Cg Programming/Unity/Transparency

This tutorial covers blending of fragments (i.e. compositing them) using Cg shaders in Unity. It assumes that you are familiar with the concept of front and back faces as discussed in.

More specifically, this tutorial is about rendering transparent objects, e.g. transparent glass, plastic, fabrics, etc. (More strictly speaking, these are actually semitransparent objects because they don't need to be perfectly transparent.) Transparent objects allow us to see through them; thus, their color “blends” with the color of whatever is behind them.

Blending
As mentioned in, the fragment shader computes an RGBA color (i.e. red, green, blue, and alpha components in the fragment output parameter with semantic ) for each fragment (unless the fragment is discarded). The fragments are then processed as discussed in. One of the operations is the blending stage, which combines the color of the fragment (as specified in the fragment output parameter), which is called the “source color”, with the color of the corresponding pixel that is already in the framebuffer, which is called the “destination color” (because the “destination” of the resulting blended color is the framebuffer).

Blending is a fixed-function stage, i.e. you can configure it but not program it. The way it is configured, is by specifying a blend equation. You can think of the blend equation as this definition of the resulting RGBA color:

where  is the RGBA color computed by the fragment shader and   is the RGBA color that is currently in the framebuffer and   is the blended result, i.e. the output of the blending stage. and  are configurable RGBA colors (of type  ) that are multiplied component-wise with the fragment output color and the pixel color. The values of  and   are specified in Unity's ShaderLab syntax with this line:

{code for } {code for  }

The most common codes for the two factors are summarized in the following table (more codes are mentioned in Unity's ShaderLab reference about blending):

As discussed in,  is just a short way of writing. Also note that all components of all colors and factors in the blend equation are clamped between 0 and 1.

Alpha Blending
One specific example for a blend equation is called “alpha blending”. In Unity, it is specified this way:

which corresponds to:

This uses the alpha component of  as an opacity. I.e. the more opaque the fragment output color is, the larger its opacity and therefore its alpha component, and thus the more of the fragment output color is mixed in the result and the less of the pixel color in the framebuffer. A perfectly opaque fragment output color (i.e. with an alpha component of 1) will completely replace the pixel color.

This blend equation is sometimes referred to as an “over” operation, i.e. “ over  ”, since it corresponds to putting a layer of the fragment output color with a specific opacity on top of the pixel color. (Think of a layer of colored glass or colored semitransparent plastic on top of something of another color.)

Due to the popularity of alpha blending, the alpha component of a color is often called opacity even if alpha blending is not employed. Moreover, note that in computer graphics a common formal definition of transparency is 1 − opacity.

Premultiplied Alpha Blending
There is an important variant of alpha blending: sometimes the fragment output color has its alpha component already premultiplied to the color components. (You might think of it as a price that has VAT already included.) In this case, alpha must not be multiplied again to the colors (corresponding to a rule that VAT must not be added more than once) and the correct blending is:

which corresponds to:

Two common mistakes in computer graphics are 1) to use the blend equation for standard alpha blending with premultiplied colors and 2) to use the blend equation for premultiplied alpha blending with colors that are not premultiplied. The visual errors caused by these mistakes can be quite subtle; usually they show up as either darkened or lightened silhouettes of blended objects. Thus, whenever you program a blend equation, make sure that you know whether you are dealing with premultiplied colors or not.

Additive Blending
Another example for a blend equation is:

This corresponds to:

which just adds the fragment output color to the color in the framebuffer. Note that the alpha component is not used at all; nonetheless, this blend equation is very useful for many kinds of transparent effects; for example, it is often used for particle systems when they represent fire or something else that is transparent and emits light. Additive blending is discussed in more detail in.

More examples of blend equations are given in Unity's ShaderLab reference about blending.

Shader Code
Here is a simple shader which uses alpha blending to render a green color with opacity 0.3:

Apart from the blend equation, which has been discussed above, there are only two lines that need more explanation:  and.

deactivates writing to the depth buffer. As explained in, the depth buffer keeps the depth of the nearest fragment and discards any fragments that have a larger depth. In the case of a transparent fragment, however, this is not what we want since we can (at least potentially) see through a transparent fragment. Thus, transparent fragments should not occlude other fragments and therefore the writing to the depth buffer is deactivated. See also Unity's ShaderLab reference about culling and depth testing.

The line  specifies that the meshes using this subshader are rendered after all the opaque meshes were rendered. The reason is partly because we deactivate writing to the depth buffer: one consequence is that transparent fragments can be occluded by opaque fragments even though the opaque fragments are farther away. In order to fix this problem, we first draw all opaque meshes (in Unity´s “opaque queue”) before drawing all transparent meshes (in Unity's “transparent queue”). Whether or not a mesh is considered opaque or transparent depends on the tags of its subshader as specified with the line. More details about subshader tags are described in Unity's ShaderLab reference about subshader tags.

It should be mentioned that this strategy of rendering transparent meshes with deactivated writing to the depth buffer does not always solve all problems. It works perfectly if the order in which fragments are blended does not matter; for example, if the fragment color is just added to the pixel color in the framebuffer, the order in which fragments are blended is not important; see. However, for other blending equations, e.g. alpha blending, the result will be different depending on the order in which fragments are blended. (If you look through almost opaque green glass at almost opaque red glass you will mainly see green, while you will mainly see red if you look through almost opaque red glass at almost opaque green glass. Similarly, blending almost opaque green color over almost opaque red color will be different from blending almost opaque red color over almost opaque green color.) In order to avoid artifacts, it is therefore advisable to use additive blending or (premultiplied) alpha blending with small opacities (in which case the destination factor  is close to 1 and therefore alpha blending is close to additive blending).

Including Back Faces
The previous shader works well with other objects but it actually doesn't render the “inside” of the object. However, since we can see through the outside of a transparent object, we should also render the inside. As discussed in, the inside can be rendered by deactivating culling with. However, if we just deactivate culling, we might get in trouble: as discussed above, it often matters in which order transparent fragments are rendered but without any culling, overlapping triangles from the inside and the outside might be rendered in a random order which can lead to annoying rendering artifacts. Thus, we would like to make sure that the inside (which is usually farther away) is rendered first before the outside is rendered. In Unity's ShaderLab this is achieved by specifying two passes, which are executed for the same mesh in the order in which they are defined:

In this shader, the first pass uses front-face culling (with ) to render the back faces (the inside) first. After that the second pass uses back-face culling (with ) to render the front faces (the outside). This works perfect for convex meshes (closed meshes without dents; e.g. spheres or cubes) and is often a good approximation for other meshes.

Summary
Congratulations, you made it through this tutorial! One interesting thing about rendering transparent objects is that it isn't just about blending but also requires knowledge about culling and the depth buffer. Specifically, we have looked at:
 * What blending is and how it is specified in Unity.
 * How a scene with transparent and opaque objects is rendered and how objects are classified as transparent or opaque in Unity.
 * How to render the inside and outside of a transparent object, in particular how to specify two passes in Unity.