Thanks for contributing these patches! I wasn't even aware there was a film being made with Cycles as the renderer.

My impression is that these are mostly features to make rendering without GI more efficient, whereas for Cycles the design goal has always been render with GI and optimize around that. While all these things are good ways to keep render times very fast, it's using Cycles in a way that doesn't really fit the original vision. I fear that adding back too many of these tricks will make it harder to do certain optimizations and improvements in the future, and will lead to bad workflows for users.

Light linking is a feature that is typically used when shaders do not react reliable under different lighting conditions, either because they have some kind of energy conservation problem or because GI is missing and important to get consistent lighting. For example hair without multiple scattering can look very wrong, and in Sintel light groups where used to light the hair separately to compensate for that. While we can improve the hair shaders a lot still, with more recent Cycles open movies the hair behaves more consistently under lighting.

What we should aim for is to improve the shaders so that light linking isn't necessary.

You're very welcome! Blender has been an awesome tool for the studio and this is the least we can do! I'll get someone from production to comment but basically it's a film vision vs. realism tradeoff thing.

UPDATE: Here's what production said...

"It's not that there are issues with shaders - it's that there's still a want to provide for character based lighting (or supplemental lighting for any assets) without affecting the surrounding elements. Think about a combination of environment lights + simple 3 point lighting to enhance the characters."

"Adding a bit of rim lighting to a character without affecting the environment is very important for Features. If we can't do this, then it's unlikely that we'd use the renderer as a feature based renderer, even with GI turned on."

Light linking is essential for product visualization. We often get requests for lighting that would be impossible in a studio but possible in CG (if there was light linking). Right now we can hack it somewhat by using diffuse shaders and glossy shaders with 100% roughness and making a distinction via the light path node, but there are too many situations where this approach does not help.

@Brecht Van Lommel (brecht) light linking has a lot of use cases. The one you mentioned is just one out of many. We're living in the age of GI and people are still faking rim lights in Cycles using the normals trick because due to the lack of light linking there is no way to get a proper rim :(

An even better system than light linking would be ray tagging so visibility options would work for any object or material, not just for lights.

I agree there's some specific cases where it's useful, but also know it gets used a lot to work around deeper issues. I'm not against adding this feature in principle, but also like to see the specific use cases enumerated so that we can understand it better and implement it the right way. Rim lighting for specific characters seems like a good use case, but would also be interesting to hear what shadow linking was used for.

Regarding the implementation, one important optimization could be to have a different light distribution CDF for each combination of lights that is used in the scene. Otherwise you end up sampling a lot of lights only to discard them immediately. Though if you're using branched path tracing with "sample all lights" it's not so much a problem to loop over a few lights that you discard quickly.

Adding a function like object_in_shadow_linking right in the middle of BVH traversal likely also has a significant performance impact for scenes that do not use shadow linking. It might be faster on average to do this check after the intersection has been found, or even to force relevant objects to be instanced so the check can be done once per object.

Ray tagging to me sounds incompatible with bidirectional methods and light sampling optimizations. So if there's important use cases for that again it would be interesting to hear actual examples of why it's needed.

Is there any progress for this very important feature?
Arnold has a very nice light groups implementation, not sure about linking, but probably it's there also if someone needs an example.

Any progress here? I ask, because this is a useful feature. And there seems to be no activity here.

This is very useful feature, want to see it in Blender. For example, in V-Ray (3ds Max) there is an option in light source settings, that allows specific for each light source to exclude or include certain objects from/in the lighting. I used this function, when I created сhandeliers with mate glass shade. In that case there are 3 light sources for each lamp. The first light source needs only for bright spot on the shade and reflections on other objects in scene (it is a mesh with emission shader), it does not emit the light. The second light source affects only on the shade. This allows accurately adjust the brightness of the shade without overexposure (or vice versa). The third light source affects on the whole scene, except the shade (affected by the second light source).

would love this to be implemented... even if just in experimental

I am SO EXCITED that I found this thread. Please continue working on this! I actually released a video on my Youtube Channel begging for this Feature YESTERDAY so I'm pretty excited that its in the works! https://youtu.be/T9RfN8TwWw8 Also, to completely fullfill my wishes, maybe add an option to exclude the bounce light, can have JUST the bounce light of an object, without the original light source. This would be SOOOOOOOO usefull for VFX.

In D1985#181767, @Jonathan Macher (KiwinBlue) wrote:

I am SO EXCITED that I found this thread. Please continue working on this! I actually released a video on my Youtube Channel begging for this Feature YESTERDAY so I'm pretty excited that its in the works! https://youtu.be/T9RfN8TwWw8 Also, to completely fullfill my wishes, maybe add an option to exclude the bounce light, can have JUST the bounce light of an object, without the original light source. This would be SOOOOOOOO usefull for VFX.

Gosh, I'm not alone... I was searching how to achieve that and I was sent on this topic.
So still nobody has figure a way to implant that feature on Blender? It's weird, because this feature is really important for so many case. It's a must for products images, for architectures etc...