2D Mirror/Water reflections (Shader and script by Hedgefield)

Note: this is a shader and script combo to allow 2D water/mirror reflections in your game. (Why it's here? because it's great! And was made by a member of the AC community!). This was originally intended for water-like reflections but if you set the resolution higher in the shader you will get a mirror effect instead of a water reflection effect.

> This solution consists of two components: a shader for the groundwork, and a script to render it on a 2D plane (or quad in this case).
>
>
> The shader has a texture slot and a color picker. The texture is optional, you can just change the water color using the color picker.
>
>
>
> The script has settings for the reflection resolution, offset, and which layers should be reflected. I keep the resolution intentionally low (256) because otherwise the reflection is too perfect and it doesn't look like water anymore. And I like the way it shimmers when you walk.
>
>

If you use this for mirrors it's also good to note the following:

> Make sure you rotate/move your mirror slightly, otherwise the reflection is completely hidden behind the character which might at first make you think it's not actually there. - Rem
>
>

Shader Code:

```
Shader "FX/Mirror"
{
	Properties
	{
		_MainTex ("Sprite Texture", 2D) = "white" {}
		_Color ("Tint", Color) = (1,1,1,1)
		[HideInInspector] _ReflectionTex ("", 2D) = "white" {}
	}
	SubShader
	{
		Tags  { "RenderType"="Opaque" }
		LOD 100
		Pass
		{
			CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag
			#include "UnityCG.cginc"

			struct appdata_t
			{
				float2 uv : TEXCOORD0;
				float4 refl : TEXCOORD1;
				float4 pos : POSITION;
				float4 col: COLOR;
			};
			struct v2f
			{
				half2 uv : TEXCOORD0;
				float4 refl : TEXCOORD1;
				float4 pos : SV_POSITION;
				fixed4 col: COLOR;
			};
			float4 _MainTex_ST;
			fixed4 _Color;
			v2f vert(appdata_t i)
			{
				v2f o;
				o.pos = mul (UNITY_MATRIX_MVP, i.pos);
				o.uv = TRANSFORM_TEX(i.uv, _MainTex);
				o.col = i.col * _Color;
				o.refl = ComputeScreenPos (o.pos);
				return o;
			}
			sampler2D _MainTex;
			sampler2D _ReflectionTex;
			fixed4 frag(v2f i) : SV_Target
			{
				fixed4 tex = tex2D(_MainTex, i.uv) * i.col;
				tex.rgb *= tex.a;
				fixed4 refl = tex2Dproj(_ReflectionTex, UNITY_PROJ_COORD(i.refl));
				return tex * refl;
			}
			ENDCG
	    }
	}
}

