Creating a game before your eyes - part 4: Shader for fade in the palette (a la NES)

Shader "Custom/Palette Shader" 
{
	Properties {
		_MainTex ("Base (RGB)", 2D) = "white" {}
	}
	SubShader {
		Pass {
			ZTest Always Cull Off ZWrite Off Fog { Mode off }
			CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag
			#pragma fragmentoption ARB_precision_hint_fastest
			#include "UnityCG.cginc"
			#pragma target 3.0
			struct v2f 
			{
				float4 pos      : POSITION;
				float2 uv       : TEXCOORD0;
			};
			uniform sampler2D _MainTex;
			v2f vert(appdata_img v)
			{
				v2f o;
				o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
				o.uv = MultiplyUV(UNITY_MATRIX_TEXTURE0, v.texcoord);
				return o;
			}
			half4 frag(v2f i): COLOR
			{
				half4 color = tex2D(_MainTex, i.uv);
				// здесь будет код, преобразующий цвет
				half4 rc = color;
				return rc;
			}
			ENDCG
		}
	}
	FallBack "Diffuse"
}

using UnityEngine;
[ExecuteInEditMode]
[RequireComponent(typeof(Camera))]
public class PaletteShader : MonoBehaviour 
{
	public Shader shader;
	private Material _material;
	////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
	protected Material material
	{
		get
		{
			if (_material == null)
			{
				_material = new Material(shader);
				_material.hideFlags = HideFlags.HideAndDontSave;
			}
			return _material;
		}
	}
	////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
	private void OnRenderImage(RenderTexture source, RenderTexture destination)
	{
		if (shader == null) return;
		Material mat = material;
		Graphics.Blit(source, destination, mat);
	}
	////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
	void OnDisable()
	{
		if (_material)	DestroyImmediate(_material);
	}
	////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
}

Shader "Custom/Palette Shader" 
{
	Properties {
		_MainTex ("Base (RGB)", 2D) = "white" {}
		_Br("Brightness", Float) = 0
		_PaletteTex ("Pelette texture", 3D) = "white" {} 
	}
	SubShader {
		Pass {
			ZTest Always Cull Off ZWrite Off Fog { Mode off }
			CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag
			#pragma fragmentoption ARB_precision_hint_fastest
			#include "UnityCG.cginc"
			#pragma target 3.0
			struct v2f 
			{
				float4 pos      : POSITION;
				float2 uv       : TEXCOORD0;
			};
			uniform sampler2D _MainTex;
			uniform sampler3D _PaletteTex;
			uniform float _Br;
			v2f vert(appdata_img v)
			{
				v2f o;
				o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
				o.uv = MultiplyUV(UNITY_MATRIX_TEXTURE0, v.texcoord);
				return o;
			}
			half4 frag(v2f i): COLOR
			{
				half4 color = tex2D(_MainTex, i.uv);
				half4 rc = tex3D(_PaletteTex, color.rgb * _Br);
				float d = abs(Luminance(color) - Luminance(rc));
				if ((d < 0.15) || (_Br == 1)) rc = color;
				return rc;
			}
			ENDCG
		}
	}
	FallBack "Diffuse"
}

using UnityEngine;
[ExecuteInEditMode]
[RequireComponent(typeof(Camera))]
public class PaletteShader : MonoBehaviour 
{
	public Shader shader;
	private Material _material;
	[Range(0, 1)] public float brightness = 0.0f;
	[Range(0, 1)] public float random = 1f;
	private float _r = 0f;
	private Texture3D _tex;
	////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
	protected Material material
	{
		get
		{
			if (_material == null)
			{
				_material = new Material(shader);
				_material.hideFlags = HideFlags.HideAndDontSave;
			}
			return _material;
		}
	}
	////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
	private Texture3D GeneratePaletteTexture()
	{
		const int dim = 16; // кол-во градаций цвета в исходной палитре
		const int depth = 3; // кол-во градаций цвета в результирующей палитре
		const float f_dim = 1.0f / (1.0f * dim - 1.0f);
		const float f_depth = 1.0f / (1.0f * depth - 1.0f);
		Texture3D tex = new Texture3D(dim, dim, dim, TextureFormat.RGB565, false);
		tex.filterMode = FilterMode.Point;
		tex.wrapMode = TextureWrapMode.Clamp;
		Color[] palette = new Color[depth*depth*depth];
		float[] palette_grey = new float[depth*depth*depth];
		// заполняем палитру используемых цветов
		for (int r = 0; r < depth; r++)
		{
			for (int g = 0; g < depth; g++)
			{
				for (int b = 0; b < depth; b++)
				{
					Color c = new Color(r * f_depth / 2, g * f_depth, b * f_depth, 1);
					int n = r*depth*depth + g*depth + b;
					palette[n] = c;
					palette_grey[n] = c.r*0.21f + c.g*0.72f + c.b*0.07f;
				}
			}
		}
		Color[] t = new Color[dim*dim*dim];
		// заполняем текстуру конвертирования
		for (int r = 0; r < dim; r++)
		{
			for (int g = 0; g < dim; g++)
			{
				for (int b = 0; b < dim; b++)
				{
					float grey = (r * 0.21f + g * 0.72f + b * 0.07f) * f_dim;
					// теперь найдем самый ближайший цвет по яркости серого
					int idx = 0;
					float min_d = grey;
					for (int i = 1; i < palette_grey.Length; i++)
					{
						float d = Mathf.Abs(palette_grey[i] - grey);
						if (d < min_d)
						{
							min_d = d;
							idx = i;
						}
					}
					t[r * dim * dim + g * dim + b] = palette[idx]; // заполним палитру конвертации
				}
			}
		}
		tex.SetPixels(t);
		tex.Apply();
		return tex;
	}
	////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
	private void OnRenderImage(RenderTexture source, RenderTexture destination)
	{
		if (shader == null) return;
		Material mat = material;
		mat.SetFloat("_Br", brightness);
		if (_tex == null) _tex = GeneratePaletteTexture();
		if (random != _r)
		{
			_r = random;
			_tex = GeneratePaletteTexture();
		}
		mat.SetTexture("_PaletteTex", _tex);
		Graphics.Blit(source, destination, mat);
	}
	////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
	void OnDisable()
	{
		if (_material)	DestroyImmediate(_material);
	}
	////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
}