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Unity Engine

How Unity compiles branching shaders

Choose dynamic branching or shader variants.
読み終わるまでの所要時間 3 分最終更新 1ヶ月前
When you use a keyword, you can choose how Unity compiles the shader code. The options are:
  • Dynamic branching: Unity keeps branching code in one compiled shader program.
  • Shader variants: Unity compiles a separate shader program called a shader variant for each branch. This is a form of static branching, and can help minimize your shader code.

Dynamic branching

To use dynamic branching, use 
#pragma dynamic_branch
to declare keywords. For example:
#pragma dynamic_branch RED GREEN...if (RED){    // Output red}else if (GREEN){    // Output green}
At compile time, Unity converts each keyword into a uniform integer variable in the compiled shader code, which can have a value of either 
0
or 
1
. At runtime, for each draw call, Unity sends the state of each keyword integer to the GPU.
You can use 
dynamic_branch
if your shaders run on a fast GPU, and don't have asymmetric code branches where one branch is longer or more complex than the other.
重要
Use 
if
with 
dynamic_branch
keywords, not preprocessor directives such as 
#if
or 
#ifdef
. If you use 
#if
or 
#ifdef
, the statement always evaluates as false.

Shader variants

To use shader variants, use 
#pragma multi_compile
or 
#pragma shader_feature
to declare keywords. For example:
#pragma multi_compile RED GREENif (RED){    // Output red}else if (GREEN){    // Output green}
At compile time, Unity splits the previous code into 2 separate but similar shader files called shader variants. One file contains the code that outputs red, and the other contains the code that outputs green. Each draw call, Unity sends the GPU the correct shader variant.
During compilation of each shader variant, Unity does the following:
  • Sets an enabled keyword as 
    1
    in a preprocessor macro. For example 
    RED
    becomes 
    #define RED 1
    , so 
    if (RED)
    becomes 
    if (1)
    .
  • Sets a disabled keyword as 
    0
    in a uniform integer variable. For example 
    GREEN
    becomes 
    static const int GREEN = 0;
    , so 
    if (GREEN)
    becomes 
    if (0)
    and the compiler removes the branch.

Choosing a shader variant type

If you only need a shader to branch based on material properties, use 
shader_feature
. Unity compiles shader variants for keyword combinations that materials in your build use, and removes other shader variants. This keeps build times low and file sizes small.
注
At runtime, if you use a keyword combination you didn't use at build time, Unity tries to find the shader variant that closely matches. You can highlight missing shaders at runtime instead, or include a shader variant collection in the list of preloaded shaders.
If you need to use a C# script to make the shader branch at runtime, use 
multi_compile
. Unity compiles shader variants regardless of whether they're used by materials in your build. For example, you can use 
multi_compile
to give users dynamic control over whether fog appears in a game.
重要
If you use 
multi_compile
, Unity might spend a long time compiling, because each combination of keywords becomes a shader variant. For example, if you have 8 keyword sets with 3 keywords each, Unity might compile over 6,000 shader variants. Use 
shader_feature
instead, or use different ways to strip shader variants.

Additional resources