Torus shape converted to mesh light (rollover image). Model kindly provided by Lee Perry Smith.
In situations where conventional light shapes will not suffice, mesh lights are more suitable. Mesh lights can be used to create interesting lighting effects that would not be possible any other way. For example, effects such as neon lighting or a car light motion trail effect can be achieved more easily with mesh lights.
Geometry converted to Mesh light
An example mesh light scene can be found here.
The mesh light is provided as an alternate translator for a mesh. This means that shape parameters, such as visibility flags, and the mesh light parameters are accessible from the Modify Panel of the mesh.
The current limitations when using a mesh light are:
- NURBS surfaces do not currently work with Mesh Lights.
Mesh light translator is available in the Modify Panel
It is not currently possible to make a mesh light visible to camera rays. A workaround is to add emission to a standard_surface shader assigned to the object. This will give the impression that the geometry is incandescent. Set the indirect_diffuse to 0 for the standard_surface shader assigned to mesh as the mesh light is already emitting light.
Setting indirect_diffuse to 0 disables GI bounces for the mesh
You may need to increase the number of subdivision iterations for the mesh light if the color texture is clearly visible in specular reflections. For example, this may be evident in a scene where a TV screen is reflected in a glass window.
Mesh Light Vs Emission
You can see that even with diffuse samples = 16, the emission is noisier than a mesh_light with diffuse samples = 2.
Below is another comparison test between a mesh light (left image), and a sphere with a highly emissive standard_surface shader assigned to it (right image). As you can see in the mesh light image, light is bounced around the scene, whereas there is no bounced light in the emissive sphere render. This is because the rays from the mesh light work in conjunction with the Diffuse rays, whereas, only the primary rays are considered when using emission only. Even with 6 Diffuse samples, the render using Emission only contains far more noise.
In the example below, Diffuse samples have been increased to ten to get a clean result using only Emission in the scene. With mesh light, the scene only requires three Diffuse samples.