Vertex shader that rotates around the center

I have the following shader that allows me to apply a color gradient :

Code (CSharp):

1.  Shader "Custom/LinearGradient"
2. {
3.      Properties{
4.          _TopColor("Color1", Color) = (1,1,1,1)
5.          _BottomColor("Color2", Color) = (1,1,1,1)
6.          _MainTex ("Main Texture", 2D) = "white" {}
7.          _Rotation("Rotation", Float) = 0
8.      }
9.          SubShader
10.      {
11.          Tags { "RenderType" = "Transparent" "Queue" = "Transparent" "PreviewType" = "Plane"}
12.          Pass
13.          {
14.              ZWrite Off
15.              Blend SrcAlpha OneMinusSrcAlpha
16.  
17.              CGPROGRAM
18.              #pragma vertex vert
19.              #pragma fragment frag
20.              #include "UnityCG.cginc"
21.  
22.              sampler2D _MainTex;
23.              float4 _MainTex_ST;
24.              fixed4 _TopColor;
25.              fixed4 _BottomColor;
26.              half _Value;
27.              float4x4 _Rotation;
28.  
29.              struct v2f {
30.                  float4 position : SV_POSITION;
31.                  fixed4 color : COLOR;
32.                  float2 uv : TEXCOORD0;
33.              };
34.  
35.              v2f vert (appdata_full v)
36.              {
37.                  v2f o;
38.                  o.uv = TRANSFORM_TEX (v.texcoord, _MainTex);
39.                  v.texcoord.xy = mul(_Rotation, v.texcoord.xy);
40.                  o.position = mul (UNITY_MATRIX_MVP, v.vertex);
41.                  o.color = lerp (_TopColor,_BottomColor, v.texcoord.y);
42.                  return o;
43.              }
44.  
45.              fixed4 frag(v2f i) : SV_Target
46.              {
47.                  float4 color;
48.                  color.rgb = i.color.rgb;
49.                  color.a = tex2D (_MainTex, i.uv).a * i.color.a;
50.                  return color;
51.              }
52.              ENDCG
53.          }
54.      }
55. }

Looks like this in Unity when applied to a sprite :


I'm also calling the following code from Update() to adjust the rotation of the gradient :

Code (CSharp):

1.  
2.     Vector2 pivot = new Vector2(0.5f, 0.5f);
3.     float rotateSpeed = 30f;
4.     Material m;
5.  
6.     void Update()
7.     {
8.         if (m != null)
9.         {
10.             Matrix4x4 t = Matrix4x4.TRS(-pivot, Quaternion.identity, Vector3.one);
11.  
12.             Quaternion rotation = Quaternion.Euler(0, 0, Time.time * rotateSpeed);
13.             Matrix4x4 r = Matrix4x4.TRS(Vector3.zero, rotation, Vector3.one);
14.  
15.             Matrix4x4 tInv = Matrix4x4.TRS(pivot, Quaternion.identity, Vector3.one);
16.             sprites[0].material.SetMatrix("_Rotation", tInv * r * t);
17.         }
18.     }

The only thing is that it doesn't rotate from the center of the sprite but from a point located here :


Any idea on how I could offset the rotation point?

 

Hm, if it really is the green point than I would change it like this:

Code (CSharp):

1.              v2f vert (appdata_full v)
2.              {
3.                  v2f o;
4.                  o.uv = TRANSFORM_TEX (v.texcoord, _MainTex);
5.                  v.texcoord.x = v.texcoord.x + 0.5;
6.                  v.texcoord.xy = mul(_Rotation, v.texcoord.xy);
7.                  v.texcoord.x = v.texcoord.x - 0.5;
8.                  o.position = mul (UNITY_MATRIX_MVP, v.vertex);
9.                  o.color = lerp (_TopColor,_BottomColor, v.texcoord.y);
10.                  return o;
11.              }

From my feeling however I would have guessed you need to move u and v in +0.5 before rotating.

 

Thanks a lot for your answer!
I just had to offset xy by -0.5 before rotating then add 0.5 back like so:

Code (CSharp):

1. v2f vert (appdata_full v)
2.              {
3.                  v2f o;
4.                  o.uv = TRANSFORM_TEX (v.texcoord, _MainTex);
5.                  v.texcoord.xy -= float2(0.5, 0.5);
6.                  v.texcoord.xy = mul(_Rotation, v.texcoord.xy);
7.                  v.texcoord.xy += float2(0.5, 0.5);
8.                  o.position = mul (UNITY_MATRIX_MVP, v.vertex);
9.                  o.color = lerp (_TopColor, _BottomColor, v.texcoord.y);
10.                  return o;
11.              }

 

Perfect! Always happy to help