Shader "Custom/ColorCube"
{
  Properties
  {
    _MainTex ("Texture", 3D) = "white" {}
    _Alpha ("Alpha", float) = 0.02
    _StepSize ("Step Size", float) = 0.01
  }
  SubShader
  {
    Tags { "Queue" = "Transparent" "RenderType" = "Transparent" }
    Blend One OneMinusSrcAlpha
    LOD 100

    Pass
    {
      CGPROGRAM
      #pragma vertex vert
      #pragma fragment frag

      #include "UnityCG.cginc"

      // Maximum amount of raymarching samples
      #define MAX_STEP_COUNT 128

      // Allowed floating point inaccuracy
      #define EPSILON 0.00001f

      struct appdata
      {
        float4 vertex : POSITION;
      };

      struct v2f
      {
        float4 vertex : SV_POSITION;
        float3 objectVertex : TEXCOORD0;
        float3 vectorToSurface : TEXCOORD1;
      };

      sampler3D _MainTex;
      float4 _MainTex_ST;
      float _Alpha;
      float _StepSize;

      v2f vert (appdata v)
      {
        v2f o;

        // Vertex in object space this will be the starting point of raymarching
        o.objectVertex = v.vertex;

        // Calculate vector from camera to vertex in world space
        float3 worldVertex = mul(unity_ObjectToWorld, v.vertex).xyz;
        o.vectorToSurface = worldVertex - _WorldSpaceCameraPos;

        o.vertex = UnityObjectToClipPos(v.vertex);
        return o;
      }

      float4 BlendUnder(float4 color, float4 newColor)
      {
        color.rgb += (1.0 - color.a) * newColor.a * newColor.rgb;
        color.a += (1.0 - color.a) * newColor.a;
        return color;
      }

      fixed4 frag(v2f i) : SV_Target
      {
        // Start raymarching at the front surface of the object
        float3 rayOrigin = i.objectVertex;

        // Use vector from camera to object surface to get ray direction
        float3 rayDirection = mul(unity_WorldToObject, float4(normalize(i.vectorToSurface), 1));

        float4 color = float4(0, 0, 0, 0);
        float3 samplePosition = rayOrigin;

        // Raymarch through object space
        for (int i = 0; i < MAX_STEP_COUNT; i++)
        {
          // Accumulate color only within unit cube bounds
          if(max(abs(samplePosition.x), max(abs(samplePosition.y), abs(samplePosition.z))) < 0.5f + EPSILON)
          {
            float4 sampledColor = tex3D(_MainTex, samplePosition + float3(0.5f, 0.5f, 0.5f));
            sampledColor.a *= _Alpha;
            color = BlendUnder(color, sampledColor);
            samplePosition += rayDirection * _StepSize;
          }
        }

        return color;
      }
      ENDCG
    }
  }
}