chameleon/Assets/Shaders/MeshPaintBrush.shader

// GPU brush: paints a 3D sphere seamlessly into the paint texture.
//
// In the vertex shader the mesh is "unwrapped" into the texture layout using its
// UVs (uv -> clip space), while the actual local 3D position of the vertex is
// passed through to the fragment shader. There, per texel, the distance to the
// brush center (_BrushCenter, in local mesh space) is tested against _BrushRadius.
//
// This bridges UV seams automatically: triangles on both sides of a seam share
// the same 3D edge and are captured by the same brush stroke, no matter where
// they lie in the UV layout (identical behavior to the CPU path).
Shader "Hidden/MeshPaintBrush"
{
    Properties
    {
        _BrushColor ("Brush Color", Color) = (1,0,0,1)
    }
    SubShader
    {
        Tags { "RenderType" = "Opaque" }
        Pass
        {
            ZTest Always
            ZWrite Off
            Cull Off
            Blend Off

            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #include "UnityCG.cginc"

            float4 _BrushColor;
            float3 _BrushCenter;   // brush center in local mesh space
            float  _BrushRadius;   // brush radius in local mesh space

            struct appdata
            {
                float4 vertex : POSITION;   // local 3D position
                float2 uv     : TEXCOORD0;  // UV (target in the texture layout)
            };

            struct v2f
            {
                float4 pos   : SV_POSITION;
                float3 local : TEXCOORD0;   // interpolated local 3D position
            };

            v2f vert (appdata v)
            {
                v2f o;
                // UV (0..1) -> clip space (-1..1). This makes the GPU rasterize
                // each triangle exactly at its place in the texture.
                float2 p = v.uv * 2.0 - 1.0;
                #if UNITY_UV_STARTS_AT_TOP
                    p.y = -p.y; // compensate the RenderTexture Y-flip (D3D etc.)
                #endif
                o.pos = float4(p, 0.0, 1.0);
                o.local = v.vertex.xyz;
                return o;
            }

            fixed4 frag (v2f i) : SV_Target
            {
                // Only color texels inside the 3D brush sphere.
                float d = distance(i.local, _BrushCenter);
                clip(_BrushRadius - d);
                // Alpha = 1 marks "painted by the brush" (source for the edge padding).
                return fixed4(_BrushColor.rgb, 1.0);
            }
            ENDCG
        }
    }
    Fallback Off
}