Out of curiosity - why ghost lights and not DS lights with render emitter = off

cgidesign

I am often using the build in DS lights (point, flood etc.) with adjusted physical dimensions and then set "render emitter" = off to hide them from the camera. For me that gives all the desired results. But in the forum a lot of users talk about ghost lights. Is there something that can be achieved with them but not with the build in ones?

Matt_Castle

Note that Render Emitter only hides the light from the primary rays of the camera. (Perhaps counter intuitively, the rays start from the camera, not from the lights, as it's more efficient and simpler to assume that rays always encounter the camera than always encounter a light).

This means that lights with render emitter hidden will still have their own shadow if they occlude another light source, and will show up in reflections. Ghost lights (at least as of DS 4.16.0.3, but I understand there's been changes to how they work in the latest Betas*, as opacity and emission are apparently now linked rather than independent) don't do either.

* That said, the newer Betas do now allow *any* object to be hidden from Primary Rays, allowing cameras to be placed in or behind walls, which is pretty neat.

cgidesign

Ok, got it. Never had the demand for ghost lights obviously and therefore did not notice the difference.

Just tested with 16.1.21 beta. Mesh light with "visible to primary rays" = off acts exactly like the build in light with "render emitter = off".

Reducing opacity on the mesh light reduces the emission as well. So, the trick to set opacity 0.00001 or so does not work anymore.

EDIT:

Initial assumption seems to be wrong. Need to test more.

Matt_Castle

Reducing opacity on the mesh light reduces the emission as well. So, the trick to set opacity 0.00001 or so does not work anymore.

I think you now need to use the Iray Matte advanced option for this, although I've not yet tested all of this thoroughly myself.

cgidesign

Some more testing in 16.1.21 beta - interesting result (at least for me who did not use ghost tricks so far):

Scene:

 

DS light with "render emitter = off"

 

Mesh light (no special iray property)
Opacity = 0.0000001 (or better said the lowest possible value)
Luminance = 1999999991808.0