切線空間的法線貼圖使用

1. 法線紋理TextureType設置爲Normal Map（使用Unity內置的Shader必須設置爲Normal Map）
2. 設置爲Normal Map的貼圖在不同平臺Unity會採用不同的壓縮方式
3. 法線的z方向始終爲正，法線紋理存儲的是單位向量，所以只需要保存法線的x, y方向
4. 對法線貼圖進行紋理採樣後需要使用UnpackNormal來獲得真正的法線（DXT5nm格式壓縮的紋理a對應x，g對應y，r，b被捨棄）
5. 法線紋理每個分量取值範圍是[0,1]，而法線每個分量取值範圍是[-1, 1]，所以normal = pixel * 2 - 1

Shader "Achonor/NormalMapInTangentSpace"
{
	Properties
	{
		_Color ("Color", Color) = (1.0, 1.0, 1.0, 1.0)
		_MainTex ("Main Tex", 2D) = "white" {}
		_BumpMap ("Normal Map", 2D) = "bump" {}
		_BumpScale("Bump Scale", Float) = 1.0
		_Specular("Specular", Color) = (1.0, 1.0, 1.0, 1.0)
		_Gloss("Gloss", Range(8, 256)) = 20
	}
	SubShader
	{
		Pass{
			Tags{"LightMode"="ForwardBase"}

			CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag

			#include "Lighting.cginc"

			struct a2v{
				float4 vertex : POSITION;
				float3 normal : NORMAL;
				float4 tangent : TANGENT;
				float4 texcoord : TEXCOORD0;
			};

			struct v2f{
				float4 pos : SV_POSITION;
				fixed3 tangentViewDir : TEXCOORD0;
				fixed3 tangentLightDir : TEXCOORD1;
				float4 uv : TEXCOORD2;
			};

			fixed3 _Color;
			sampler2D _MainTex;
			float4 _MainTex_ST;
			sampler2D _BumpMap;
			float4 _BumpMap_ST;
			float _BumpScale;
			fixed3 _Specular;
			float _Gloss;

			v2f vert(a2v v){
				v2f o;
				o.pos = UnityObjectToClipPos(v.vertex);

				//計算uv
				o.uv.xy = TRANSFORM_TEX(v.texcoord, _MainTex);
				o.uv.zw = TRANSFORM_TEX(v.texcoord, _BumpMap);

				TANGENT_SPACE_ROTATION;
				o.tangentViewDir =  normalize(mul(rotation, ObjSpaceViewDir(v.vertex)));
				o.tangentLightDir =  normalize(mul(rotation, ObjSpaceLightDir(v.vertex)));

				return o;
			}

			fixed4 frag(v2f o) : SV_Target{
				fixed3 tangentViewDir = normalize(o.tangentViewDir);
				fixed3 tangentLightDir = normalize(o.tangentLightDir);

				//紋理採樣
				fixed3 albedo = tex2D(_MainTex, o.uv.xy).rgb * _Color.rgb;

				//環境光
				fixed3 ambient = UNITY_LIGHTMODEL_AMBIENT.rgb * albedo.rgb;

				//計算法線
				//fixed3 tangentNormal = tex2D(_BumpMap, o.uv.zw).rgb;
				//tangentNormal.xy = (tangentNormal.xy * 2 - 1) * _BumpScale;
				fixed3 tangentNormal = UnpackNormal(tex2D(_BumpMap, o.uv.zw));
				tangentNormal.xy *= _BumpScale;
				tangentNormal.z = sqrt(1 - saturate(dot(tangentNormal.xy, tangentNormal.xy)));

				//漫反射
				fixed3 duffise = _LightColor0.rgb * albedo.rgb * saturate(dot(tangentLightDir, tangentNormal));

				//高光反射
				fixed3 halfDir = normalize(tangentViewDir + tangentLightDir);
				fixed3 specular = _LightColor0.rgb * _Specular.rgb * pow(saturate(dot(halfDir, tangentNormal)), _Gloss);

				return fixed4(ambient + duffise + specular, 1);
			}
			ENDCG
		}
	}
}