To pick what is behind mouse pointer in 3D you get the vector from near plane to far plane and iterate through it in world coordinates and check from world data if anything is supposed to be in those coordinates.

Assumed we operate in world that has coordinate significance/resolution of full integers.

In order not to miss any coordinates the vector intercepts (but minimize the amount of iterations), how to determine the step size?

Is a safe step size 0.5*0.4*0.3? Intuitively it feels that when close to a intersection of different coordinate volumes the significant step size should get infinitely small, or you jump over some corners of slightly intersecting coordinates.

Inverse of draw canvas pixel size could be a good threshold. One cannot pick more accurately than how many pixels you have available for the mouse pointer to rest on.

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