One of the materials that I might use as a kind of place holder material in threejs would be the normal material, in fact I often seem to use if for that kind of task. One nice thing about it is that it is a way to quickly show some depth without having to do much of anything with textures and light sources when using the basic or standard materials for exmaple. However there are still a few other options for that sort of task such as the depth material.
The normal material will render colors to the faces of a geometry by way of the state of the normal attribute of the buffer geometry geometry instance used with the mesh object. The normal attribute is an array of values that corresponds with the position attribute that is used to set an alternative direction independent of the position direction from the origin when it comes to the position attribute of geometry. The normal attribute is a must have attribute when it comes to using any material that has to do with light such as with the standard material, but it is also needed for a material such as the normal material which will help show what the deal is with the state of this kind of attribute.
The normal material can be used as a way to find out if there are problems with the normal attribute of a geometry as there is a certain look that an object should have when using it. However it might not be the best tool for the job as there are other things to work with in the core of the threejs library such as arrow helpers. In addition there are additional external files that can be used on top of threejs that will add a kind of normal helper which might be a better tool for debugging normals.
It might be a good idea to take some time to gain a deeper understanding of the normal attribute of a buffer geometry instance in general. I have wrote a post on the topic of the normal attribute alone that might be worth reading as one way to go about doing so. Crossing that bridge is something that one will just need to do sooner or later when it comes to making custom geometry, also when it comes to sticking to the built in geometry constructors there are some visitations in which I might want to flip the normals, or make use of the side property of a material to get a desired outcome.
So there is the normal attribute of a buffer geometry instance, but then there are other major attributes of a buffer geometry such as the position and uv attributes along with a number of other attributes that might come into play also. There is also a wide range of additional prototype methods and properties of a buffer geometry instance that are also worth looking into more at some point sooner or later.
In this post the main focus is just simply the normal material, and not so much creating or updating a normals attribute of a buffer geometry. I have wrote a post on the compute vertex normals method of the buffer geometry instance. This compute vertex normals method will work fine in most cases for the task of creating a normals attribute of one is not there, or updating one in the event that the position attribute chances a little. In some cases however I will need to work out a way to update the state manually though, in any case I might touch base on this a little here but for the most part that is a whole other can of worms as the expression goes.
The mesh normal material is just one of many material options in threejs so it might be a good idea to read some post that serves as a general overview of all the options when it comes to materials. The main feature of interest with the normal material is just rendering textures for the faces of a geometry using the state of the normals of a geometry, but not taking into account anything that might be going on when it comes to light sources.
The normal material is something that I often used as just a place holder material, and also it is one of several tools when it comes to debugging issues with the normal attribute of a geometry as well. However I can not say that it is the kind of material that I would use for any kind of real final product in terms of most of not all idea that come to mind with projects.
Often the normal material might be used as a way to help debug the state of a normal attribute. That is not such a bad idea as there is a certain way it should look if the normals are in a state that is often what is desired. However I will often use the normal material on top of the vertex normal helper that is my first and foremost tool that I would go with when it comes to the task of debugging vertex normals of a geometry.
When I first wrote this post I was using r127 of threejs, and the last time I came around to doing a little editing here I was using r140.
The source code examples that I am writing about in this post can be found in my test threejs repository on Github.
In this section I am going to be writing about a simple hello world style example of mesh normal material. So in a way this is just a very basic getting started example of threejs in general actually as I do still like to start out my threejs posts with very basic examples before getting into anything that might be a bit more advanced.
After I have my core set of objects set up I will want to create a mesh object and add that mesh object to the scene as a child by using the add method of the scene object. When creating a mesh object I am going to want to pass a geometry as the first argument, and then a material as the second argument. So for this basic example I will be using one of the built in geometry constructors such as the THREE.BoxGeometry constructor, this will have the normal attribute set up to begin with.
After I have my geometry and pass it as the first argument to the mesh constructor, I then just need a material to use with the geometry of the mesh object, and for this I am of course using the Normal Material. For this example I am just calling the THREE.MeshNormalMaterial constructor by itself without passing any options to it. I then pass this normal material instance as the second argument for the mesh object.
So now that I have my scene object, camera, renderer, and mesh object all set up just the way I like it now I just need to call the render method off of the renderer object. When doing so I pass the scene object as the first argument and the camera as the second argument.
2 - Mutating the normal attribute to see how that changes the appearance when using the Normal Material
The normals are set up the way that they should be typically when using a built in geometry constructor such as the Box Geometry constructor that I am using in these examples. However when it comes to debugging problems with the normal attribute of a geometry there is knowing how it should look. To gain a sense of what this looks like there is taking a moment to just mutate a few values in the normal attribute of the geometry, just for the sake of seeing what happens when the normals are not in a state in which they should be in.
Although this example might help to show what happens when the normals are not set in a way in which they should be maybe, using the normal material alone might not be the best way to debug problems with the state of the normal material. There are a few additional tools in the core of the threejs library as well as some additional files in the repository that can be used as a way to really get to the bottom of what is going on with the state of this attribute of a buffer geometry instance.
If I need to debug something that is going on with a normals attribute the use of the normal material is a good start, but in order to really get a good visual idea of what is going on I will want to use the THREE.VertexNormalsHelper. This helper is not baked into the core of the threejs library and as such must be added to a project example by way of an additional file that can be found in the examples folder of the threejs Github repository. In this example I am also making use of the orbit controls which is another such external file that must be added to a project on top of that of threejs by itself.
One thing to be aware of when it comes to normals and a material such as the normal material is the side property of a material. When it comes to the side property the default value is the value of the THREE.FrontSide constant in the threejs library. That is that it will only be the front side of each face that will be renderered. There is setting the side property to that of something like THREE.BackSide, or THREE.DoubleSide. However in any case the question might come up as to how to go about defining what side is the front side to begin with. Well the way to do so will be to change the values of the of the normal attribute which would be the alternative to changing what the value of the side property is.
The normal material is often my default go to material when I am working out things that do not have to do with materials and textures and lighting just yet. The normal material is often a step up from using the basic material with just a solid color, and not much of anything else as just results in a blob of color in the canvas rather than something that looks like a solid shape. The mesh normal material is one way to show that there are sides to an object, however there are some additional options when it comes to a simple place holder material such as the depth material, or using the basic material just adding a simple texture to it by way of a data texture, or a canvas texture.
I can not say that I use the normal material when it comes to making any kind of final product though, unless that final product is to outline the nature of the normal attribute of a geometry, then it goes without saying that this is the idea material to use. Even then I think I should add more to the example that just simply use the normal material, such as arrow helpers or the vertex helper to really show what the current state of things are with the normals.
The vertex normals helper is the best tool for the job when it comes to showing the direction of each vertex in the geometry. Most of the time I would want all of them to just point outward form the center of the geometry, but in some cases I might need to do something weird.