Ray based picking

[ffreyer]

Our current pick functionality works by rendering a plot id and a plot element / instance id to a buffer and reading that buffer at the appropriate pixel position when pick is called. An alternative to that is casting a ray and finding plots by checking which bounding boxes intersect with it.

The benefits of ray based picking are that it can be implemented on the CPU, i.e. it can be largely backend independent, and it can pick multiple objects. This means that occluded and transparent objects can be picked. What suffers is accuracy and potentially performance.