Problem with multipass rendering

[chi]

hey....there isnt a problem...in you render settings look for the field labeled seperate lights...you can then choose...all...or selected...

after that you are given the option of which pass you want for the light....availible options are diffuse....shadow...and specular...

oops...miss read your post...reflection\refraction....technicaly these attributes are not affected by lights...so if you had a channel for each light they would all look the same

[astrofish]

Ok, here's an example. Suppose I have a scene involving multiple lights and reflective objects.

If I manually turn on just one light at a time and render out a standard image, then I get a set of images that show me the complete contribution to the scene from each light source.

This is the simplest use of a multipass rendering approach, but as far as I can tell I can't get C4D's multipass rendering option to do this.

To use some mathsy notation, if D is the diffuse channel for my scene, R is the reflective channel, L1 and L2 are lights, and output is O, and I ignore all other channels, then a normal render would give me:
O = L1*(D+R) + L2*(D+R)

By manually turning lights on and off, I can render out two images:
O1 = L1*(D+R)
and
O2 = L2*(D+R)
as seperate images.

If I want to halve the brightness of the second light in my final image, I simply (in my compositor) do:
O = O1 + 0.5*O2.

Unfortunately, C4D won't let my render O1 and O2 as described above in its multipass settings.

The closest I can get is:-
O1' = L1*D
O2' = L2*D
O3' = (L1+L2)*R

The problem is that for the reflectance pass (O3'), the effects of both lights are pre-combined.

Given these outputs to work with I can't do the simple task of halving the brightness of a light...

Cheers - Steve

[chi]

ok...I..see but you already solved your problem then...guess you cant do it automaticly...stick with the two renders...
we all learn something new....

[astrofish]

Well doing it manually does at least mean that you can get the passes you need, but it's a real pain making sure you get everything set right for each pass. Multipass render really should be able to do this. Hopefully it will get added...

(Off to the suggestion box again.)

Cheers - Steve

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