Here's an interesting term that comes to us from the world of 3D rendering. Anisotropic shading, as it applies to pinup art, can be seen as those wavy highlights in shiny hair. The highlights are irregular, as they conform to the grain of the curved follicles. They appear as a band when viewed straight-on or as a circle/semicircle when viewed from above. On curlier hair, they get even crazier!
You'll see a lot of anisotropic hair shading in comic books/manga, usually as a bowed gradient strip or a simple jagged line. Of course, not all hair requires this kind of treatment. It's just nice to know why it looks the way it looks.
Here are some real-life and illustrated examples of anisotropic shading.
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