As JohnnyRandom mentioned, the PRT Hair object will always place a particle on the knots of the source spline no matter what spacing setting you choose.
The scheme for seeding particles in PRT Hair is as follows:
- It solves for where a particle would be placed in order to be “spacing” units away from the last particle.
- If the view adaptive controls are enabled, it solves for the pixel distance between this point and the last particle. If it exceeds the specified pixel spacing, the particle is placed to make it exactly “pixel spacing” pixels away.
- If the chosen position has crossed into the next spline segment, the particle is forced to be placed at the end of the current spline.
So its essentially the lessor of the “spacing”, “pixel spacing” and current spline segment’s length.
The PRT Hair system sets the Density of the particle to the inverse of the inter-particle spacing in order to accommodate varying seeding density. Its not exactly perfect, but it appears to be close enough from an empirical perspective.