图形学笔记 -- OpenGL 图形学 / 视差贴图（Parallax Mapping）

// 计算 uv 的偏移 delta
inline float2 ParallaxUvDelta(v2f i)
{
    // 高度图中描述的高度数据
    half h = tex2D(_ParallaxMap, i.uvMain).r;
    // 切线空间中的视线方向
    float3 viewDir = normalize(i.viewDir);
    // 将三维的视线向量投影到二维的 uv 平面，乘以高度数据
    // _ParallaxScale 是一个用户可调节的值，根据效果需要进行调节，数值太大造成视觉上的严重错误
    float2 delta = viewDir.xy / viewDir.z * h * _ParallaxScale;
    return delta;
}

float2 uvDelta = ParallaxUvDelta(i);
i.uvMain += uvDelta;
i.uvBump += uvDelta;

uniform vec3 uEyePosition; // 相机坐标
varying vec3 vEyeVector; // 相机向量

void main()
{
    vec3 EyeVector = uEyePosition - inVertexPosition;

    vEyeVector.x = dot(inVertexTangent, EyeVector);
    vEyeVector.y = dot(inVertexBinormal, EyeVector);
    vEyeVector.z = dot(inVertexNormal, EyeVector);
    vEyeVector *= vec3(1.0, -1.0, -1.0);
}

uniform float uFactor; // 纹理颜色的乘积参数
varying vec3 vEyeVector; // 相机向量

void main()
{
    vec4 TempFetch = texture2D(uTextureUnit1, vTexCoord) *  2.0 - 1.0;
    TempFetch *= uFactor;

    vec3 EyeVector = normalize(vEyeVector);
    vec2 TexCoord = EyeVector.xy * TempFetch.w + vTexCoord;

    // vec4 Color  = texture2D(uTextureUnit0, vTexCoord);
    vec4 Color  = texture2D(uTextureUnit0, TexCoord);
    // vec3 Normal = texture2D(uTextureUnit1, vTexCoord).xyz *  2.0 - 1.0;
    vec3 Normal = texture2D(uTextureUnit1, TexCoord).xyz *  2.0 - 1.0;
}

inline float2 ParallaxUvDelta(Input i)
{
    float3 viewDir = normalize(i.viewDir);

    // 细分的层数
    const float numLayers = 20;

    // 单层步进的高度
    float layerHeight = 1.0 / numLayers;
    // 最高的高度值
    float currentLayerHeight = 1.0;
    // delta 最大值
    float2 P = viewDir.xy * _ParallaxScale;
    // delta 单步逼近值
    float2 deltaTexCoords = P / numLayers;

    // 开始一步步逼近，直到找到合适的红点
    float2 currentTexCoords = i.uv_MainTex;
    float currentDepthMapValue = tex2D(_ParallaxMap, currentTexCoords).r;
    while (currentLayerHeight > currentDepthMapValue)
    {
        currentTexCoords -= deltaTexCoords;
        currentDepthMapValue = tex2D(_ParallaxMap, currentTexCoords).r;
        currentLayerHeight -= layerHeight;
    }

    // 计算 h1 和 h2
    float2 prevTexCoords = currentTexCoords + deltaTexCoords;
    float afterHeight  = currentDepthMapValue - currentLayerHeight;
    float beforeHeight = currentLayerHeight + layerHeight - tex2D(_ParallaxMap, prevTexCoords).r;
    // 利用 h1 h2 得到权重，在两个红点间使用权重进行差值
    float weight = afterHeight / (afterHeight + beforeHeight);
    float2 finalTexCoords = prevTexCoords * weight + currentTexCoords * (1.0 - weight);

    return finalTexCoords - i.uv_MainTex;
}

float2 uvDelta = ParallaxUvDelta(i);
i.uvMain += uvDelta;
i.uvBump += uvDelta;