TrailShader

Shader "Trail"
{
    Properties
    {
        _Color("Main Color", Color) = (1,1,1,1)
        _Width("Width", float) = 0.2
    }

    SubShader
    {
        Tags
        {
            "Queue" = "Transparent"
            "IgnoreProjector" = "True"
            "RenderType" = "Opaque"
        }

        Pass
        {
            Cull Back
            Lighting Off
            ZWrite Off
            Blend OneMinusDstColor OneMinusSrcColor

            CGPROGRAM

            #pragma vertex vert
            #pragma fragment frag

            #include "MyriadCG.cginc"

            uniform fixed4 _Color;
            uniform float2 _Trail[600];
            uniform int _Samples;
            uniform float _Width;
            uniform float2 _Ref;

            struct v2f
            {
                float4 vertex : SV_POSITION;
            };

            inline float unlerp(float min, float max, float value)
            {
                return clamp((value - min) / (max - min), 0, 1);
            }

            v2f vert(uint index : SV_VertexID)
            {
                int sign = ((index % 2) * 2) - 1;
                int current = clamp(floor((float)index * 0.5), 0, _Samples);
                int prev = current + 1;

                float2 prevPosition = mul(unity_WorldToObject, _Trail[current] - _Ref);
                float2 currentPosition= mul(unity_WorldToObject, _Trail[prev] - _Ref);

                float2 direction = normalize(currentPosition - prevPosition);
                float2 perpendicular = float2(-direction.y, direction.x) * sign;

                v2f output;
                output.vertex = UnityObjectToClipPos(float4(currentPosition + (perpendicular * 0.2 * 0.5 * unlerp(0, _Samples, current)), 0, 0));
                return output;
            }

            fixed4 frag(void) : COLOR
            {
                return _Color;
            }

            ENDCG
        }
    }
}