google street viewの画像をSkybox/Panoramicのシェーダで表示しつつ、順番に切り替えていきたい。
切り替え時は瞬間的に変更するのではなく、次の画像とブレンドした画像を表示しながら切り替えていきたい。
しかし、なぜかPanoramic画像をブレンドするシェーダが見当たらない。

ということで、ビルトインのSkybox/Panoramicシェーダを改造して、2つの画像をブレンドするようにした。
以下の_MainTexAと_MainTexBに画像入れると、valueの割合でブレンドされる。

// Unity built-in shader source. Copyright (c) 2016 Unity Technologies. MIT license (see license.txt)

Shader "Panoramic-Skybox-Blend" 
{
Properties 
{
    _Tint ("Tint Color", Color) = (.5, .5, .5, .5)
    [Gamma] _Exposure ("Exposure", Range(0, 8)) = 1.0
    _Rotation ("Rotation", Range(0, 360)) = 0
    [NoScaleOffset] _MainTexA ("Spherical1", 2D) = "grey" {}
    [NoScaleOffset] _MainTexB ("Spherical2", 2D) = "grey" {}
    _value ("Value", Range (0, 1)) = 0.5
    [KeywordEnum(6 Frames Layout, Latitude Longitude Layout)] _Mapping("Mapping", Float) = 1
    [Enum(360 Degrees, 0, 180 Degrees, 1)] _ImageType("Image Type", Float) = 0
    [Toggle] _MirrorOnBack("Mirror on Back", Float) = 0
    [Enum(None, 0, Side by Side, 1, Over Under, 2)] _Layout("3D Layout", Float) = 0
}

SubShader {
    Tags { "Queue"="Background" "RenderType"="Background" "PreviewType"="Skybox" }
    Cull Off ZWrite Off

    Pass {

        CGPROGRAM
        #pragma vertex vert
        #pragma fragment frag
        #pragma target 2.0
        #pragma multi_compile_local __ _MAPPING_6_FRAMES_LAYOUT

        #include "UnityCG.cginc"

        sampler2D _MainTexA;
        sampler2D _MainTexB;
        float _value;
        float4 _MainTex_TexelSize;
        half4 _MainTex_HDR;
        half4 _Tint;
        half _Exposure;
        float _Rotation;
# ifndef _MAPPING_6_FRAMES_LAYOUT
        bool _MirrorOnBack;
        int _ImageType;
        int _Layout;
# endif

# ifndef _MAPPING_6_FRAMES_LAYOUT
        inline float2 ToRadialCoords(float3 coords)
        {
            float3 normalizedCoords = normalize(coords);
            float latitude = acos(normalizedCoords.y);
            float longitude = atan2(normalizedCoords.z, normalizedCoords.x);
            float2 sphereCoords = float2(longitude, latitude) * float2(0.5/UNITY_PI, 1.0/UNITY_PI);
            return float2(0.5,1.0) - sphereCoords;
        }
# endif

# ifdef _MAPPING_6_FRAMES_LAYOUT
        inline float2 ToCubeCoords(float3 coords, float3 layout, float4 edgeSize, float4 faceXCoordLayouts, float4 faceYCoordLayouts, float4 faceZCoordLayouts)
        {
            // Determine the primary axis of the normal
            float3 absn = abs(coords);
            float3 absdir = absn > float3(max(absn.y,absn.z), max(absn.x,absn.z), max(absn.x,absn.y)) ? 1 : 0;
            // Convert the normal to a local face texture coord [-1,+1], note that tcAndLen.z==dot(coords,absdir)
            // and thus its sign tells us whether the normal is pointing positive or negative
            float3 tcAndLen = mul(absdir, float3x3(coords.zyx, coords.xzy, float3(-coords.xy,coords.z)));
            tcAndLen.xy /= tcAndLen.z;
            // Flip-flop faces for proper orientation and normalize to [-0.5,+0.5]
            bool2 positiveAndVCross = float2(tcAndLen.z, layout.x) > 0;
            tcAndLen.xy *= (positiveAndVCross[0] ? absdir.yx : (positiveAndVCross[1] ? float2(absdir[2],0) : float2(0,absdir[2]))) - 0.5;
            // Clamp values which are close to the face edges to avoid bleeding/seams (ie. enforce clamp texture wrap mode)
            tcAndLen.xy = clamp(tcAndLen.xy, edgeSize.xy, edgeSize.zw);
            // Scale and offset texture coord to match the proper square in the texture based on layout.
            float4 coordLayout = mul(float4(absdir,0), float4x4(faceXCoordLayouts, faceYCoordLayouts, faceZCoordLayouts, faceZCoordLayouts));
            tcAndLen.xy = (tcAndLen.xy + (positiveAndVCross[0] ? coordLayout.xy : coordLayout.zw)) * layout.yz;
            return tcAndLen.xy;
        }
# endif

