Greyscale Filter

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////////////////////CPU SIDE////////////////////
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using UnityEngine;
using System.Collections;

[ExecuteInEditMode]
public class Greyscale : MonoBehaviour
{

    public float intensity;
    private Material material;

    void Awake()
    {
        material = new Material(Shader.Find("My Shaders/Greyscale")); //Use the shader.
    }

    void OnRenderImage(RenderTexture source, RenderTexture destination) //Before the buffer is sent to the camera, modify it with above shader.
    {
        if (intensity == 0)
        {
            Graphics.Blit(source, destination); //Intensity is 0. Blit the image straight to the screen.
            return;
        }

        material.SetFloat("_Scale", intensity); //Uniform the value to the shader.
        Graphics.Blit(source, destination, material); //Blit the modified image to the screen.
    }
}

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////////////////////GPU SIDE////////////////////
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Shader "My Shaders/Greyscale"
{
    Properties
    {
        _MainTex ("Buffer", 2D) = "white" {}
        _Scale ("Intensity", Range (0, 1)) = 0
    }
    SubShader
    {
        Pass
        {
            CGPROGRAM
            #pragma vertex vert_img //Standard vertex shader as we don't need to make a custom one.
            #pragma fragment frag

            #include "UnityCG.cginc"

            uniform sampler2D _MainTex;
            uniform float _Scale;

            float4 frag(v2f_img input) : COLOR
            {
                float4 textureMap = tex2D(_MainTex, input.uv);

                float lum = textureMap.r * 0.2126 + textureMap.g * 0.7152 + textureMap.b * 0.0722; //Standard values for HDTV greyscale calibration. It is important that each channel is modified by a seperate amount due to luminosity.
                float3 greyscale = float3(lum, lum, lum); //A vec3 result of the above.

                float4 result = textureMap;
                result.rgb = lerp(textureMap.rgb, greyscale, _Scale); //Mix between the original buffer and the greyscale modification based off the weight of _Scale.
                return result;
            }
            ENDCG
        }
    }
}