ambient

• Make the ambient color vary directionally, e.g. using spherical harmonics (SH) or a small cubemap, and looking up the color in a shader based on each vertex's or pixel's normal vector. This allows some visual differentiation between surfaces of different orientations, even where no direct light reaches them.

http://computergraphics.stackexchange.com/questions/3955/physically-based-shading-ambient-indirect-lighting

https://unionassets.com/blog/working-with-lighting-in-unity-theory-and-practice-462

http://kayru.org/articles/spherical-harmonics-in-games/

https://docs.unity3d.com/Manual/SL-UnityShaderVariables.html

http://www.gamasutra.com/blogs/DavidRosen/20100420/87163/ImageBased_Ambient_Lighting.php

http://sunandblackcat.com/tipFullView.php?l=eng&topicid=31&topic=Advanced-GLSL-Lighting-Models

https://seblagarde.wordpress.com/tag/ambient-lighting/

Depends on the use case, complex SH lighting can take a really long time to precompute but is fast to evaluate at runtime. Cube map lighting is of the same complexity as doing a texture sample.

https://www.gamedev.net/topic/630814-spherical-harmonics-lighting-questions/

http://computergraphics.stackexchange.com/questions/233/how-is-a-light-probe-different-than-an-environmental-cube-map

http://www.artisticexperiments.com/cg-shaders/cg-shaders-ambient-cubemaps