In this section, you'll learn how to create a custom shader that you can use in postprocessing. We'll create two different shaders. The first one will convert the current image into a grayscale image, and the second one will convert the image into an 8-bit image by reducing the number of colors that are available. Note that creating vertex and fragment shaders is a very broad subject. In this section, we only touch the surface of what can be done by these shaders and how they work. For more in-depth information, you can find the WebGL specification at http://www.khronos.org/webgl/. An additional good resource full of examples is Shadertoy at https://www.shadertoy.com/.
To create a