A material capture or 'matcap' shader is often used in modelling applications, where an image of a sphere is used to control the appearance of geometry in the viewport. The most frequent use case is to use an image of a sphere made of clay, so that sculpting applications can better approximate sculpting in real world materials.
Here’s several matcap textures (the spheres), and the test pig geo with the matcap texture applied.
Note that the above image is from the 3d viewport and runs at realtime framerates. While the matcap shader is supported in mantra, it’s main use is in the viewport to help visualise model curvature.
Because of how matcap materials work (where model normals are used to directly access pixels in the sphere texture), matcaps don’t use scene lights, and don’t display in the high quality viewport modes.
Texture to use as matcap, a photo or render of a sphere, cropped as tightly as pssible. The normals of the geometry will be matched with the implied normals of the sphere image, reading the pixel colour from the matching position.
Use MatCap Alpha
If an alpha channel is in the texture, also map that into the shaded geometry.