Soulburn Studios Art Lessons

Emission
By Neil Blevins
Created On: Dec 15th 2024
Updated On: Dec 15th 2024
Software: Any

As well as the standard material properties of Diffuse, Specular, Transparency and Subsurface, there is one 5th property to consider which is Emission. This attribute is different from the other 4 because it doesn't follow the rules of Energy Conservation. Energy Conservation says that the light that bounces off of, is absorbed by, and travels through a surface can't be more than the light that strikes a surface. And so the 4 other attributes added together must be equal to the energy that the surface recieves. But emissive surfaces create their own energy, which can be seen as a brightness or glow that is not associated at all with other light sources. In a 100% black room, the other 4 attributes of a material would return black (or a value of 0.0), wheras an emissive surface will return a color even if no other light sources exist in the space.

Practical examples of emissive materials include lit lightbulbs, glow in the dark paint and bioluminescent fungus.

Constant or Self Illuminated Material

Many materials in CG software have a mode that's frequently called Self Illumination or Constant. This is the first step toward emissive, the standard diffuse and specular models are blended away to a pure representation of the surface texture with no shading. Basically, if the pixel value of your texture is 50% grey, the rendered result will be an identical 50% grey.

Here's a texture map of marble.

And here's the marble assigned to a sphere, see how the shading (the darker shadow on the surface of the sphere) goes away as we increase self illumination from 0% to 50% to 100%.

Self illumination is frequently used in Non-photoreal rendering (NFR) or when you want to place an image in a 3d scene where the lighting has been painted directly into the texture, and you don't need any contribution to the shading from the lights in your 3d scene.

Glow

The next step is increasing the output values of your image beyond 1.0. A lightbulb for example can produce energy (light) far beyond the 0-255 color values our monitors display. Just like how the sun can be thousands of times brighter than the color white. Increasing the emission beyond 1.0 allows us to create materials / textures that can be used as light sources themselves.

Conclusion

Check your favorite renderers for details about their specific implementation of these features.