Shader "Hidden/VFX/GradientBorder"
{
    Properties
    {
        _Border("Border",float) = 1
        _Radius("Radius",float) = 1
        _PixelScale("PixelScale",float) = 1
        _Size("Size",Vector) = (100,100,0,0)
        _ColorStart("ColorStart",Color) = (1,1,0,1)
        _ColorEnd("ColorEnd", Color) = (0,1,1,1)
    }
    SubShader
    {
        Tags { "RenderType"="Transparent" "Queue"="Transparent"}
        LOD 100
        Cull Off
        ZWrite Off
        ZTest Always
        Blend SrcAlpha OneMinusSrcAlpha

        Pass
        {
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            // make fog work
            #pragma multi_compile_fog

            #include "UnityCG.cginc"

            struct appdata
            {
                float4 vertex : POSITION;
                float2 uv : TEXCOORD0;
            };

            struct v2f
            {
                float2 uv : TEXCOORD0;
                float4 vertex : SV_POSITION;
                float4 pos : TEXCOORD2;
                float2 clipUV : TEXCOORD1;
                float height : TEXCOORD3;
            };

            float _Border;
            float _Radius;
            float _PixelScale;
            float2 _Size;
            fixed4 _ColorStart;
            fixed4 _ColorEnd;

            uniform float4x4 unity_GUIClipTextureMatrix;
            sampler2D _GUIClipTexture;

            //#define _Radius 34
            //#define _Border 4

            v2f vert (appdata v)
            {
                v2f o;

                float2 size = _Size - float2(_Radius,_Radius);

                float margingScale = 2 + (_Border/_Radius /_PixelScale);

                o.pos = float4(v.vertex.xy * size + v.uv* margingScale * v.vertex.xy* _Radius, 0, 0);
                o.height = (v.vertex.y + 1)* 0.5;
                o.vertex = UnityObjectToClipPos(o.pos);
                o.uv = v.uv*margingScale;
                float3 eyePos = UnityObjectToViewPos(o.pos );
                o.clipUV = mul(unity_GUIClipTextureMatrix, float4(eyePos.xy, 0, 1.0));

                return o;
            }

            fixed4 frag (v2f i) : SV_Target
            {
                float pixelScale = 1.0f/abs(ddx(i.pos.x));

                float realRadius = (_Radius - _Border * 0.5 - 0.5); // radius at the center of the line. -0.5 to keep space for AA
                float2 uvCenter = i.uv * _Radius / realRadius; // uv expressed in realRadius instead of _Radius
                //float uvDist = 1-abs(1-length(uvCenter)); //
                float uvDist = length(uvCenter); // distance to center expressed in realRadius
                float uvBorder = _Border*0.5f / realRadius; // half border width expressed in realdRadius
                float borderDist = abs((uvDist-1) / uvBorder); // distance from center of line expressed in half border
                /*
                if( borderDist > 1) // possible optim : is the early discard is more profitable than the branch ?
                    discard;
                */
                float clipA = tex2D(_GUIClipTexture, i.clipUV).a;
                float pixelBorderSize = _Border*0.5 * pixelScale; // half border expressed on transformed pixel
                borderDist = pixelBorderSize * (1 - borderDist) + 0.5; // signed distance from edge of line in transformed pixel

                //float height = 0.5 + i.pos.y / i.height * 0.5; // height expressed in size.y

                fixed4 color = lerp(_ColorStart, _ColorEnd,i.height);
                return float4(color.rgb,color.a*saturate(borderDist)*clipA);
            }
            ENDCG
        }
    }
}