Try out some other blending modes to create some desired effect. The specific choice of the effect is up to you, but it should include some custom blending configuration. Do write in the submission, what effect were you trying to aim for and how you achieved it.

For example, you could create an overlay that will create kind of an vignette or that the camera is looking to the scene via a clean spot on a dirty window.

This is done with the same smoke.png texture that is used for the soft particles. You don't have to restrict yourself to that texture, but using another one might create some extra work for you in order to load it to your application.

In this example the overlay is just a quad placed close to the near plane of the camera. That quad has the texture mapped to it and specific blending configured.

This is a creative bonus task, so no base code. Feel free to use any of the previous base codes to build upon.

JavaScript

In Three.js materials can have a specific blending configured to them. Check out the documentation for the Material, the Blending Mode Constants, and the Custom Blending Equation Constants.

C++

For OpenGL check out the glBlendEquation() and the glBlendFunc(). Don't forget to also enable the blending with glEnable().