Writing shaders requires an understanding of the individual stages involved in the rendering a scene with the GPU, and how we can use GLSL to program them. The sequence of these stages is called the "graphical pipeline" in the technical parlance of computer graphics, and we will now take a look at the main stages in the pipeline from the perspective of a Processing sketch.

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<p>Note that the goal of this document is not to provide a programming guide to GLSL, but to describe in detail the new shader API in Processing so that users already familiar with GLSL can write their own custom shaders and then use them in Processing. There are several resources, such as <a href="http://www.lighthouse3d.com/tutorials/">online</a> <a href="http://ogldev.atspace.co.uk/">tutorials</a> and <a href="http://www.opengl.org/discussion_boards/forum.php">forums</a>, <a href="http://www.amazon.com/OpenGL-Shading-Language-3rd-Edition/dp/0321637631/ref=sr_1_1?">books</a>, and web coding sandboxes (for example <a href="https://www.shadertoy.com/">Shader Toy</a>, <a href="http://glsl.heroku.com/">GLSL Sandbox</a>, and <a href="https://www.vertexshaderart.com/">Vertex Shader Art</a>), that can be recommended for learning GLSL programming. Furthermore, the GLSL experience gained using a different programming interface, platform or toolkit (openFrameworks, Cinder, webGL, iOS, Android, etc.) can be easily translated over to Processing.<p>

<p>Let's start with a simple 3D sketch as the model to understand the relationship between the Processing functions and variables and the underlying pipeline running on the GPU. This sketch draws a quad with lights and some geometric transformations applied to it: