Sobel Filter

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

[ExecuteInEditMode]
public class Sobel : MonoBehaviour
{

    public float threshold;
    private Material material;

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

    void OnRenderImage(RenderTexture source, RenderTexture destination) //Before the buffer is sent to the camera, modify it with above shader.
    {
        if (threshold >= 1.0)
        {
            Graphics.Blit(source, destination); //Intensity is 0. Blit the image straight to the screen.
            return;
        }
        if (threshold < 0.0)
            threshold = 0.0f;

        material.SetFloat("_ScreenWidth", Screen.width);
        material.SetFloat("_ScreenHeight", Screen.height);
        material.SetFloat("_Threshold", threshold); //Uniform the value to the shader.
        Graphics.Blit(source, destination, material); //Blit the modified image to the screen.
    }
}

////////////////////////////////////////////////
////////////////////GPU SIDE////////////////////
////////////////////////////////////////////////
Shader "My Shaders/Sobel"
{
    Properties
    {
        _MainTex ("Buffer", 2D) = "white" {}
        _Threshold ("Outline Threshold", Range (0, 1)) = 0
        _ScreenWidth ("Screen Width", float) = 0.0
        _ScreenHeight ("Screen Height", float) = 0.0
    }
    SubShader
    {
        Pass
        {
            CGPROGRAM
            #pragma vertex vert_img
            #pragma fragment frag

            #include "UnityCG.cginc"

            uniform sampler2D _MainTex;
            uniform float _Threshold;
            uniform float _ScreenWidth;
            uniform float _ScreenHeight;

            float4 frag(v2f_img input) : COLOR
            {
                float offsetX = 1.0 / _ScreenWidth;
                float offsetY = 1.0 / _ScreenHeight;

                float4 result = float4(0.0, 0.0, 0.0, 0.0);
                float4 sumX = float4(0.0, 0.0, 0.0, 0.0);
                float4 sumY = float4(0.0, 0.0, 0.0, 0.0);
                float sum = 0.0;
                float4 temp = float4(0.0, 0.0, 0.0, 0.0);

                temp = tex2D(_MainTex, input.uv + float2(-offsetX, -offsetY));
                sumX += temp * -3.0;

                temp = tex2D(_MainTex, input.uv + float2(offsetX, -offsetY));
                sumX += temp * 3.0;

                temp = tex2D(_MainTex, input.uv + float2(-offsetX, 0.0));
                sumX += temp * -10.0;

                temp = tex2D(_MainTex, input.uv + float2(-offsetX, 0.0));
                sumX += temp * 10.0;

                temp = tex2D(_MainTex, input.uv + float2(-offsetX, offsetY));
                sumX += temp * -3.0;

                temp = tex2D(_MainTex, input.uv + float2(offsetX, offsetY));
                sumX += temp * 3.0;


                temp = tex2D(_MainTex, input.uv + float2(-offsetX, -offsetY));
                sumY += temp * 3.0;

                temp = tex2D(_MainTex, input.uv + float2(0.0, -offsetY));
                sumY += temp * 10.0;

                temp = tex2D(_MainTex, input.uv + float2(offsetX, -offsetY));
                sumY += temp * 3.0;

                temp = tex2D(_MainTex, input.uv + float2(-offsetX, offsetY));
                sumY += temp * -3.0;

                temp = tex2D(_MainTex, input.uv + float2(0.0, offsetY));
                sumY += temp * -10.0;

                temp = tex2D(_MainTex, input.uv + float2(offsetX, offsetY));
                sumY += temp * -3.0;

                sum = sqrt((sumX * sumX) + (sumY * sumY));

                float4 textureMap = tex2D(_MainTex, input.uv);

                //Result of the Sobel Filter.
//              result = float4(sum, sum, sum, 1.0);

                if(sum > _Threshold)
                    result = float4(0.0, 0.0, 0.0, 1.0);
                else
                    result = float4(textureMap.rgb, 1.0);

                return result;
            }
            ENDCG
        }
    }
}