```

Script code:

```
using UnityEngine;
using System.Collections;

// This is in fact just the Water script from Pro Standard Assets,
// just with refraction stuff removed.

[ExecuteInEditMode] // Make mirror live-update even when not in play mode
public class MirrorReflection : MonoBehaviour
{
	public bool m_DisablePixelLights = true;
	public int m_TextureSize = 256;
	public float m_ClipPlaneOffset = 0.07f;

	public LayerMask m_ReflectLayers = -1;

	private Hashtable m_ReflectionCameras = new Hashtable(); // Camera -> Camera table

	private RenderTexture m_ReflectionTexture = null;
	private int m_OldReflectionTextureSize = 0;

	private static bool s_InsideRendering = false;

	// This is called when it's known that the object will be rendered by some
	// camera. We render reflections and do other updates here.
	// Because the script executes in edit mode, reflections for the scene view
	// camera will just work!
	public void OnWillRenderObject()
	{
		var rend = GetComponent<Renderer>();
			return;

		Camera cam = Camera.current;
		if( !cam )
			return;

		// Safeguard from recursive reflections.
		if( s_InsideRendering )
			return;
		s_InsideRendering = true;

		Camera reflectionCamera;
		CreateMirrorObjects( cam, out reflectionCamera );

		// find out the reflection plane: position and normal in world space
		Vector3 pos = transform.position;
		Vector3 normal = transform.up;

		// Optionally disable pixel lights for reflection
		int oldPixelLightCount = QualitySettings.pixelLightCount;
		if( m_DisablePixelLights )
			QualitySettings.pixelLightCount = 0;

		UpdateCameraModes( cam, reflectionCamera );

		// Render reflection
		// Reflect camera around reflection plane
		float d = -Vector3.Dot (normal, pos) - m_ClipPlaneOffset;
		Vector4 reflectionPlane = new Vector4 (normal.x, normal.y, normal.z, d);

		Matrix4x4 reflection = Matrix4x4.zero;
		CalculateReflectionMatrix (ref reflection, reflectionPlane);
		Vector3 oldpos = cam.transform.position;
		Vector3 newpos = reflection.MultiplyPoint( oldpos );
		reflectionCamera.worldToCameraMatrix = cam.worldToCameraMatrix * reflection;

		// Setup oblique projection matrix so that near plane is our reflection
		// plane. This way we clip everything below/above it for free.
		Vector4 clipPlane = CameraSpacePlane( reflectionCamera, pos, normal, 1.0f );
		//Matrix4x4 projection = cam.projectionMatrix;
		Matrix4x4 projection = cam.CalculateObliqueMatrix(clipPlane);
		reflectionCamera.projectionMatrix = projection;

		reflectionCamera.cullingMask = ~(1<<4) & m_ReflectLayers.value; // never render water layer
		reflectionCamera.targetTexture = m_ReflectionTexture;
		GL.invertCulling = true;
		reflectionCamera.transform.position = newpos;
		Vector3 euler = cam.transform.eulerAngles;
		reflectionCamera.transform.eulerAngles = new Vector3(0, euler.y, euler.z);
		reflectionCamera.Render();
		reflectionCamera.transform.position = oldpos;
		GL.invertCulling = false;
		Material[] materials = rend.sharedMaterials;
		foreach( Material mat in materials ) {
			if( mat.HasProperty("_ReflectionTex") )
				mat.SetTexture( "_ReflectionTex", m_ReflectionTexture );
		}

		// Restore pixel light count
		if( m_DisablePixelLights )
			QualitySettings.pixelLightCount = oldPixelLightCount;

		s_InsideRendering = false;
	}

	// Cleanup all the objects we possibly have created
	void OnDisable()
	{
		if( m_ReflectionTexture ) {
			DestroyImmediate( m_ReflectionTexture );
			m_ReflectionTexture = null;
		}
		foreach( DictionaryEntry kvp in m_ReflectionCameras )
			DestroyImmediate( ((Camera)kvp.Value).gameObject );
		m_ReflectionCameras.Clear();
	}

	private void UpdateCameraModes( Camera src, Camera dest )
	{
		if( dest == null )
			return;
		// set camera to clear the same way as current camera
		dest.clearFlags = src.clearFlags;
