Implementing heatmaps with arrays

One characteristic that makes shaders hard to master is the lack of a proper documentation. Most developers learn shaders by messing up with the code, without having a deep knowledge of what's going on. The problem is amplified by the fact that Cg/HLSL makes a lot of assumptions, some of which are not properly advertised. Unity3D allows C# scripts to communicate with shaders using methods such as SetFloat, SetInt, SetVector, and so on. Unfortunately, Unity3D doesn't have a SetArray method, which led many developers to believe that Cg/HLSL doesn't support arrays either. This is not true. This post will show you how it's possible to pass arrays to shaders. Just remember that GPUs are highly optimized for parallel computations, and using for loops in a shader will dramatically drop its performance.

For this recipe, we will implement a heatmap, as shown in the following image:

Getting ready

The effect in this recipe creates a heatmap from a set of points. This...