Legacy Documentation: Version 2017.1 (Go to current version)
ShaderLab: UsePass
ShaderLab: SubShader Tags
Other Versions

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.


The GrabPass belongs inside a subshader. 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 scene.

Additionally, GrabPass can use Name and Tags commands.


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

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

        // Grab the screen behind the object into _BackgroundTexture

        // Render the object with the texture generated above, and invert the colors
            #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;


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

See Also

Did you find this page useful? Please give it a rating:

ShaderLab: UsePass
ShaderLab: SubShader Tags
Copyright © 2020 Unity Technologies
优美缔软件(上海)有限公司 版权所有
"Unity"、Unity 徽标及其他 Unity 商标是 Unity Technologies 或其附属机构在美国及其他地区的商标或注册商标。其他名称或品牌是其各自所有者的商标。