[Need Help] Billboarding Geometry Shader

Hi everyone,

I want to make a billboard geometry shader to all tree mesh will be face towards camera positon. But i don't have experience with shaderlab, i hope someone can help me fix for me.

Below is billboard geometry shader code that i am using:
i get from here:
https://forum.unity.com/threads/billboard-geometry-shader.169415/

Code (CSharp):

1.  
2. Shader "Custom/GS Billboard"
3. {
4.    Properties
5.    {
6.       _SpriteTex ("Base (RGB)", 2D) = "white" {}
7.       _Size ("Size", Range(0, 10)) = 1
8.    }
9.  
10.    SubShader
11.    {
12.       Pass
13.       {
14.          Tags { "RenderType"="Opaque" }
15.          LOD 200
16.  
17.          CGPROGRAM
18.             #pragma target 5.0
19.             #pragma vertex VS_Main
20.             #pragma fragment FS_Main
21.             #pragma geometry GS_Main
22.             #include "UnityCG.cginc"
23.  
24.             // **************************************************************
25.             // Data structures                                    *
26.             // **************************************************************
27.             struct GS_INPUT
28.             {
29.                float4   pos      : POSITION;
30.                float3   normal   : NORMAL;
31.                float2  tex0   : TEXCOORD0;
32.             };
33.  
34.             struct FS_INPUT
35.             {
36.                float4   pos      : POSITION;
37.                float2  tex0   : TEXCOORD0;
38.             };
39.  
40.             // **************************************************************
41.             // Vars                                             *
42.             // **************************************************************
43.  
44.             float _Size;
45.             float4x4 _VP;
46.             Texture2D _SpriteTex;
47.             SamplerState sampler_SpriteTex;
48.  
49.             // **************************************************************
50.             // Shader Programs                                    *
51.             // **************************************************************
52.  
53.             // Vertex Shader ------------------------------------------------
54.             GS_INPUT VS_Main(appdata_base v)
55.             {
56.                GS_INPUT output = (GS_INPUT)0;
57.  
58.                output.pos =  mul(unity_ObjectToWorld, v.vertex);
59.                output.normal = v.normal;
60.                output.tex0 = float2(0, 0);
61.  
62.                return output;
63.             }
64.  
65.             // Geometry Shader -----------------------------------------------------
66.             [maxvertexcount(4)]
67.             void GS_Main(point GS_INPUT p[1], inout TriangleStream<FS_INPUT> triStream)
68.             {
69.                float halfWidth = _Size / 2.0f;
70.                
71.                
72.                float3 planeNormal = p[0].pos - _WorldSpaceCameraPos;
73.                planeNormal.y = 0.0f;
74.                planeNormal = normalize(planeNormal);
75.  
76.                float3 upVector = float3(0.0f, 1.0f, 0.0f);
77.                float3 rightVector = normalize(cross(planeNormal, upVector));
78.                rightVector = rightVector * halfWidth;
79.                upVector = float3(0, _Size, 0);
80.  
81.                float4 v[4];
82.                v[0] = float4(p[0].pos - rightVector, 1.0f);
83.                v[1] = float4(p[0].pos + rightVector, 1.0f);
84.                v[2] = float4(p[0].pos - rightVector + upVector, 1.0f);
85.                v[3] = float4(p[0].pos + rightVector + upVector, 1.0f);
86.  
87.  
88.                FS_INPUT pIn;
89.                pIn.pos = UnityObjectToClipPos(v[0]);
90.                pIn.tex0 = float2(1.0f, 0.0f);
91.                triStream.Append(pIn);
92.  
93.                pIn.pos = UnityObjectToClipPos(v[1]);
94.                pIn.tex0 = float2(0.0f, 0.0f);
95.                triStream.Append(pIn);
96.  
97.                pIn.pos = UnityObjectToClipPos(v[2]);
98.                pIn.tex0 = float2(1.0f, 1.0f);
99.                triStream.Append(pIn);
100.  
101.                pIn.pos = UnityObjectToClipPos(v[3]);
102.                pIn.tex0 = float2(0.0f, 1.0f);
103.                triStream.Append(pIn);
104.             }
105.  
106.             // Fragment Shader -----------------------------------------------
107.             float4 FS_Main(FS_INPUT input) : COLOR
108.             {
109.                return _SpriteTex.Sample(sampler_SpriteTex, input.tex0);
110.             }
111.  
112.          ENDCG
113.       }
114.    }
115. }
116.  

I want to using this shader for my combined tree mesh (which have many individual child mesh) like below:

This is screenshot when i using standard shader for it.

But, when i using above GS Billboard shader for it, the position and scale of original mesh was changed like below:


As you can see, when apply to GS billboard shader, all position and scale was changed.
I want to keep original position and scale of all individual tree mesh like when i using Standard Shader, but i don't known how i can fix it and fix from where.
I hope someone can help me fix this shader !

Thank in advanced !