In threejs there are a few things to know about when it comes to making transparent materials, so I think it is called for to write a post on the topic. When it comes to working with just the Basic material for example the process is not that hard at all actually. When creating the basic material I just need to set the transparent property of the material to true. Once I have the transparency property value of a basic material set to true, it is then just a matter of setting the desired opacity value for the material.
However there is a great deal more to write about when it comes to transparency in threejs. One example of an advanced topic with this sort of thing might be adding what is called an alpha map for the material. When doing so there is not just working out the texture, but in some cases I might also need to address things with the uv attribute of the buffer geometry as well.
Also things can get a little involved when we start bringing light sources into the mix with materials like the Phong and standard materials, and how that should be handled. So in this post I will be covering the very basics of getting started with transparency when working with materials in threejs, but I will also have to touch base on a few more things beyond that.
If I want to just make a mesh object, or just about any display object based off of the object3d class not visible for a while I can always just set the visible boolean of the object to false. There are also a number of other topics to cover when it comes to making an object completely not visible or not beyond just that of the visibility boolean of an object3d based object. For example there is adding one or more mesh objects to one or more groups and then adding and removing the groups to the main scene object that is used with the render function of a renderer as needed. Yet another option for this would be the layers property of the Object3d class also which is yet another option for that sort of thing.
When it comes to the material options in threejs that support transparency it should be, well all of them. The reason why is both the transparency and opacity properties and features of the Base material class used by all Mesh materials. Also sense the same base class is used for the points, and line materials options this feature should work with those as well. So one can more or less start with any materials option that will worm well for the object that they want to skin.
Although I might touch base on the subject of alpha maps, and uv mapping in this post. The main focus here is just what there is to be aware of with in the base material class. Transparency alone is easy enough to get started with, but things can quickly get drop off into the deep end when it comes to alpha maps and everything that branches off from them. There is not just want one will need to adjusted with respect to the material options, but there is also how to go about making a texture, and some times how to make changes to the geometry that is being used also.
The source code examples that I am writing about in this post can be found in my test theejs repository on Guthub. This is also where I park the source code examples for my many other posts on threejs. This post on transparency like many of my other posts is still very much a work in progress, every so often I do get around to expanding posts with more examples while also improving the over all quality of older examples while I am at it. If you see something that should be fixed, or of you can thing of something that should be added there is the comments section in the bottom of this post, and also the test threejs repository where pull requests can be made.
So now to start out with a basic example of transparency in threejs, and as such If I just want to make a whole mesh transparent, and I am not doing anything fancy with lights, alpha maps, and so forth then transparency is not that hard to get up and running. All I have to do is set the transparency boolean of the basic materials to true, and then after that I set the opacity property of the mesh to set the level of transparency that I want for the material.
When it comes to drawing to a canvas element with the 2d drawing context there are a number of options when it comes to setting the color for the fill and stroke style. One option is to use RGBA which allows for me to set what the values should be fore red, green, blue, and the alpha channel. So in this example I am using this when drawing to the canvas element.
Just as before from the beginning I am going to want to set the transparent option of the material to true. However if I want I can leave the opacity of the material to 1. That is if I want to do everything with transparency with the 2d drawing context alone anyway, otherwise I might want to set it to something else.
There are some other options that I might want to set when setting up the material that I use with this canvas texture such as the side property of the material. If I am making a texture that I would like to show up an both sides of a face then I will want to set it to THREE.DoubleSide rather than the default THREE.FrontSide constant.
Things get a little more involve when using a light source, when this is the case I will have to use a material that will respond to light so I can not use the basic material as it will not do that. There are a lot of options with materials that will respond to light to choose form, one such material is the standard material that strokes me as a good middle of the road type chose when it comes to this sort of thing when making choices between performance and how things look. Now that I have made a choice when it comes to materials I am going to need to add at least one light source, there are a number of options for that also of course, the one I often go for is a point light. This is is a nice kind of light that will shine light in all directions form the position at which it is located which works well for many of this kind of examples.
So once again I added a bit more to what was once my basic transparency example by adding a method that will help me to make a texture for a alpha map using a canvas element. When drawing to the canvas element I will want to stick to using gray scale color values, when a color is black that will mean that area in the texture will be fully transparent, while white will mean fully opaque. So I need to just set the stroke color, and or fill color values of the 2d canvas drawing context as needed when working gout the texture for the alpha map.
Things will get a little involve beyond just that of adding the alpha map to the a material though, I will want to play around with some additional material properties to get things to work the way that I want them to. For example there is the question of drawing just the front sides of a cube, or all sides. I might want to also play around with the depth function that is used for the material.
For this example I also added an ambient light to make sure that there is always a certain amount of base light for all surfaces, on top of the ushual point light that I like to use.
On top of setting a transparent material there is also the question of how to go about having a transparent background in the threejs project as well. For this there is setting the alpha option for the renderer to true when calling the web gl renderer constructor function. After that I can use the set clear color method of the renderer to adjust what the clear color is if needed.
So then transparency is something that can be set for a material as a whole, but it can also be set in at a texture level also when it comes to alpha maps. So then transparency can be something that can be very simple if we are taking the use of the basic material, and just setting the transparency of the whole material at a global level. However things can get a little involved when it comes to adding alpha maps to the mix, as that can lead to all kinds of additional things when it comes to sides, and directional light.
I still thing I need to work out some more examples on this topic, and also how to deal with various tings that will come up when working with light and various maps that are used in a material to skin a geometry that are used together for a single mesh object.