ShaderLab syntax: SubShader Tags

Subshaders use tags to tell how and when they expect to be rendered to the rendering engine.

Syntax

Tags { "TagName1" = "Value1" "TagName2" = "Value2" }: Specifies TagName1 to have Value1, TagName2 to have Value2. You can have as many tags as you like.

Details

Tags are basically key-value pairs. Inside a SubShader tags are used to determine rendering order and other parameters of a subshader. Note that the following tags recognized by Unity must be inside SubShader section and not inside Pass!

Rendering Order - Queue tag

You can determine in which order your objects are drawn using the Queue tag. A Shader decides which render queue its objects belong to, this way any Transparent shaders make sure they are drawn after all opaque objects and so on.

There are four pre-defined render queues, but there can be more queues in between the predefined ones. The predefined queues are:

Background - this render queue is rendered before any others. It is used for skyboxes and the like.
Geometry (default) - this is used for most objects. Opaque geometry uses this queue.
AlphaTest - alpha tested geometry uses this queue. It's a separate queue from Geometry one since it's more efficient to render alpha-tested objects after all solid ones are drawn.
Transparent - this render queue is rendered after Geometry and AlphaTest, in back-to-front order. Anything alpha-blended (i.e. shaders that don't write to depth buffer) should go here (glass, particle effects).
Overlay - this render queue is meant for overlay effects. Anything rendered last should go here (e.g. lens flares).

Shader "Transparent Queue Example" {
     SubShader {
        Tags {"Queue" = "Transparent" }
        Pass {
            // rest of the shader body...
        }
    }
}

An example illustrating how to render something in the transparent queue

Geometry render queue optimizes the drawing order of the objects for best performance. All other render queues sort objects by distance, starting rendering from the furthest ones and ending with the closest ones.

For special uses in-between queues can be used. Internally each queue is represented by integer index; Background is 1000, Geometry is 2000, AlphaTest is 2450, Transparent is 3000 and Overlay is 4000. If a shader uses a queue like this:

Tags { "Queue" = "Geometry+1" }

This will make the object be rendered after all opaque objects, but before transparent objects, as render queue index will be 2001 (geometry plus one). This is useful in situations where you want some objects be always drawn between other sets of objects. For example, in most cases transparent water should be drawn after opaque objects but before transparent objects.

RenderType tag

RenderType tag categorizes shaders into several predefined groups, e.g. is is an opaque shader, or an alpha-tested shader etc. This is used by Shader Replacement and in some cases used to produce camera's depth texture.

ForceNoShadowCasting tag

If ForceNoShadowCasting tag is given and has a value of "True", then an object that is rendered using this subshader will never cast shadows. This is mostly useful when you are using shader replacement on transparent objects and you do not wont to inherit a shadow pass from another subshader.

IgnoreProjector tag

If IgnoreProjector tag is given and has a value of "True", then an object that uses this shader will not be affected by Projectors. This is mostly useful on semitransparent objects, because there is no good way for Projectors to affect them.