Custom grayscale shader

To create a custom shader for Three.js (and also for other WebGL libraries), you have to create two components: a vertex shader and a fragment shader. The vertex shader can be used to change the position of individual vertices, and the fragment shader can be used to determine the colors of individual pixels. For a postprocessing shader, we only need to implement a fragment shader, and we can keep the default vertex shader provided by Three.js. An important point to make before looking at the code is that GPUs support multiple shader pipelines. This means that the vertex shaders run in parallel on multiple vertices at the same time, and the same goes for the fragment shaders.

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