- To add your new texture to the MatCap, simply click on the little texture icon in the material modifiers and choose the texture from the popup.
A further note about twin sphere textures:
If you want to capture these exactly then the MatCap modifiers need setting for each sphere separately. For the left sphere, set Intensity A to 1, Intensity B to 0, Cavity Detection to 0 and Cavity transition to -1. For the right sphere, set Intensity A to 0, Intensity B to 1, Cavity Detection to 0 and Cavity transition to 1. You might also want to reduce Obj Shadow to 0 in the Render:Preview Shadows submenu.*
*Thanks to Svengali for pointing out how to adjust the sliders for twin spheres.
I hope you find this tutorial useful. The MatCap shown in the images - adapted from the default RedWax - is in the zip file.