This article has been written to help you understand how things are drawn on screen, and how transparency is handled in BabylonJS. Feel free to contribute in whatever way seems fit to you!
A general rule of thumb in real-time 3D rendering is that drawing several overlapping opaque objects is always easier than non-opaque ones. This is due to the fact that opaque objects are compatible with the use of a depth buffer, and thus no specific order is required when rendering them.
A depth buffer is a surface using the same dimensions as the screen, and holding for every pixel the following information: how far from the camera was the last pixel drawn here. With this information, we can draw as many objects as we want and always be sure that we will never draw something that was supposed to be hidden by another object. BabylonJS offers access to this information with a special DepthRenderer object.
Rendering objects without a depth buffer would require resorting to an old-school technique called Painter's Algorithm, which is extremely simple: draw further objects first. Sky, then backdrop, etc. all the way to foreground objects. This is basically ordering objects by distance from camera (a.k.a. depth), and clearly not enough for most cases.
Testing against a depth buffer during render is a very common technique, simple to implement and performance-inexpensive. However, things get more complicated for non-opaque objects, as a depth buffer can't be used anymore (since these objects don't completely hide what's behind them).
This is what a depth buffer looks like for a scene which contains only opaque meshes:
Before actually drawing meshes on the screen, BabylonJS puts them in the following categories, which are presented in the order in which they are drawn:
Depth pre-pass meshes
Alpha tested meshes
Alpha blended meshes, sorted by depth (= distance to camera)
Sprites (handled by SpriteManager)
Particles (handled by ParticleSystem)
Categories 5 and 6 are self-explanatory. Note that they are always drawn after all the other meshes, and that they do not use any depth buffer that may have been previously created. These simply cannot be hidden by regular meshes.
Renderable objects can be organized into rendering groups, which act as layers. In each of these rendering groups, the order above will be used.
To use rendering groups, you simply need to set the property
.renderingGroupId on the objects you want to put in other layers than the default one (which has the ID of 0).
This property exists on meshes, particle systems and sprite managers.
Rendering groups are rendered by ascending ID, starting with the default one. There can be no more than 4 rendering groups in total, meaning that the only valid IDs are 0, 1, 2 and 3.
Meshes have another property that has an influence on the rendering order:
By default, this property is set to
Number.MAX_VALUE, which is the highest value that a numerical variable can hold (around 1.79E+308).
Unlike opaque and alpha-tested meshes, the BabylonJS rendering engine sorts alpha-blended meshes by depth before drawing them on screen (see below). The
.alphaIndex property allows you to override this sorting, as one mesh which has a lower alpha index than another will always be rendered before it, regardless of their respective depth.
To phrase it more simply: alpha-blended are sorted first by alpha index, and then by depth (distance to camera).
Keep in mind that this property works only for alpha-blended mesh, and has absolutely no effect for opaque and alpha-tested ones.
Note: this property can be manually set on meshes in 3DS Max with the BabylonJS exporter plugin.
How your meshes are categorized may be very important for the final aspect of your scene. Let's take a closer look at the way categories 1 to 4 are defined.
The depth pre-pass meshes define an additional rendering pass for meshes. During this pass, meshes are only rendered to depth buffer. The depth pre-pass meshes are not exclusive. A mesh can have a depth pre-pass and an opaque or alpha blend pass. The enable the depth pre-pass for a mesh, jsut call
mesh.material.needDepthPrePass = true.
The goal is either to optimize the scene by rendering meshes to the depth buffer to reduce overdraw or to help reducing alpha blending sorting issues.
These will be the easiest to render: their polygons are fully drawn on screen with their colors and textures. A depth buffer will be used to make sure nothing is drawn over something that is closer to the camera.
Same as opaque meshes, except that some parts of these meshes can be defined as completely transparent. Alpha test means that each pixel of the mesh can be either opaque (and then drawn on screen and in the depth buffer) or transparent, which means the pixel is completely discarded. Although very efficient, this type of render usually produces aliased borders and does not allow for smooth transparency effects.
A pixel is considered transparent if its alpha value is < 0.4, and opaque if not. This value is currently hardcoded.
These meshes have translucent parts that may have an alpha value of 0.0 (completely transparent) to 1.0 (completely opaque). Their color is blended with what's behind them to reflect that. These meshes are sorted by depth, based on the center of their bounding sphere. This does not prevent some problems when several alpha-blended meshes overlap.
Also, note that backface culling is pretty much obligatory for alpha blended meshes, otherwise polygons from the front and the back of the objects will be garbled (unless you use a depth pre-pass)
This is what a depth buffer looks like for a scene that contains each of those type of meshes:
In this scene, the sphere is alpha tested, the base blocks are opaque and the pillars are alpha blended.
The following list will help you understand which categories your meshes will be put into. For more information on each of the properties mentioned here, take a look at the Materials Tutorial.
Alpha blended meshes:
hasVertexAlphaset to true (automatically set for exported meshes if vertices have individual alpha (transparency) values)
.visibilityvalue < 1
Alpha tested meshes:
Occasionally, you may have some of your meshes falling into the wrong category, e.g. an alpha tested mesh unnecessarily marked as alpha blended, or a mesh staying opaque when it shouldn't. This will give you weird glitches, which can sometimes be very annoying. You should refer to this article to check how your meshes and materials properties are set.
You're welcome to use this playground example -
engine = new BABYLON.Engine(canvas, false); // built-in smoothing will be disabled
This may help you with visible seams between meshes and other similar issues.
needDepthPrePassto help fixing issues with self transparency.
separateCullingPasson materials to force the engine to render the transparent objects in 2 passes: first the back faces and then the front faces. This can help a lot with self transparency.
The transparent concave meshes render obvisouly with the same rules than explained before : https://www.babylonjs-playground.com/#1PLV5Z -
BABYLON.Mesh.DOUBLESIDE: https://www.babylonjs-playground.com/#1PLV5Z#2 -
(to be expanded)