In threejs there is an option to use directional light as one of several types of light to choose from when adding light to a scene object. Other options that I find myself using the most thus far include point lights, and ambient light, but in some cases I might want to use directional light in place of or on top of these other options.
A directional light is like ambient light in the sense that it is a good way to go about simulating day light. With ambient light a base light intensity is just applied for all materials in a scene and the location of the ambient light in world space does not really matter. Directional light however as the name implies is a situation in which light is coming in from a given direction, but not in the same way as with a point light, or spot light. The rays are all coming in at a parallel, uniform direction, rather from a fixed point outward. Directional light alone might not still be the best way to simulate daylight though, I often find myself using a combination of directional light and ambient light to do so.
So then directional light is kind of like that of point lights and spotlights in that I want to set a desired value for the position property of the object that is returned when calling the THREE.DirectionalLight constructor. However when doing so I can also make use of a normalized Vector3 instance value, as the direction and not so much the magnitude of that directional that matters with directional light. When it comes to spot lights and point lights it is both direction and position that matter.
Not all materials will respond to light sources, for example mesh material options such as the mesh basic material, and the mesh normal material just to name a few. When I work with light I often go with the standard material, but there are some additional options such as the Lambert material and the Phong material that one should also take into account.
Light is also a major thing to be aware of when it comes to getting into writing custom GLSL shaders with the shader material as well. Getting into every little detail with this would of course be way off topic though so there is checking out one or more of these materials or starting out with some kind of main post on the subject of materials if you feel as though you may need to research more on them first.
At this time you should know at least a little about the object3d class, and mainly the position property of the object3d class. This position property of the object3d base class stores an instance of the Vector3 class as the value of the property. When it comes to a directional light the position property is what is used to set the direction of the light. Although it may be the position property it is really just the direction of the vector that matters and not so much the unit length.
The source code exmaples in this post can also be found in my test threejs repository on Github. This is also where I park the source code examples, and various other assets that I write about in my many other blog posts on threejs.
When I made the code examples for this post I was using threejs revision 127, and the last time I came around to do some editing here I was using r146. I do not think that much has changed with directional light for a real long time, however I can not say the same for a whole lot of other features of threejs. At some point in the future it is possible that the code examples here might break, so it is a good idea to always take into account what the version number is that you are using as well as the version number that was used by the author of a code example on the open web.
So here I have a basic example of a directional light as a way to have some light in a scene that will work with a material that responds to light such as the standard material. To create an instance of directional light in threejs I just need to call the THREE.DirectionalLight constructor and add the resulting instance of that constructor to the scene object. When calling the constructor I can pass a color as the fist argument, and an intensity level as a value between zero and one as the second argument.
By default the position of the directional light is 0,1,0 and the target of the directional light is the origin at 0,0,0. The position of the light can be changed by way of the Vector3 object stored at the position propery just as with any other object3d class based object. This will change the direction at which the light is coming from.
A directional light like most lights and objects that are placed in a scene in threejs inherits from the object 3d class. So it has a position property than can be used to set the position of the directional light to a point other than that of the default position. However when it comes to directional light it is really just the direction and not the unit length, or distance from the origin that matters. I have found that the light does not seem to need to be normalized, but it would not hurt to do so anyway.
So I am thinking that directional light might be one of the best choices when it comes to simulating sunlight. Still I think that I might only want to use directional light in conjunction with at least one additional light source such as ambient light. So in these examples I am working out the beginnings of a very simple outdoor scene that will just make use of a mesh object made with the Box geometry as a way to represent a house, and a plain geometry based mesh to represent ground. On top of using directional light as before, and moving that directional light around I am also using the ambient light to set a kind of base amount of light that will always be in effect regardless of the current direction of the directional light source. Also while I am at it I will be making use of other threejs features with these examples to create something that starts to look like a final project of some kind such as shadows, textures, the intensity values of lights, and so forth.
This is then an example that is more or less the same as my movement example above. However now I am making use of ambient light as a way to set a base amount of light that will always be in effect no matter what. On top of this I am also now adding shadows to the scene, and I am also now using more than one material as well while I am at it.
When this is up and running I have directional light working as expected, but when the direction light is not longer hitting the surface of a mesh the ambient light is still doing its thing to make sure that there is still a base amount of light.
In any case once a texture is there to work with, there are a lot of options when it comes to using that texture with a material. What options there are to work with will change from one material to the next. With the standard material the map option can be used to set a texture that will work with a light source. Also there is making use of the emissive map option of the standard material to define a texture that will show up regardless of what the situation is with light.
There is also playing around with intensity both with the light sources as well as the emissive intensity options of the materials. This can be done in such a way that as the intensity of the lights goes down to zero, the intensity of the emmisve values of the materials goes up to the max. This allows for defining effects where I have two sets of textures ones that will show up in the light, and the others that will show up when there is no light.
So then direction light is one of a few options for light sources that can be used with, or as a replacement for ambient light in a scene. However I would always use ambient light and then maybe directional light, as I have found that it is still a good idea to always have a base amount of light for materials.
I went a little farther with the house example of this post and ended up making a slightly more advanced model of a house that is not just a box geometry but a group of meshes being used together to create something that I am calling a house. If you would like to check it out it is one of my many three.js example posts, I have it listed there along with all the other basic project examples I have together thus far.