Billboard with single axis rotation

Hello,

I am trying to make a billboard shader with two conditions:
1) The angle between the view vector and the local y axis of the model is minumum (i.e. the model always faces the camera)
2) The model keeps its orientation to z azis when rotated or translated

The best I have got so far is the folowing:

Code (CSharp):

1. Shader "Custom/BillboardZ"
2. {
3.     Properties
4.     {
5.         _MainTex ("Texture Image", 2D) = "white" {}
6.         _Color("Tint Color", Color) = (0.5,0.5,0.5,0.5)
7.     }
8.     SubShader
9.     {
10.         Tags { "Queue" = "Transparent" "IgnoreProjector" = "False" "RenderType" = "Transparent" }
11.         Cull Off
12.         ZTest Less
13.         Blend SrcAlpha OneMinusSrcAlpha
14.         ColorMask RGB
15.         Cull Off Lighting Off Fog{ Color(0,0,0,0) }
16.         ZWrite Off
17.         Cull Back
18.         Pass
19.         {
20.             CGPROGRAM
21.             #include "UnityCG.cginc"
22.             #pragma shader_feature IGNORE_ROTATION_AND_SCALE
23.             #pragma vertex vert
24.             #pragma fragment frag
25.      
26.             uniform sampler2D _MainTex;
27.             fixed4 _Color;
28.          
29.             float4 _MainTex_ST;
30.             struct vertexInput
31.             {
32.                 float4 vertex : POSITION;
33.                 float4 tex : TEXCOORD0;
34.             };
35.             struct vertexOutput
36.             {
37.                 float4 pos : SV_POSITION;
38.                 float2 tex : TEXCOORD0;
39.             };
40.  
41.             vertexOutput vert(vertexInput input)
42.             {
43.                 float3 worldPos = mul(unity_ObjectToWorld, float4(0, 0, 0, 1)).xyz;
44.                 float3 posCamVector = _WorldSpaceCameraPos - worldPos;
45.  
46.                 float w = atan2(posCamVector.y, posCamVector.x);
47.  
48.  
49.                 float cosw = cos(w);
50.                 float sinw = sin(w);            
51.  
52.                 float3x3 rotMatrix;          
53.                 rotMatrix[0].xyz = float4(cosw, -sinw, 0, 0);
54.                 rotMatrix[1].xyz = float4(sinw, cosw, 0, 0);
55.                 rotMatrix[2].xyz = float4(0, 0 ,1, 0);
56.                 float4 newPos = float4(mul(rotMatrix, input.vertex * float4(0, 1, 1, 0)), 1);
57.  
58.                 vertexOutput output;
59.                 output.pos = mul(UNITY_MATRIX_VP, mul(unity_ObjectToWorld, newPos));
60.                 output.tex = TRANSFORM_TEX(input.tex, _MainTex);
61.                 return output;
62.             }
63.  
64.             float4 frag(vertexOutput input) : COLOR
65.             {
66.                 return tex2D(_MainTex, input.tex.xy) * _Color;
67.             }
68.             ENDCG
69.         }
70.     }
71. }

This does exactly what I want except when the model is rotated around an axis the 1st condition doesn't hold (it does not face the camera).
I think the solution would be to apply the rotation to the z axis locally to the model after the model matrix is applied (and not before as in the above code) but I don't know how to do it. Anty ideas?
The model used with the above shader is a cube.

I would appreciate any ideas.

cheers,
Sotos

 

I'm not sure I follow exactly what you're asking for. I assume you want the mesh to face the camera by rotating around the (world?) Y axis, but also want keep the rotation around the local Z axis so it can spin? If you can guarantee the game object itself is always only ever rotated in world Z then you should be able to apply the objec to world matrix as a rotation matrix to the initial vertex position prior to rotating it towards the camera.

float3 vertPos = mul((float3x3)unity_ObjectToWorld, v.vertex.xyz);
float3 newPos = mul(rotMatrix, vertPos) + worldPos;
output.pos = mul(UNITY_MATRIX_VP, float4(newPos, 1));

That should apply the gameobject's rotation and scale, then rotate towards the camera. You may also want to use a quad mesh instead of a cube so you're not doing calculations for vertices you don't need.

 

Thank you for the reply,

What I wanted to do is make the billboard face always the camera but with the additional constraint that the billboard is allowed to rotate only around to a local axis say z.

I solved the problem by decomposing the model matrix in scale rotation and translation therefore T * R * S * v and then multiplying the rotation with another matrix Rz which is a rotation around the local z axis represented in the world coordinates with an angle that depends on the projection of the position to Camera vector to the x local axis.

therefore the it goes T * Rz * R * S * v

cheers,
Sotos