Weighted Reservoir Sampling（GPU Shader 实现）

一、说明

这是一个 Weighted Reservoir Sampling 算法的 GPU 实现版本，运行在 GPU Shader Pipeline 中，因此源码采用 .glsl 文件格式，而不是 .cpp。

二、Pipeline

1. 读取历史 reservoir
2. 读取邻域 reservoir
3. 选择最大权重 reservoir
4. 计算当前 sample 权重
5. 更新 reservoir
6. 写回 reservoir

三、算法实现

输出变量定义

out vec4 FragColor;

// 结构示意
// struct Reservoir
// {
//     sample
//     weightSum
// }

输入变量 分辨率、注视点位置、历史样本缓冲区

uniform vec2 resolution;
uniform vec2 gazePos;
uniform sampler2D reservoirBuffer;

定义随机数，用于选择当前样本 or 历史样本

float rand(vec2 co)
{
    return fract(
        sin(dot(co,
        vec2(12.9898,78.233)))
        *43758.5453
    );
}

定义比较函数，用于比较两个 sample，返回权重较大的一个

vec4 pickBetter(vec4 A, vec4 B)
{
    if(B.a > A.a)
        return B;

    return A;
}

在主函数中，首先获取当前像素坐标，范围 [0, 1]

vec2 uv = gl_FragCoord.xy / resolution;

读取当前水库样本和周围四个邻近的样本

vec4 reservoir = texture(reservoirBuffer, uv);

vec4 left  = texture(reservoirBuffer, uv + vec2(-1.0/resolution.x,0));
vec4 right = texture(reservoirBuffer, uv + vec2( 1.0/resolution.x,0));
vec4 up    = texture(reservoirBuffer, uv + vec2(0, 1.0/resolution.y));
vec4 down  = texture(reservoirBuffer, uv + vec2(0,-1.0/resolution.y));

选择权重最大的样本作为当前水库样本

reservoir = pickBetter(reservoir,left);
reservoir = pickBetter(reservoir,right);
reservoir = pickBetter(reservoir,up);
reservoir = pickBetter(reservoir,down);

提取历史样本和权重

vec3 historySample = reservoir.rgb;
float weightSum = reservoir.a;

基于gaze，计算当前像素的权重

float dist = distance(uv, gazePos);
float weight = exp(-dist*6.0);

生成当前权重

vec3 currentSample = vec3(weight);

合并当前 sample 与历史 sample，更新权重总和

weightSum += weight;

根据权重概率随机选择当前样本或历史样本

float probability = weight / weightSum;
float r = rand(gl_FragCoord.xy);

vec3 finalSample;

更新sample，写回水库

if(r < probability)
    finalSample = currentSample;
else
    finalSample = historySample;

输出，写回 reservoir

FragColor = vec4(finalSample, weightSum);