// Upgrade NOTE: replaced '_World2Object' with 'unity_WorldToObject'
// Upgrade NOTE: replaced 'mul(UNITY_MATRIX_MVP,*)' with 'UnityObjectToClipPos(*)'

Shader "Unity Shaders Book/Chapter 11/Billboard" {
	Properties {
		_MainTex ("Main Tex", 2D) = "white" {}
		_Color ("Color Tint", Color) = (1, 1, 1, 1)
		_VerticalBillboarding ("Vertical Restraints", Range(0, 1)) = 1 
	}
	SubShader {
		// Need to disable batching because of the vertex animation
		Tags {"Queue"="Transparent" "IgnoreProjector"="True" "RenderType"="Transparent" "DisableBatching"="True"}
		
		Pass { 
			Tags { "LightMode"="ForwardBase" }
			
			ZWrite Off
			Blend SrcAlpha OneMinusSrcAlpha
			Cull Off
		
			CGPROGRAM
			
			#pragma vertex vert
			#pragma fragment frag
			
			#include "Lighting.cginc"
			//
			sampler2D _MainTex;
			float4 _MainTex_ST;
			fixed4 _Color;
			fixed _VerticalBillboarding;
			
			struct a2v {
				float4 vertex : POSITION;
				float4 texcoord : TEXCOORD0;
			};
			
			struct v2f {
				float4 pos : SV_POSITION;
				float2 uv : TEXCOORD0;
			};
			
			v2f vert (a2v v) {
				v2f o;
				
				// Suppose the center in object space is fixed
				float3 center = float3(0, 0, 0);
				float3 viewer = mul(unity_WorldToObject,float4(_WorldSpaceCameraPos, 1));
				
				float3 normalDir = viewer - center;
				// If _VerticalBillboarding equals 1, we use the desired view dir as the normal dir
				// Which means the normal dir is fixed
				// Or if _VerticalBillboarding equals 0, the y of normal is 0
				// Which means the up dir is fixed
				normalDir.y =normalDir.y * _VerticalBillboarding;
				normalDir = normalize(normalDir);
				// Get the approximate up dir
				// If normal dir is already towards up, then the up dir is towards front
				float3 upDir = abs(normalDir.y) > 0.999 ? float3(0, 0, 1) : float3(0, 1, 0);
				float3 rightDir = normalize(cross(upDir, normalDir));
				upDir = normalize(cross(normalDir, rightDir));
				
				// Use the three vectors to rotate the quad
				float3 centerOffs = v.vertex.xyz - center;
				float3 localPos = center + rightDir * centerOffs.x + upDir * centerOffs.y + normalDir * centerOffs.z;
              
				o.pos = UnityObjectToClipPos(float4(localPos, 1));
				o.uv = TRANSFORM_TEX(v.texcoord,_MainTex);

				return o;
			}
			
			fixed4 frag (v2f i) : SV_Target {
				fixed4 c = tex2D (_MainTex, i.uv);
				c.rgb *= _Color.rgb;
				return c;
			}
			
			ENDCG
		}
	} 
	FallBack "Transparent/VertexLit"
}