The mask condition is based on a 3D Mask
. This type of mask is in 3D, and has a value of 0% (where it masks nothing) and a value of 100% (where it masks fully). The values are interpolated from 0 to 100 in-between.
The particle event will be triggered if the particle crosses a threshold (entering or going out of the mask).
As an example, if a threshold = 50% and the mask is a Linear mask
, starting at (0, 0, 0) and ending at (0, 0, 10), the threshold will be crossed if a particle goes from (0, 0, 4.5) to (0, 0, 5.5).
This is because the threshold is at 50% of the linear mask.
This can work with any kind of 3D Mask: you could check if a particle enters or leaves a sphere, a cube, a composite mask made out of many different 3D masks, etc., etc.