Writing Surface Shaders
Writing shaders that interact with lighting is complex. There are different light types, different shadow options, different rendering paths (forward and deferred rendering), and the shader should somehow handle all that complexity.
Surface Shaders in Unity is a code generation approach that makes it much easier to write lit shaders than using low level vertex/pixel shader programs. Note that there is no custom languages, magic or ninjas involved in Surface Shaders; it just generates all the repetitive code that would have to be written by hand. You still write shader code in Cg / HLSL.
For some examples, take a look at Surface Shader Examples and Surface Shader Custom Lighting Examples.
How it works
You define a "surface function" that takes any UVs or data you need as input, and fills in output structure SurfaceOutput. SurfaceOutput basically describes properties of the surface (it's albedo color, normal, emission, specularity etc.). You write this code in Cg / HLSL.
Surface Shader compiler then figures out what inputs are needed, what outputs are filled and so on, and generates actual vertex&pixel shaders, as well as rendering passes to handle forward and deferred rendering.
Standard output structure of surface shaders is this:
struct SurfaceOutput {
half3 Albedo;
half3 Normal;
half3 Emission;
half Specular;
half Gloss;
half Alpha;
};
Samples
See Surface Shader Examples and Surface Shader Custom Lighting Examples pages.
Surface Shader compile directives
Surface shader is placed inside CGPROGRAM..ENDCG block, just like any other shader. The differences are:
- It must be placed inside SubShader block, not inside Pass. Surface shader will compile into multiple passes itself.
- It uses
#pragma surface ...directive to indicate it's a surface shader.
The #pragma surface directive is:
#pragma surface surfaceFunction lightModel [optionalparams]
Required parameters:
- surfaceFunction - which Cg function has surface shader code. The function should have the form of
void surf (Input IN, inout SurfaceOutput o), where Input is a structure you have defined. Input should contain any texture coordinates and extra automatic variables needed by surface function. - lightModel - lighting model to use. Built-in ones are
Lambert(diffuse) andBlinnPhong(specular). See Custom Lighting Models page for how to write your own.
Optional parameters:
alpha- Alpha blending mode. Use this for semitransparent shaders.alphatest:VariableName- Alpha testing mode. Use this for transparent-cutout shaders. Cutoff value is in float variable with VariableName.vertex:VertexFunction- Custom vertex modification function. See Tree Bark shader for example.exclude_path:prepassorexclude_path:forward- Do not generate passes for given rendering path.addshadow- Add shadow caster & collector passes. Commonly used with custom vertex modification, so that shadow casting also gets any procedural vertex animation.dualforward- Use dual lightmaps in forward rendering path.fullforwardshadows- Support all shadow types in Forward rendering path.decal:add- Additive decal shader (e.g. terrain AddPass).decal:blend- Semitransparent decal shader.softvegetation- Makes the surface shader only be rendered when Soft Vegetation is on.noambient- Do not apply any ambient lighting or spherical harmonics lights.novertexlights- Do not apply any spherical harmonics or per-vertex lights in Forward rendering.nolightmap- Disables lightmap support in this shader (makes a shader smaller).noforwardadd- Disables Forward rendering additive pass. This makes the shader support one full directional light, with all other lights computed per-vertex/SH. Makes shaders smaller as well.approxview- Computes normalized view direction per-vertex instead of per-pixel, for shaders that need it. This is faster, but view direction is not entirely correct when camera gets close to surface.halfasview- Pass half-direction vector into the lighting function instead of view-direction. Half-direction will be computed and normalized per vertex. This is faster, but not entirely correct.
Additionally, you can write #pragma debug inside CGPROGRAM block, and then surface compiler will spit out a lot of comments of the generated code. You can view that using Open Compiled Shader in shader inspector.
Surface Shader input structure
The input structure Input generally has any texture coordinates needed by the shader. Texture coordinates must be named "uv" followed by texture name (or start it with "uv2" to use second texture coordinate set).
Additional values that can be put into Input structure:
float3 viewDir- will contain view direction, for computing Parallax effects, rim lighting etc.float4withCOLORsemantic - will contain interpolated per-vertex color.float4 screenPos- will contain screen space position for reflection effects. Used by WetStreet shader in Dark Unity for example.float3 worldPos- will contain world space position.float3 worldRefl- will contain world reflection vector if surface shader does not write to o.Normal. See Reflect-Diffuse shader for example.float3 worldNormal- will contain world normal vector if surface shader does not write to o.Normal.float3 worldRefl; INTERNAL_DATA- will contain world reflection vector if surface shader writes to o.Normal. To get the reflection vector based on per-pixel normal map, useWorldReflectionVector (IN, o.Normal). See Reflect-Bumped shader for example.float3 worldNormal; INTERNAL_DATA- will contain world normal vector if surface shader writes to o.Normal. To get the normal vector based on per-pixel normal map, useWorldNormalVector (IN, o.Normal).