struct v2f {
    float3 worldPos : TEXCOORD0;
    // these three vectors will hold a 3x3 rotation matrix
    // that transforms from tangent to world space
    half3 tspace0 : TEXCOORD1; // tangent.x, bitangent.x, normal.x
    half3 tspace1 : TEXCOORD2; // tangent.y, bitangent.y, normal.y
    half3 tspace2 : TEXCOORD3; // tangent.z, bitangent.z, normal.z
    // texture coordinate for the normal map
    float2 uv : TEXCOORD4;
    float4 pos : SV_POSITION;
};

// vertex shader now also needs a per-vertex tangent vector.
// in Unity tangents are 4D vectors, with the .w component used to
// indicate direction of the bitangent vector.
// we also need the texture coordinate.
v2f vert (float4 vertex : POSITION, float3 normal : NORMAL, float4 tangent : TANGENT, float2 uv : TEXCOORD0)
{
    v2f o;
    o.pos = UnityObjectToClipPos(vertex);
    o.worldPos = mul(_Object2World, vertex).xyz;
    half3 wNormal = UnityObjectToWorldNormal(normal);
    half3 wTangent = UnityObjectToWorldDir(tangent.xyz);
    // compute bitangent from cross product of normal and tangent
    half tangentSign = tangent.w * unity_WorldTransformParams.w;
    half3 wBitangent = cross(wNormal, wTangent) * tangentSign;
    // output the tangent space matrix
    o.tspace0 = half3(wTangent.x, wBitangent.x, wNormal.x);
    o.tspace1 = half3(wTangent.y, wBitangent.y, wNormal.y);
    o.tspace2 = half3(wTangent.z, wBitangent.z, wNormal.z);
    o.uv = uv;
    return o;
}

// normal map texture from shader properties
sampler2D _BumpMap;

fixed4 frag (v2f i) : SV_Target
{
    // sample the normal map, and decode from the Unity encoding
    half3 tnormal = UnpackNormal(tex2D(_BumpMap, i.uv));
    // transform normal from tangent to world space
    half3 worldNormal;
    worldNormal.x = dot(i.tspace0, tnormal);
    worldNormal.y = dot(i.tspace1, tnormal);
    worldNormal.z = dot(i.tspace2, tnormal);

    // rest the same as in previous shader
    half3 worldViewDir = normalize(UnityWorldSpaceViewDir(i.worldPos));
    half3 worldRefl = reflect(-worldViewDir, worldNormal);
    half4 skyData = UNITY_SAMPLE_TEXCUBE(unity_SpecCube0, worldRefl);
    half3 skyColor = DecodeHDR (skyData, unity_SpecCube0_HDR);
    fixed4 c = 0;
    c.rgb = skyColor;
    return c;
}

// ...
// 当前uv的右上角相邻格子的uv存入uv[0]
o.uv[0] = i.texcoord + _MainTex_TexelSize.xy * half2(1,1);