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Question by chirp · Aug 18, 2016 at 03:28 PM · camerashadermatrix

What is the difference between UNITY_MATRIX_MVP and (Camera.projectionMatrix * Camera.worldToCameraMatrix) * unity_ObjectToWorld ?

Hey, I try to obtain camera MVP matrix via C#Script and set to global Matrix. But the results of calculation are different from the built-in UNITY_MATRIX_MVP. Why ?

 using UnityEngine;
 using System.Collections;
 
 public class LightProjector : MonoBehaviour {
     private Camera _lightCam;
     private Shader _replaceSdr;
     void Awake () {
         _lightCam = GetComponent<Camera>();
         _replaceSdr = Shader.Find("Hidden/LightPorject/Replace");
         if (_replaceSdr != null)
         {
             _lightCam.SetReplacementShader(_replaceSdr, "RenderType");
         }
     }
     
     void Update () {    
         Shader.SetGlobalMatrix("_LightProjectMatrix", _lightCam.projectionMatrix * _lightCam.worldToCameraMatrix);
     }
 }

ReplaceShader with UNITY_MATRIX_MVP :

 Shader "Hidden/LightPorject/Replace"
 {
     SubShader
     {
         Tags { "RenderType" = "Opaque" }
         LOD 100
 
         Pass
         {
             CGPROGRAM
             #pragma vertex vert
             #pragma fragment frag
             // make fog work
             #pragma multi_compile_fog
 
             #include "UnityCG.cginc"
 
             struct appdata
             {
                 float4 vertex : POSITION;
                 float3 normal : NORMAL;
             };
 
             struct v2f
             {
                 float4 vertex : SV_POSITION;
                 float3 normal : TEXCOORD0;
             };
 
             float4x4 _LightProjectMatrix;
 
             v2f vert (appdata v)
             {
                 v2f o;
                 o.vertex = mul(UNITY_MATRIX_MVP, v.vertex);
                 o.normal = mul(unity_ObjectToWorld, float4(v.normal,0)).xyz;
                 return o;
             }
             
             fixed4 frag (v2f i) : SV_Target
             {
                 fixed4 col = fixed4(i.normal,1);
                 return col;
             }
             ENDCG
         }
     }
 }

alt text

ReplaceShader with (Camera.projectionMatrix Camera.worldToCameraMatrix) unity_ObjectToWorld :

 Shader "Hidden/LightPorject/Replace"
 {
     SubShader
     {
         Tags { "RenderType" = "Opaque" }
         LOD 100
 
         Pass
         {
             CGPROGRAM
             #pragma vertex vert
             #pragma fragment frag
             // make fog work
             #pragma multi_compile_fog
 
             #include "UnityCG.cginc"
 
             struct appdata
             {
                 float4 vertex : POSITION;
                 float3 normal : NORMAL;
             };
 
             struct v2f
             {
                 float4 vertex : SV_POSITION;
                 float3 normal : TEXCOORD0;
             };
 
             float4x4 _LightProjectMatrix;
 
             v2f vert (appdata v)
             {
                 v2f o;
                 float4 wPos = mul(unity_ObjectToWorld, v.vertex);
                 o.vertex = mul(_LightProjectMatrix, wPos);
                 o.normal = mul(unity_ObjectToWorld, float4(v.normal,0)).xyz;
                 return o;
             }
             
             fixed4 frag (v2f i) : SV_Target
             {
                 fixed4 col = fixed4(i.normal,1);
                 return col;
             }
             ENDCG
         }
     }
 }

alt text

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