Render Mesh Light additively over diffuse material on the same mesh

[nikolan8855W]

Hi there,
I have an LED screen surface which I'd like to be a mesh light, and the texture content I am using has black values, for example, some coloured shapes on a black background. I am trying to get an additive light setup like a material with emission texture, but I need the AOV of the light cast as well as the emission surface visibility.

I tick the visible light option on the mesh light tag. However, the mesh is no longer a diffuse surface, it has no material, and you see pure black where the content is black, rather than diffuse surface. Im not sure how to approach it.

this is the desired look (when using material with luminance texture) 

this is what I get when I use the mesh light tag

I tried using an alpha channel in the texture and having the diffuse material tag on the object as well, but that didnt work.

One suggest workaround was to do an AOV using a diffuse material and an AOV mask of the mesh, then in comp to blend them additively. I made a test scene to check if its possible to do an AOV of a custom material, I cant find a way to plug the shader into the AOV input into the material since it only accepts color as an input.

the current version we are locked into is:
C4DtoA 4.4.1 [f852d447]
Arnold core 7.1.3.2 [2a385cac]

[peter_horvath]

It is technically possible in Arnold to render the diffuse surface of the shape and use it as a mesh light at the same time. However when adding a mesh light tag, the plugin currently overrides the shader and visibility of the shape, eliminating the shading basically. I can add an option in a future release to control this override and allow the look you want to achieve.

For now your best option might be to duplicate the geo, although it can be an overkill. One geo with the mesh light tag, light visibility turned off. The other geo with a material that mixes a diffuse and emissive surface driven by your texture, visible only to camera rays.