There are two basic shadowing techniques that are commonly used in real time rendering: stencil shadows and texture shadows. The stencil shadow technique (also called shadow volumes) works directly on the geometry to project shadow volumes through the scene. The texture shadow technique works by rendering the scene to a texture (jargon that simply means image) from the light’s point of view and using that texture to determine where shadows fall during the rendering process of the output image (what you see on the screen). Both methods have distinct advantages and disadvantages and the myriad array of sub-techniques and specialisations of these two umbrella techniques makes choosing a shadowing technique suited to ones own uses a bit of a task. Describing, or even enumerating all of the sub-techniques that have been used or proposed is outside the scope of this article (read: outside the scope of my evening). I may write some articles in the future about the sub-techniques that I think are wonderfully clever (and some of them are deliciously clever) but for now, the basics. How does anyone get any shadows on the screen in the first place?

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I remember the first time I started looking into shadows. It was back when I first started working with 3D graphics and I had this naive idea that 3D graphics were so awesome because they were simulating nature. Learning about real time shadow rendering techniques quickly killed that idea. Learning about other aspects of real time 3D rendering like geometry and lighting I suspended the realization that it is all just smoke and mirrors, by imagining that if my computer were just faster, I might be able to take into account enough detail to actually be simulating how light works, rather than just making a nice picture with a pseudo-approximation of how light works. But when it comes to shadows, that rationalisation rolls over and dies. The methods used to create the appearance of shadows in real time rendering (and as far as I’ve researched, in offline rendering as well) are far removed from the real world physics that create shadows.

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