unity3d shader BIllboard 使模型始终朝向摄像机

修改unity 内置shader Particles/Additive 为增加 BIllboard功能
上代码

Shader "Particles/AddtiveBillboard" {Properties {    _TintColor ("Tint Color", Color) = (0.5,0.5,0.5,0.5)    _MainTex ("Particle Texture", 2D) = "white" {}    _InvFade ("Soft Particles Factor", Range(0.01,3.0)) = 1.0}Category {    Tags { "Queue"="Transparent" "IgnoreProjector"="True" "RenderType"="Transparent" }    Blend SrcAlpha One    AlphaTest Greater .01    ColorMask RGB    Cull Off Lighting Off ZWrite Off Fog { Color (0,0,0,0) }    SubShader {        Pass {            CGPROGRAM            #pragma vertex vert            #pragma fragment frag            #pragma multi_compile_particles            #include "UnityCG.cginc"            sampler2D _MainTex;            fixed4 _TintColor;            struct appdata_t {                float4 vertex : POSITION;                fixed4 color : COLOR;                float2 texcoord : TEXCOORD0;            };            struct v2f {                float4 vertex : SV_POSITION;                fixed4 color : COLOR;                float2 texcoord : TEXCOORD0;                #ifdef SOFTPARTICLES_ON                float4 projPos : TEXCOORD1;                #endif            };            float4 _MainTex_ST;            v2f vert (appdata_t v)            {                v2f o;                //o.vertex = mul(UNITY_MATRIX_MVP, v.vertex);//这里是原有的vertex 赋值代码,注释掉                //****************这里修改的vertex赋值代码*******************                float4 ori=mul(UNITY_MATRIX_MV,float4(0,0,0,1));                float4 vt=v.vertex;                vt.y=vt.z;                vt.z=0;                vt.xyz+=ori.xyz;                o.vertex=mul(UNITY_MATRIX_P,vt);                //******************************                #ifdef SOFTPARTICLES_ON                o.projPos = ComputeScreenPos (o.vertex);                COMPUTE_EYEDEPTH(o.projPos.z);                #endif                o.color = v.color;                o.texcoord = TRANSFORM_TEX(v.texcoord,_MainTex);                return o;            }            sampler2D_float _CameraDepthTexture;            float _InvFade;            fixed4 frag (v2f i) : SV_Target            {                #ifdef SOFTPARTICLES_ON                float sceneZ = LinearEyeDepth (SAMPLE_DEPTH_TEXTURE_PROJ(_CameraDepthTexture, UNITY_PROJ_COORD(i.projPos)));                float partZ = i.projPos.z;                float fade = saturate (_InvFade * (sceneZ-partZ));                i.color.a *= fade;                #endif                return 2.0f * i.color * _TintColor * tex2D(_MainTex, i.texcoord);            }            ENDCG         }    }   }}