alpha visibility on Emissive Volume

[JoachimArt]

Currently when you render using Corona Volumetric Material you can get the alpha channel of the volume. But If you only use the emissive part, there's no alpha information. Is this something that is possible to add?

[maru]

Hi, can you explain what exactly you are doing that you would need a feature like this?

I think overlaying a solid/transparent object is a different operation than overlaying a solid/transparent light.

In case of an object, you only need to put it on top of some background and use alpha as the mask (so if it's solid it will cover everything and if it's transparent it will show some background behind).

In case of a light, you need to put it on top of the background but blend using add operation (since light is additive, like in blending LightMix layers). So I think in case of volumes you would have to isolate the illumination and apply it separately with the add operation. 

[JoachimArt]

Hi, yes I will try to explain. (see attachment) There has been many instances where I would like to use this, but right now it is for a helicopter with a searchlight. And I want to simply/fake/exaggerate and not add a fog volume to the entire scene, but just have a cone coming out of where the light is, so that I don't waste rendering time and also I don't want fog in other parts of the image as I'm going to deliver the helicopter as a separated animation. Also I do want the cone to work from a front angle, if not I could just have mapped a cylinder with a self.ill opacity map.

Does that make sense?

[TomG]

This looks a case where CESSENTIAL_Emission render element would be best? Saved to 32 bit as EXR, it could be Linear Added and have Exposure and everything adjusted, all in post.

[JoachimArt]

Thanks for the suggestion. I have managed to work around it. But I don't understand the logic of not getting an alpha because I'm only using emissive volume. Even when using a "light material only", you do have the option to render an alpha, even when it is "only" a light. Wouldn't this be the same logic? Also emissive volume can be used in a lot of useful cases, when not rendering completely realistic stuff - so it would be nice with a alpha channel for it as well.