        float3 RotateAroundYInDegrees (float3 vertex, float degrees)
        {
            float alpha = degrees * UNITY_PI / 180.0;
            float sina, cosa;
            sincos(alpha, sina, cosa);
            float2x2 m = float2x2(cosa, -sina, sina, cosa);
            return float3(mul(m, vertex.xz), vertex.y).xzy;
        }

        struct appdata_t {
            float4 vertex : POSITION;
            UNITY_VERTEX_INPUT_INSTANCE_ID
        };

        struct v2f {
            float4 vertex : SV_POSITION;
            float3 texcoord : TEXCOORD0;
# ifdef _MAPPING_6_FRAMES_LAYOUT
            float3 layout : TEXCOORD1;
            float4 edgeSize : TEXCOORD2;
            float4 faceXCoordLayouts : TEXCOORD3;
            float4 faceYCoordLayouts : TEXCOORD4;
            float4 faceZCoordLayouts : TEXCOORD5;
# else
            float2 image180ScaleAndCutoff : TEXCOORD1;
            float4 layout3DScaleAndOffset : TEXCOORD2;
# endif
            UNITY_VERTEX_OUTPUT_STEREO
        };

        v2f vert (appdata_t v)
        {
            v2f o;
            UNITY_SETUP_INSTANCE_ID(v);
            UNITY_INITIALIZE_VERTEX_OUTPUT_STEREO(o);
            float3 rotated = RotateAroundYInDegrees(v.vertex, _Rotation);
            o.vertex = UnityObjectToClipPos(rotated);
            o.texcoord = v.vertex.xyz;
# ifdef _MAPPING_6_FRAMES_LAYOUT
            // layout and edgeSize are solely based on texture dimensions and can thus be precalculated in the vertex shader.
            float sourceAspect = float(_MainTex_TexelSize.z) / float(_MainTex_TexelSize.w);
            // Use the halfway point between the 1:6 and 3:4 aspect ratios of the strip and cross layouts to
            // guess at the correct format.
            bool3 aspectTest =
                sourceAspect >
                float3(1.0, 1.0f / 6.0f + (3.0f / 4.0f - 1.0f / 6.0f) / 2.0f, 6.0f / 1.0f + (4.0f / 3.0f - 6.0f / 1.0f) / 2.0f);
            // For a given face layout, the coordinates of the 6 cube faces are fixed: build a compact representation of the
            // coordinates of the center of each face where the first float4 represents the coordinates of the X axis faces,
            // the second the Y, and the third the Z. The first two float componenents (xy) of each float4 represent the face
            // coordinates on the positive axis side of the cube, and the second (zw) the negative.
            // layout.x is a boolean flagging the vertical cross layout (for special handling of flip-flops later)
            // layout.yz contains the inverse of the layout dimensions (ie. the scale factor required to convert from
            // normalized face coords to full texture coordinates)
            if (aspectTest[0]) // horizontal
            {
                if (aspectTest[2])
                { // horizontal strip
                    o.faceXCoordLayouts = float4(0.5,0.5,1.5,0.5);
                    o.faceYCoordLayouts = float4(2.5,0.5,3.5,0.5);
                    o.faceZCoordLayouts = float4(4.5,0.5,5.5,0.5);
                    o.layout = float3(-1,1.0/6.0,1.0/1.0);
                }
                else
                { // horizontal cross
                    o.faceXCoordLayouts = float4(2.5,1.5,0.5,1.5);
                    o.faceYCoordLayouts = float4(1.5,2.5,1.5,0.5);
                    o.faceZCoordLayouts = float4(1.5,1.5,3.5,1.5);
                    o.layout = float3(-1,1.0/4.0,1.0/3.0);
                }
            }
            else
            {
                if (aspectTest[1])
                { // vertical cross
                    o.faceXCoordLayouts = float4(2.5,2.5,0.5,2.5);
                    o.faceYCoordLayouts = float4(1.5,3.5,1.5,1.5);
                    o.faceZCoordLayouts = float4(1.5,2.5,1.5,0.5);
                    o.layout = float3(1,1.0/3.0,1.0/4.0);
                }
                else
                { // vertical strip
                    o.faceXCoordLayouts = float4(0.5,5.5,0.5,4.5);
                    o.faceYCoordLayouts = float4(0.5,3.5,0.5,2.5);
                    o.faceZCoordLayouts = float4(0.5,1.5,0.5,0.5);
                    o.layout = float3(-1,1.0/1.0,1.0/6.0);
                }
            }
            // edgeSize specifies the minimum (xy) and maximum (zw) normalized face texture coordinates that will be used for
            // sampling in the texture. Setting these to the effective size of a half pixel horizontally and vertically
            // effectively enforces clamp mode texture wrapping for each individual face.
            o.edgeSize.xy = _MainTex_TexelSize.xy * 0.5 / o.layout.yz - 0.5;
            o.edgeSize.zw = -o.edgeSize.xy;
# else // !_MAPPING_6_FRAMES_LAYOUT
            // Calculate constant horizontal scale and cutoff for 180 (vs 360) image type
            if (_ImageType == 0)  // 360 degree
                o.image180ScaleAndCutoff = float2(1.0, 1.0);
            else  // 180 degree
                o.image180ScaleAndCutoff = float2(2.0, _MirrorOnBack ? 1.0 : 0.5);
            // Calculate constant scale and offset for 3D layouts
            if (_Layout == 0) // No 3D layout
                o.layout3DScaleAndOffset = float4(0,0,1,1);
            else if (_Layout == 1) // Side-by-Side 3D layout
                o.layout3DScaleAndOffset = float4(unity_StereoEyeIndex,0,0.5,1);
            else // Over-Under 3D layout
                o.layout3DScaleAndOffset = float4(0, 1-unity_StereoEyeIndex,1,0.5);
# endif
            return o;
        }

