Here you will find some tutorials I wrote on the issue of digital illustration and rendering. Actually most of these were created as visual aid to forum discussions on the McNeel newsgroups.
How to blend refraction and reflection from multiple renderings in order to change the hierarchy of transparency vs. mirror in a render engine.
How to post-process a rendering using sperate layers.
How to render digital geometry into an existing scene (photo).
How to create a pseudo shiny material, without using highlights or scene reflection.
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A series of images showing how to add photorealistic soft/area shadows to cartoon, sketch or cell renderings.