Subtractive mode

Subtractive mode is a lighting mode shared by all Mixed Lights in a Scene. To set Mixed lighting to Subtractive, open the Lighting window (menu: Window > 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 > Lighting Settings), click the SceneA Scene contains the environments and menus of your game. Think of each unique Scene file as a unique level. In each Scene, you place your environments, obstacles, and decorations, essentially designing and building your game in pieces. More info
See in Glossary tab, navigate to Mixed LightingA Light Mode for creating indirect lighting, shadowmasks and subtractive lighting. Indirect lighting gets baked into lightmaps and light probes. Shadowmasks and light probes occlusion get generated for baked shadows. More info
See in Glossary and set the Lighting Mode to Subtractive. See documentation on Mixed lighting to learn more about this lighting mode, and see documentation on Light modesA Light property that defines the use of the Light. Can be set to Realtime, Baked and Mixed. More info
See in Glossary to learn more about the other modes available.

Subtractive is the only Mixed lighting mode that bakes direct lighting into the light map, and discards the information that Unity uses to composite dynamic and static shadows in other Mixed lighting modes. Because the Light is already baked into the lightmapA 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, Unity cannot perform any direct lighting calculations at run time.

In Subtractive mode:

• Static GameObjectsThe fundamental object in Unity scenes, which can represent characters, props, scenery, cameras, waypoints, and more. A GameObject’s functionality is defined by the Components attached to it. More info
  See in Glossary do not show any specular or glossy highlights at all from Mixed Lights. They also cannot receive any shadows from dynamic GameObjects, except for the main Directional Light (see paragraph below for more information on this).

• Dynamic GameObjects receive real-time lighting, and support glossy reflections. However, they can only receive shadows from static GameObjects via Light ProbesLight probes store information about how light passes through space in your scene. A collection of light probes arranged within a given space can improve lighting on moving objects and static LOD scenery within that space. More info
  See in Glossary.

In Subtractive mode, the main Directional Light (which is usually the sun) is the only light source which casts real-time shadows from dynamic GameObjects onto static GameObjects. Shadows cast from static GameObjects onto other static GameObjects are baked into the lightmap, even for the main Light, so Unity cannot guarantee correct composition of baked and real-time shadows. Subtractive mode therefore has a Realtime Shadow Color field. Unity uses this color in 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 to composite real-time shadows with baked shadows. To do this, it reduces the effect of the light map in areas shadowed by dynamic GameObjects. Because there is no correct value that the engine can predetermine, choosing a value that works for any given Scene is down to your own artistic choice.

A good example of when Subtractive mode might be useful is when you are building a cel-shaded (that is, cartoon-style) game with outside levels and very few dynamic GameObjects.

Shadows

The following table shows how static and dynamic GameObjects cast and receive shadows when using Subtractive mode:

Advantages and disadvantages of Subtractive mode

The performance requirements of Subtractive mode make it a good option for building to low-end mobile devices. These are the most significant advantages and disadvantages of using Subtractive mode:

Advantages

• It provides high-quality shadows between static GameObjects in lightmaps at no additional performance requirement.

• One Texture operation in the Shader handles all lighting and shadows between static GameObjects.

• It provides indirect lighting.

Disadvantages

• It does not provide real-time direct lighting, and therefore does not provide specular lighting.

• It does not provide dynamic shadows on static GameObjects, except for one Directional Light (the main Light).

• It only provides low-resolution shadows from static GameObjects onto dynamic GameObjects, via Light Probes.

• It provides inaccurate composition of dynamic and static shadows.

• It has increased memory requirements for the light map texture set (compared to no lightmaps).

• 2017–06–08 Page published with limited editorial review

• Light Modes added in 5.6