Shaders define both how an object looks by itself (its material properties) and how it reacts to the light. Because lighting calculations must be built into the shader, and there are many possible light & shadow types, writing quality shaders that “just work” would be an involved task. To make it easier, Unity has Surface Shaders, where all the lighting, shadowing, lightmapping, forward vs. deferred rendering things are taken care of automatically.
This document describes the pecularities of Unity’s lighting & rendering pipeline and what happens behind the scenes of Surface Shaders.
How lighting is applied and which Passes of the shader are used depends on which Rendering Path is used. Each pass in a shader communicates its lighting type via Pass Tags.
ForwardAddpasses are used.
Deferredpass is used.
PrepassFinalpasses are used.
VertexLMpasses are used.
ShadowCasterpass is used.
ForwardBase pass renders ambient, lightmaps, main directional light and not important (vertex/SH) lights at once.
ForwardAdd pass is used for any additive per-pixel lights; one invocation per object illuminated by such light is done. See Forward Rendering for details.
If forward rendering is used, but a shader does not have forward-suitable passes (i.e. neither
ForwardAdd pass types are present), then that object is rendered just like it would in Vertex Lit path, see below.
Deferred pass renders all information needed for lighting (in built-in shaders: diffuse color, specular color, smoothness,
world space normal, emission). It also adds lightmaps, reflection probes and ambient lighting into the emission channel. See Deferred Shading for details.
PrepassBase pass renders normals & specular exponent;
PrepassFinal pass renders final color by combining textures, lighting & emissive material properties. All regular in-scene lighting is done separately in screen-space. See Deferred Lighting for details.
Since vertex lighting is most often used on platforms that do not support programmable shaders, Unity can’t create multiple shader variants internally to handle lightmapped vs. non-lightmapped cases. So to handle lightmapped and non-lightmapped objects, multiple passes have to be written explicitly.
Vertexpass is used for non-lightmapped objects. All lights are rendered at once, using a fixed function OpenGL/Direct3D lighting model (Blinn-Phong)
VertexLMRGBMpass is used for lightmapped objects, when lightmaps are RGBM encoded (PC and consoles). No realtime lighting is applied; pass is expected to combine textures with a lightmap.
VertexLMMpass is used for lightmapped objects, when lightmaps are double-LDR encoded (mobile platforms). No realtime lighting is applied; pass is expected to combine textures with a lightmap.