December 2008 Newsletter

Alpha Blending is the process used to render transparent surfaces in 3D scenes. This article will cover how textures affect the engines' rendering process and how to optimize textures to increase rendering performance.

HTML clipboardThere are two ways which textures can be used to generate transparency. Firstly, a texture can contain a pre-generated alpha channel. This extra channel stores the transparency of the pixel color (255 being opaque and 0 being transparent). Generally, if the alpha channel is required, the texture will be prefixed with "a_" in order to give a hint to the engine. The other method is to use masking. If the "m_" prefix is used; the engine will remove every pixel that is completely black (RGB: 0, 0, 0).
This article uses diagnostic shaders in order to find out information about each object.

You can obtain them: here.
 
Rendering Modes
 
The engine renders objects in a number of different passes. The three most important, and configurable, are:
 
Normal - An object is rendered conventionally, and is completely opaque (Figure 1.1).
Batched - More than one instance of this object exists. The geometry is drawn in batches to maintain GPU speed (Figure 1.2).
Transparent - Similar to normal, but the object is transparent (Figure 1.3).
 
The transparent pass is very different to all others. Every object which is to be rendered in this pass has to be sorted by depth; this is because when the blending of the transparent pixel occurs, it must know the color of the pixel behind it. If the objects are drawn in the wrong order; they will blend incorrectly leading to some very odd results. Any object with a transparent surface cannot be batched; this means that placing a number of trees will affect the game performance; we need to have as many objects as possible drawn in the first two passes.
 
 
 
 
Alpha Testing with Masks
 
If you have an object which genuinely needs transparency, such as a window, then there is little you can do about its' performance impact. However, if you have trees or any object which has pixels that are either fully opaque or fully transparent (and nowhere in between), then there is a different method we can use to help us.
 
Alpha testing is the process of discarding pixels which are not opaque. The alpha object can go through the opaque rendering pass and will have its' transparent parts removed when the GPU finishes rendering.
 
In the case of our tree in figure 1.3, we can see that the leaves are rendered in the transparent pass, and its trunk is rendered in the 'normal' pass. If we place more than one; we cannot take advantage of batching due to sorting (Figure 2.1).
 
 
In this case, the tree leaf texture is masked. This is determined with the "m_" prefix (Figure 2.2). If we remove the prefix, then the engine assumes that it is an opaque texture; causing it to be rendered the normal and batch passes!
 
 
We are missing something though; the texture is now just colored black. We need to create an alpha channel for this object to store the mask that the engine used to generate. As long as we do not prefix the texture filename with "a_" or "m_", the engine will still think that it is opaque, regardless of the alpha channels' presence.
 
You can use your favorite graphics software to generate the new texture from the old. Here are some tutorials for various packages:
 
 
 
The resulting texture will render as our masked texture used to. However, it is now being rendered in a different pass as the engine thinks that it is a solid object (Figure 2.4). Reducing the amount of blending used in a scene can help your games' FPS by quite a lot.
 
 
Converting from Realm Crafter Standard
 
If you're using an old Standard project; you shouldn't have any real issues as far as rendering is concerned. However, following the above advice is still the same. If you can edit any masked meshes so that they rendering in the normal scene pass instead of the transparent pass then you should do so.