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 > Lighting > Settings), click the Scene tab, navigate to Mixed Lighting and set the Lighting Mode to Subtractive. See documentation on Mixed lighting to learn more about this lighting mode, and see documentation on Light modes 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 lightmap, Unity cannot perform any direct lighting calculations at run time.
In Subtractive mode:
Static GameObjects 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 Probes.
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 Shader 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.
The following table shows how static and dynamic GameObjects cast and receive shadows when using Subtractive mode:
A dynamic GameObject that is receiving a shadow from another static or dynamic GameObject
A static GameObject that is receiving a shadow from another static or dynamic GameObject
|Within Shadow Distance||Beyond Shadow Distance||Within Shadow Distance||Beyond Shadow Distance|
A dynamic GameObject that is casting a shadow
|Shadow map||-||Main light shadow map||-|
A static GameObject that is casting a shadow
|Light Probes||Light Probes||Lightmap||Lightmap|
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:
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.
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).