        fixed4 frag (v2f i) : SV_Target
        {
# ifdef _MAPPING_6_FRAMES_LAYOUT
            float2 tc = ToCubeCoords(i.texcoord, i.layout, i.edgeSize, i.faceXCoordLayouts, i.faceYCoordLayouts, i.faceZCoordLayouts);
# else
            float2 tc = ToRadialCoords(i.texcoord);
            if (tc.x > i.image180ScaleAndCutoff[1])
                return half4(0,0,0,1);
            tc.x = fmod(tc.x*i.image180ScaleAndCutoff[0], 1);
            tc = (tc + i.layout3DScaleAndOffset.xy) * i.layout3DScaleAndOffset.zw;
# endif

            half4 texA = tex2D (_MainTexA, tc);
            half4 texB = tex2D (_MainTexB, tc);
            half4 tex = lerp(texA, texB, _value);
            half3 c = DecodeHDR (tex, _MainTex_HDR);
            c = c * _Tint.rgb * unity_ColorSpaceDouble.rgb;
            c *= _Exposure;
            return half4(c, 1);
        }
        ENDCG
    }
}
//CustomEditor "SkyboxPanoramicShaderGUI"
Fallback Off
}

ライセンスのlicense.txtは、以下から入手
https://unity3d.com/jp/get-unity/download/archive

切り替えの簡易スクリプト。
matに登録するマテリアルに、先のシェーダを使っておく。
あらかじめTextures変数にstreet viewのパノラマ画像をリストで保存しておく。

street viewの画像は、事前にすべて同じ方角を向くように揃えておくこと。(おそらくデフォルトだと車の進行方向を向いているので、写真によって向きがばらついているはず)
揃えていない場合は切り替えるごとに方角が変わってしまうので、切り替え時に方角を補正するように_Rotationもいじる必要がある。Street view画像のNorth rotationの値が北向きからの方角のズレなので、_Rotationをその値に設定すれば良い(rotation処理は2枚の画像で分けていないので、フェード前の画像のrotationも切り替わってしまうけれど)


using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class StreetViewWalker : MonoBehaviour
{

    public Material mat;

    //street viewパノラマ画像のリスト
    public List<Texture2D> Textures;

    //現在のテクスチャ番号
    private int nowtexture = 0;

    //切り替え時間
    private float fadetime = 0.5f;

    void Start()
    {
        RenderSettings.skybox = mat;
        mat.SetFloat("_value", 0);
    }

    public void moveMap()
    {
        nowtexture += 1;
        if (nowtexture < Textures.Count) {
            mat.SetTexture("_MainTexA", Textures[nowtexture-1]);
            mat.SetTexture("_MainTexB", Textures[nowtexture]);

            StartCoroutine(Fade());
        }
    }

    private IEnumerator Fade()
    {
        float i = 0;
        while (i < 1f)
        {
            mat.SetFloat("_value", i);
            i += Time.deltaTime / fadetime;
            yield return null;
        }
    }

    void Update()
    {
        //デバッグ時にキーボードで切り替え
        if (Input.GetKeyDown("t"))
        {
            moveMap();
        }
    }   
}