Adding Fog to a Scene in three.js is a fairly easy, and straight forward process, so this should be a quick post for today. However there are still a few basic things that a developer should be aware of when it comes to adding fog, such as the fact that one can not just use any material, and that typically the background color of a scene should be same color used for the color of the fog when creating it with the THREE.FogExp2 constructor.
A Fog is a nice effect to have for most projects as it allows for a more graceful transition from rendering within range to no longer rendering when an object is to far from the camera, as apposed to the object just all of a sudden disappearing. Even if you have the far value of the camera set to a high value so that the object is just a single pixel before it is no longer rendered, it can still be a nice additional effect on top of the object just simply getting smaller.
So in this post I will be going over a basic example of fog in a three.js project, and will also be touching base on a few other three.js rated topics such as the nature of the standard material which is one option when it comes to using a material that will work with fog.
Not all materials will work with fog, for example the Normal Material will not work with fog, but the Lambert and standard materials will. If you have not done so before hand it might be a good idea to go over all the materials and work out some basic examples of each, and also play around with lights and so forth in order to get a better sense of what the options are for materials. I have my post on materials in general, but for this post I will be sticking to the standard material as that seems to be one of the best options and not just because it supports fog.
A fog is something that can be added to a three.js Scene object by attaching an instance of the THREE.Fog, or THREE.FogExp2 constructors to the scene.fog property os a scene object instance. By default the value of the fog property of a Scene instance is null, so if you do not add one there will not be one by default.
You pass the Fog constructor three arguments the first is the color, the second is the near distance, and the final is the far distance of the fog effect. This is similar to the camera as well where there are also values that are the near and far distances for the cameras view of a scene in which it is used.
Typically you will also want to set the Fog color, as the same color as the background. You also might want to have the near, and far distances correspond to your camera as well.
The scene.for property of of course a feature of the THREE.Scene class, and there is a great deal more to know about this scene class and the objects that it creates beyond just that of the fog property. For example there is the background property of the scene object which can be used to set a solid color background, or a cube texture for the scene. There is also a great deal that branches off from the scene object such as the fact that a scene object is one of many objects in threejs that is based on the Object3d class, and that it is also a good idea to become familiar with the THREE.Color class that can be used to create a color object for the background, as well as for the fog color of a scene.
The version of three.js is something that is important when writing post on it, often many code breaking changes are introduced that will result in older examples no longer working. When I first started writing this post I am using three.js r91, and the last time I edited this post I as using r127. It would seem that not much of anything has changed with fog alone at least thus far, but still this is always something that a developer should be aware of when it comes to the fact that three.js is a fairly fast moving target in terms of development.
A full working demo will require all the usual components that make up a fully functioning three.js project. There is nothing out of the norm when it comes to setting up the renderer as compared to working with things like shadows for example where there are some special properties that have to be set for the renderer, as well as mesh objects. With fog I just need to create a Fog for the scene.fog property using the THREE.FoxExp2 constructor, setting the color and density for the fog via the arguments.
In this demo I put in a simple loop to have a mesh move back and forth from the camera as a way to show off the fog effect. The effect seems to work just as I would expect, and adjusting the density value will make the fog more or less dense when it comes to playing around with this a little.
You will want to make sure that you are using a material that can be effected my shadows. Some materials will not work with a fog, such as the MeshNormalMaterial. To help with this you can check the fog boolean which is a property of the base Material class.
There are two fog constructors in three.js, for the most part I have just covered the normal linear Fog constructor where the fog grows at a constant rate as the distance grows farther until the far distance is exceeded. There is another constructor that does the same thing, only at an exponential rate that can be defined by one of its arguments with not near, or far values. This fog just takes two arguments then, the color, and a value that represents the exponential rate at which the fog will grow denser.
That is it for now when it comes to just fog alone in three.js, which is a nice feature to have at the ready when toying around with things. Also I am sure that it is a feature that I will want to use in certain actually full projects in the future for certain scenes. There are a number of other similar features about three.js that might also be worth looking into when it comes to backgrounds and fog, for example there is having a cube texture for the background rather than just a solid color background.