Version: 2018.2 (switch to 2018.1 or 2017.4)
ShaderLab: UsePass
ShaderLab: SubShader Tags
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ShaderLab: GrabPass

GrabPass is a special pass type - it grabs the contents of the screen where the object is about to be drawn into a texture. This texture can be used in subsequent passes to do advanced image based effects.

Syntax

The GrabPass belongs inside a subshaderEach shader in Unity consists of a list of subshaders. When Unity has to display a mesh, it will find the shader to use, and pick the first subshader that runs on the user’s graphics card. More info
See in Glossary
. It can take two forms:

  • Just GrabPass { } grabs the current screen contents into a texture. The texture can be accessed in further passes by _GrabTexture name. Note: this form of grab pass will do the time-consuming screen grabbing operation for each object that uses it.
  • GrabPass { "TextureName" } grabs the current screen contents into a texture, but will only do that once per frame for the first object that uses the given texture name. The texture can be accessed in further passes by the given texture name. This is a more performant method when you have multiple objects using GrabPass in 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
    .

Additionally, GrabPass can use Name and TagsA reference word which you can assign to one or more GameObjects to help you identify GameObjects for scripting purposes. For example, you might define and “Edible” Tag for any item the player can eat in your game. More info
See in Glossary
commands.

Example

Here is an inefficient way to invert the colors of what was rendered before:

Shader "GrabPassInvert"
{
    SubShader
    {
        // Draw ourselves after all opaque geometry
        Tags { "Queue" = "Transparent" }

        // Grab the screen behind the object into _BackgroundTexture
        GrabPass
        {
            "_BackgroundTexture"
        }

        // Render the object with the texture generated above, and invert the colors
        Pass
        {
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #include "UnityCG.cginc"

            struct v2f
            {
                float4 grabPos : TEXCOORD0;
                float4 pos : SV_POSITION;
            };

            v2f vert(appdata_base v) {
                v2f o;
                // use UnityObjectToClipPos from UnityCG.cginc to calculate 
                // the clip-space of the vertex
                o.pos = UnityObjectToClipPos(v.vertex);
                // use ComputeGrabScreenPos function from UnityCG.cginc
                // to get the correct texture coordinate
                o.grabPos = ComputeGrabScreenPos(o.pos);
                return o;
            }

            sampler2D _BackgroundTexture;

            half4 frag(v2f i) : SV_Target
            {
                half4 bgcolor = tex2Dproj(_BackgroundTexture, i.grabPos);
                return 1 - bgcolor;
            }
            ENDCG
        }

    }
}

This 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
has two passes: The first pass grabs whatever is behind the object at the time of 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
, then applies that in the second pass. Note that the same effect could be achieved more efficiently using an invert blend mode.

See Also

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ShaderLab: UsePass
ShaderLab: SubShader Tags