Surface Shaders and rendering paths

In the Built-in Render Pipeline, when using a Surface ShaderUnity’s code generation approach that makes it much easier to write lit shaders than using low level vertex/pixel shader programs. More info
See in Glossary, how lighting is applied and which Passes of the shaderA small script that contains the mathematical calculations and algorithms for calculating the Color of each pixel rendered, based on the lighting input and the Material configuration. More info
See in Glossary are used depends on which rendering pathThe technique Unity uses to render graphics. Choosing a different path affects the performance of your game, and how lighting and shading are calculated. Some paths are more suited to different platforms and hardware than others. More info
See in Glossary is used. Each pass in a shader communicates its lighting type via Pass Tags.

• In Forward RenderingA rendering path that renders each object in one or more passes, depending on lights that affect the object. Lights themselves are also treated differently by Forward Rendering, depending on their settings and intensity. More info
  See in Glossary, ForwardBase and ForwardAdd passes are used.
• In Deferred ShadingA rendering path in the Built-in Render Pipeline that places no limit on the number of Lights that can affect a GameObject. All Lights are evaluated per-pixel, which means that they all interact correctly with normal maps and so on. Additionally, all Lights can have cookies and shadows. More info
  See in Glossary, Deferred pass is used.
• In legacy Deferred Lighting, PrepassBase and PrepassFinal passes are used.
• In legacy Vertex Lit, Vertex, VertexLMRGBM and VertexLM passes are used.
• In any of the above, to render ShadowsA UI component that adds a simple outline effect to graphic components such as Text or Image. It must be on the same GameObject as the graphic component. More info
  See in Glossary or a depth texture, ShadowCaster pass is used.

Forward RenderingThe process of drawing graphics to the screen (or to a render texture). By default, the main camera in Unity renders its view to the screen. More info
See in Glossary path

ForwardBase pass renders ambient, lightmapsA pre-rendered texture that contains the effects of light sources on static objects in the scene. Lightmaps are overlaid on top of scene geometry to create the effect of lighting. More info
See in Glossary, 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 ForwardBase nor ForwardAdd pass types are present), then that object is rendered just like it would in Vertex Lit path, see below.

Deferred Shading path

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.

Legacy Deferred Lighting path

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.

Legacy Vertex Lit Rendering path

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.

• Vertex pass is used for non-lightmapped objects. All lights are rendered at once, using a fixed function OpenGL/Direct3D lighting model (Blinn-Phong)
• VertexLMRGBM pass 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.
• VertexLM pass 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.