		dest.backgroundColor = src.backgroundColor;
		if( src.clearFlags == CameraClearFlags.Skybox )
		{
			Skybox sky = src.GetComponent(typeof(Skybox)) as Skybox;
			Skybox mysky = dest.GetComponent(typeof(Skybox)) as Skybox;
			if( !sky || !sky.material )
			{
				mysky.enabled = false;
			}
			else
			{
				mysky.enabled = true;
				mysky.material = sky.material;
			}
		}
		// update other values to match current camera.
		// even if we are supplying custom camera&projection matrices,
		// some of values are used elsewhere (e.g. skybox uses far plane)
		dest.farClipPlane = src.farClipPlane;
		dest.nearClipPlane = src.nearClipPlane;
		dest.orthographic = src.orthographic;
		dest.fieldOfView = src.fieldOfView;
		dest.aspect = src.aspect;
		dest.orthographicSize = src.orthographicSize;
	}

	// On-demand create any objects we need
	private void CreateMirrorObjects( Camera currentCamera, out Camera reflectionCamera )
	{
		reflectionCamera = null;

		// Reflection render texture
		if( !m_ReflectionTexture || m_OldReflectionTextureSize != m_TextureSize )
		{
			if( m_ReflectionTexture )
				DestroyImmediate( m_ReflectionTexture );
			m_ReflectionTexture = new RenderTexture( m_TextureSize, m_TextureSize, 16 );
			m_ReflectionTexture.name = "__MirrorReflection" + GetInstanceID();
			m_ReflectionTexture.isPowerOfTwo = true;
			m_ReflectionTexture.hideFlags = HideFlags.DontSave;
			m_OldReflectionTextureSize = m_TextureSize;
		}

		// Camera for reflection
		reflectionCamera = m_ReflectionCameras[currentCamera] as Camera;
		if( !reflectionCamera ) // catch both not-in-dictionary and in-dictionary-but-deleted-GO
		{
			GameObject go = new GameObject( "Mirror Refl Camera id" + GetInstanceID() + " for " + currentCamera.GetInstanceID(), typeof(Camera), typeof(Skybox) );
			reflectionCamera = go.GetComponent<Camera>();
			reflectionCamera.enabled = false;
			reflectionCamera.transform.position = transform.position;
			reflectionCamera.transform.rotation = transform.rotation;
			reflectionCamera.gameObject.AddComponent<FlareLayer>();
			go.hideFlags = HideFlags.HideAndDontSave;
			m_ReflectionCameras[currentCamera] = reflectionCamera;
		}
	}

	// Extended sign: returns -1, 0 or 1 based on sign of a
	private static float sgn(float a)
	{
		if (a > 0.0f) return 1.0f;
		if (a < 0.0f) return -1.0f;
		return 0.0f;
	}

	// Given position/normal of the plane, calculates plane in camera space.
	private Vector4 CameraSpacePlane (Camera cam, Vector3 pos, Vector3 normal, float sideSign)
	{
		Vector3 offsetPos = pos + normal * m_ClipPlaneOffset;
		Matrix4x4 m = cam.worldToCameraMatrix;
		Vector3 cpos = m.MultiplyPoint( offsetPos );
		Vector3 cnormal = m.MultiplyVector( normal ).normalized * sideSign;
		return new Vector4( cnormal.x, cnormal.y, cnormal.z, -Vector3.Dot(cpos,cnormal) );
	}

	// Calculates reflection matrix around the given plane
	private static void CalculateReflectionMatrix (ref Matrix4x4 reflectionMat, Vector4 plane)
	{
		reflectionMat.m00 = (1F - 2F*plane[0]*plane[0]);
		reflectionMat.m01 = (   - 2F*plane[0]*plane[1]);
		reflectionMat.m02 = (   - 2F*plane[0]*plane[2]);
		reflectionMat.m03 = (   - 2F*plane[3]*plane[0]);

		reflectionMat.m10 = (   - 2F*plane[1]*plane[0]);
		reflectionMat.m11 = (1F - 2F*plane[1]*plane[1]);
		reflectionMat.m12 = (   - 2F*plane[1]*plane[2]);
		reflectionMat.m13 = (   - 2F*plane[3]*plane[1]);

		reflectionMat.m20 = (   - 2F*plane[2]*plane[0]);
		reflectionMat.m21 = (   - 2F*plane[2]*plane[1]);
		reflectionMat.m22 = (1F - 2F*plane[2]*plane[2]);
		reflectionMat.m23 = (   - 2F*plane[3]*plane[2]);

		reflectionMat.m30 = 0F;
		reflectionMat.m31 = 0F;
		reflectionMat.m32 = 0F;
		reflectionMat.m33 = 1F;
	}
}

```

You can also check the Original Hedgefield's Blog Article (you can download the scripts from there too).

[Original script and shader by user: Hedgefield]