it would work.

I have seen that particle systems would be good to make this work easier on a flat plane, but this is not working on a flat plane.

Answer 1

In Geometry Nodes, input nodes (like Position, or Index) do not hold any specific information by default, they're ways of asking a question in the context of the geometry they're linked to. This means you can just use a Position node here to subtract the Camera's location from. Since it ends up being linked to the Instance on Point node, it's automatically asking the question "What's the positions of these instances created by this node?" (Question 2). If you have also manipulated this information in between, like we're doing here with getting the difference vector between positions of all relevant instances and the camera, it will apply the resultant rotation to all of them individually (Question 1).

Additionally, if trees are to stay upright, you should flatten the difference vector (by multiplying its Z value with $0$), otherwise they would also rotate in other axes as the camera moves around.