Relationship between Mental Ray and System Units?


Hello Everyone:)

Well rendering caustics is not a big deal in mental ray, when every thing is on default.
But when you start playing with the system units the mental ray output is completely changed!!:surprised

now here is render of what you will get when you render a simple scene like this!!

now here is render of the same scene when i changed the system units to Meters!! bam the caustics are gone:banghead:

scene when system units set to Millimeters.:argh:

Now my question is why does Mental Ray take system units into consideration when rendering caustics?:curious:

Even Reactor and Cloth FX does not care about system units…
Does other rendering packages care about system units??:curious:

well would love to have an explanation on this!!


NOTE:I have attached the scene


The lights in MR use inverse square decay (fallof), like in real world. This is why unit set-up has to be correct.


Thats a fast reply!!:slight_smile:

Okey but there is no inverse square decay on the MR omni light in the scene!!! and it even happens with standard omni light!!! without any decay.



The photons uses inverse square decay and the photon radius set in the rendering parameters is expressed in… system unit (even if not checked it is set as 1/100th of the scene size) !


Thanks buddy that explains a lot.:slight_smile:
But i was still unable to get caustics when working in meter scale, no matter how extreme i go with the settings. I guess the photons must be to small or too big right!!!

So is it best to work in inches than in meters?


Working in meters works perfectly, but when you convert a scene unit, you may end-up with unrealistic measures: for example in your scene, the sphere is 40 meters larges when units are converted in meters… since the caustics are a prismatic focused light effect, you will not see them unless the light is very very strong. In fact, just change the decay to zero in the GI (light properties) global rendering settings OR put the energy multiplier to 1000, and you will see them…
I recommend starting with meters from the begginning.
Btw, you also forgot to enable the “generate caustics” in the properties of your light, and did not put any photon shader to your material as you should when using MR.
Another comment: Reactor do care about units too, like any dynamic simulator or realistic rendering software; it just happens to be set-up correctly by default.
Hope this helps,


Wow!! great! i got the caustics:lightbulb working in meters, all i played was with the decay!!, Thanks for telling that:beer: … lol i was bumping up the photon samples and playing with sampling radius…

And about enabling “generate caustics” in the properties for the light, well i guess it is for the objects that i want to generate caustic right, i am saying this because it didn’t have any effect in my scene either on or off for the the light, the render output was the same!:surprised
and same thing goes for photon shader!!! well actully very new to mental ray so not familiar with all the shaders and stuff!!
For the caustics I just played around with the decay and it did fine!! i didn’t need to change anything else.
For instance if i dont want this particular light to generate caustics i can simply set its caustics photons to 0, rather than using the generate caustics under light properties, which for some reason does not have any effect. Correct me if wrong:blush:

Sorry, guess i said wrong about reactor, well i meant to say was that the value you give for
World Scale— This value is specified in world units. It specifies what distance in 3ds max units represents one meter in your reactor world, and hence determines the size of every object in your simulation. So what ever system units you are working in, you can always define 1 Meter in The Havok physics engine(Reactor) = What Units in Max.
Same goes for the cloth fx, it has its own way of defining scaling of the objects! rather than taking directly the system units into consideration.
I though there must me something like that in Mental Ray too.
Anyways thanks for pointing that out.



Okey i found one more thing, i didn’t change the decay settings in the light properties render dialogue, but i choose manual settings instead of automatic!!! and rendered the scene with system units set to Meters!!! bam :argh: i hav the caustics like what i was getting when it was in Inches!!

So what is the relation ship between the Decay under light properties and the sytem units and that Manual settings…
this is all very confusing:banghead:



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