Skybox with local cubemap correction

Hi!

I'm looking a solution about a skybox shader that implement the perspective correction as reflection probes does for reflections.

There is some topic about implementing this for reflections, (like this one http://www.gamasutra.com/blogs/Robe...lections_Based_on_Local_Cubemaps_in_Unity.php ) but I didn't find anything about using a skybox as background.

What I'm looking for is something similar to what the CSR Racing2 guys did for their showroom

 

The idea of a standard skybox is that it is infinitely far away and thus does not need perspective correction. Can you post an image of what you are trying to achieve instead?

 

I know that a skybox is meant to be used with a "sky", but sometime you need to render an interior space or something that is near the camera than the infinity

However, yesterday I found myself this solution:

Code (csharp):

1.  
2. Shader "Skybox/Cubemap Parallax" {
3. Properties {
4.     _Tint ("Tint Color", Color) = (.5, .5, .5, .5)
5.     [Gamma] _Exposure ("Exposure", Range(0, 8)) = 1.0
6.     _Rotation ("Rotation", Range(0, 360)) = 0
7.     [NoScaleOffset] _Tex ("Cubemap   (HDR)", Cube) = "grey" {}
8.     _Pos ("Position", Vector) = (0,0,0,0)
9.     _BBoxMin ("BoxSizeMin",Vector) = (1,1,1)
10.     _BBoxMax ("BoxSizeMax",Vector) = (1,1,1)
11. }
12.  
13. SubShader {
14.     Tags { "Queue"="Background" "RenderType"="Background" "PreviewType"="Skybox" }
15.     Cull Off ZWrite Off
16.  
17.     Pass {
18.        
19.         CGPROGRAM
20.         #pragma vertex vert
21.         #pragma fragment frag
22.  
23.         #include "UnityCG.cginc"
24.  
25.         samplerCUBE _Tex;
26.         half4 _Tex_HDR;
27.         half4 _Tint;
28.         half _Exposure;
29.         float _Rotation;
30.         fixed3 _Pos;
31.         fixed3 _BBoxMin;
32.         fixed3 _BBoxMax;
33.  
34.         float3 LocalCorrect(float3 origVec, float3 bboxMin, float3 bboxMax,
35.             float3 vertexPos, float3 cubemapPos)
36.         {
37.            // Find the ray intersection with box plane
38.            float3 invOrigVec = float3(1.0,1.0,1.0)/origVec;
39.            float3 intersecAtMaxPlane = (bboxMax - vertexPos) * invOrigVec;
40.            float3 intersecAtMinPlane = (bboxMin - vertexPos) * invOrigVec;
41.            // Get the largest intersection values
42.            // (we are not intersted in negative values)
43.            float3 largestIntersec = max(intersecAtMaxPlane, intersecAtMinPlane);
44.            // Get the closest of all solutions
45.            float Distance = min(min(largestIntersec.x, largestIntersec.y),
46.                                 largestIntersec.z);
47.            // Get the intersection position
48.            float3 IntersectPositionWS = vertexPos + origVec * Distance;
49.            // Get corrected vector
50.            float3 localCorrectedVec = IntersectPositionWS - cubemapPos;
51.            return localCorrectedVec;
52.         }
53.  
54.         float4 RotateAroundYInDegrees (float4 vertex, float degrees)
55.         {
56.             float alpha = degrees * UNITY_PI / 180.0;
57.             float sina, cosa;
58.             sincos(alpha, sina, cosa);
59.             float2x2 m = float2x2(cosa, -sina, sina, cosa);
60.             return float4(mul(m, vertex.xz), vertex.yw).xzyw;
61.         }
62.        
63.         struct appdata_t {
64.             float4 vertex : POSITION;
65.         };
66.  
67.         struct v2f {
68.             float4 vertex : SV_POSITION;
69.             float3 texcoord : TEXCOORD0;
70.         };
71.  
72.         v2f vert (appdata_t v)
73.         {
74.             v2f o;
75.             o.vertex = mul(UNITY_MATRIX_MVP, RotateAroundYInDegrees(v.vertex, _Rotation));
76.             o.texcoord = v.vertex.xyz;
77.             return o;
78.         }
79.  
80.         fixed4 frag (v2f i) : SV_Target
81.         {
82.             float3 r = LocalCorrect(i.texcoord,_BBoxMin,_BBoxMax,_Pos,float3(0,0,0));
83.             half4 tex = texCUBE (_Tex, r);
84.  
85.             half3 c = DecodeHDR (tex, _Tex_HDR);
86.             c = c * _Tint.rgb * unity_ColorSpaceDouble.rgb;
87.             c *= _Exposure;
88.             return half4(c, 1);
89.         }
90.         ENDCG
91.     }
92. }    
93.  
94.  
95. Fallback Off
96.  
97. }
98.  

and the C# part:

Code (csharp):

1.  
2.  
3. using UnityEngine;
4. using System.Collections;
5.  
6. [ExecuteInEditMode]
7. public class RelativePosition : MonoBehaviour {
8.  
9.  
10.     public Transform cam;
11.     Vector3 pos;
12.     public Material skyboxMaterial;
13.     public BoxCollider bCol;
14.     Vector3 BBMin;
15.     Vector3 BBMax;
16.  
17.     // Use this for initialization
18.     void Start () {
19.         if (cam == null) cam = Camera.main.transform;
20.         skyboxMaterial.SetVector("_BBoxMin",-transform.localScale*0.5f);
21.         skyboxMaterial.SetVector("_BBoxMax",transform.localScale*0.5f);
22.     }
23.    
24.     // Update is called once per frame
25.     void Update () {
26.         GetBB();
27.     }
28.  
29.     void GetBB()
30.     {
31.         pos = cam.position - bCol.center + transform.position;
32.         BBMin = bCol.center - bCol.size*0.5f;
33.         BBMax = bCol.center + bCol.size*0.5f;
34.         skyboxMaterial.SetVector("_BBoxMin",BBMin);
35.         skyboxMaterial.SetVector("_BBoxMax",BBMax);
36.         skyboxMaterial.SetVector("_Pos",pos);
37.     }
38.  
39.     void OnDrawGizmos()
40.     {
41.         Gizmos.color = new Color(1,1,0,0.25f);
42.         Gizmos.DrawCube(bCol.center+ transform.position,bCol.size);
43.         Gizmos.color = Color.yellow;
44.         Gizmos.DrawWireCube(bCol.center+ transform.position,bCol.size);
45.     }
46. }
47.  

You need a gameobject with a BoxCollider in order to work.

With this technique you can use prerendered backgrounds and match your scene camera position.

Feel free to improve and share results