BSDF modifiers in DC calculation vs 3 Phase Method

Hi all,

Up until now I’ve always used the Three Phase Method when simulating complex fenestration systems. But I realised I can also use BSDF modifiers in regular DC or enhanced DC simulations (as in Ladybug Tools for example).

So I started wondering what the difference is, whether the results differ, whether the computation time differs. I’ve seen a couple of old workshop presentations but from just the slides it’s hard to conclude much.

One big advantage of the 3PM I’m aware of is that simulating different fenestration states is as easy as swapping the transmission matrix, whereas in DC everything needs to run again. But I imagine there is more to this debate…

I’d love to learn more about what’s really going on here.

A lot will depend on the type(s) of fenestration systems you are simulating. The worst case for the DC method with a BSDF is probably prismatic panels, which redirect transmitted light in specific ways that backwards ray-tracing struggles to find. The photon map may be used in some circumstances to model these systems.

If we compare the more common case of a fenestration device that has some diffusing component and some through component (e.g., venetian blinds, roller shades), you should get similar results from 3-phase and DC methods, the latter being slightly more precise as you aren’t passing through another level of directional quantization. (You still have the sky patches in there, but you can make those denser with the Reinhart sky subdivision.)

In the end, if you are not taking advantage of the ability to swap BSDFs, there is no real advantage to the 3-phase method over DC.

I hope this helps a bit.