Make billboard shader only rotate on one axis.

Hello,

I got a billboard shader from https://en.wikibooks.org/wiki/Cg_Programming/Unity/Billboards
I adjusted it a little to work with the default plane and with textures having an alpha channel:

Code (CSharp):

1. Shader "Cg  shader for billboards" {
2.    Properties {
3.       _MainTex ("Billboard Image", 2D) = "white" {}
4.    }
5.    SubShader {
6.  
7.       Tags {"Queue"="Transparent" "RenderType"="Transparent"}
8.       Blend SrcAlpha OneMinusSrcAlpha
9.  
10.       Pass {
11.          CGPROGRAM
12.  
13.          // compilation directives
14.          #pragma vertex vert  // tells that the code contains a vertex program in the given function (vert here).
15.          #pragma fragment frag // tells that the code contains a fragment program in the given function (frag here).
16.  
17.          // User-specified uniforms          
18.          uniform sampler2D _MainTex; // define variable in order to have access to the property.
19.          struct vertexInput {
20.             float4 vertex : POSITION; // position in local space
21.             float4 tex : TEXCOORD0;
22.          };
23.          struct vertexOutput {
24.             float4 pos : SV_POSITION; // position after being transformed into projection space
25.             float4 tex : TEXCOORD0;
26.          };
27.          vertexOutput vert(vertexInput input)
28.          {
29.             vertexOutput output;
30.  
31.              float4 camDir = mul(UNITY_MATRIX_P,
32.               mul(UNITY_MATRIX_MV, float4(0.0, 0.0, 0.0, 1.0))
33.               - float4(input.vertex.x, input.vertex.z, 0.0, 0.0));
34.  
35.             output.pos = camDir;
36.             output.tex = input.tex;
37.  
38.             return output;
39.          }
40.          float4 frag(vertexOutput input) : COLOR
41.          {
42.             return tex2D(_MainTex, float2(input.tex.xy));
43.          }
44.          ENDCG
45.       }
46.    }
47. }

This shader rotates the billboard in all directions (on all axes). I'm new to shader programming (like you can see from the comments ) and don't know how to achive a rotation only on one axis. I'd like to have it only rotate on the global axis going up (y). Can you help me out?

PS: I have seen this question a lot in the forums/answers but never with a solution.

 

I also have this same exact question, arising from the same exact place as the OP. Does anyone know the matrix math to accomplish it?

 

I do always attempt to solve my own problems, and I took a long hard shot at this one, but I'm still not sure what I'm doing wrong.

This website
http://www.geeks3d.com/20140807/billboarding-vertex-shader-glsl/
shows how to do cylindrical billboarding. I attempted to copy it into HLSL. The spherical billboarding works perfectly! But the cylindrical billboarding squashes the mesh on one axis instead of preventing it from rotating.

I'm wondering if I missed something in the conversion from GLSL to HLSL, since HLSL's * operator means something different than in GLSL, and I read somewhere HLSL matrices are row major, while in GLSL are column major, but I don't know if that affects what I'm doing or how to fix that in this circumstance.

The relevant part of the code looks like this:

Code (CSharp):

1.  
2. vertexOutput vert(vertexInput input)
3.          {
4.             vertexOutput output;
5.  
6.             float4x4 modelView = UNITY_MATRIX_MV;
7.  
8.             modelView[0][0] = 1;
9.             modelView[0][1] = 0;
10.             modelView[0][2] = 0;
11.      
12.             // uncomment for working spherical billboarding
13.             //modelView[1][0] = 0;
14.             //modelView[1][1] = 1;
15.             //modelView[1][2] = 0;
16.  
17.             modelView[2][0] = 0;
18.             modelView[2][1] = 0;
19.             modelView[2][2] = 1;
20.  
21.             float4 P = mul(modelView, input.vertex * float4(_ScaleX, _ScaleY, 1.0, 1.0));
22.          
23.             output.pos = mul(UNITY_MATRIX_P, P);
24.  
25.             // alternate method for spherical billboarding, working
26.             //output.pos = mul( UNITY_MATRIX_P,
27.             //mul(UNITY_MATRIX_MV, float4(0.0, 0.0, 0.0, 1.0))
28.             //- float4(input.vertex.x, input.vertex.y, 0.0, 0.0)
29.             //* float4(_ScaleX, _ScaleY, 1.0, 1.0));
30.  
31.             output.color = input.color * _Color;
32.             output.tex = input.tex;
33.  
34.             return output;
35.          }