There are many shader tutorials out there. For example, Unity 3D and Learn OpenGL provide basics, Ronja gives some more elaborate effects. The Book of Shaders is an introduction to fragment shaders, under development. Shadertoy is at the far end of the spectrum, an interactive web environment doing everything in the fragment shader; Omar Shehata provides an introduction. Other web-based systems for experimentation include GLSL Sandbox (similar to Shadertoy), Shdr for basic inputs and meshes to play with, Shadershop for visualizing shader functions, and ISF for video DJs.

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