start findStartPoint() {
    // ray theory
    interPoint = startPos + t * dir;
    t = (interPoint - startPos) / dir;

    // aabb detestation theory
    // the cube x,y,z coord are in [0-1]
    t1 = (0.0 - startPos.x) / dir.x;
    t2 = (1.0 - startPos.x) / dir.x;
    ..
    t5 = (0.0 - startPos.z) / dir.z;
    t6 = (1.0 - startPos.z) / dir.z;

    // 最后一个进去volume才算整个完全进入
    tmin = max(max(min(t1, t2), min(t2, t3), min(t3, t4)));
    // 最早一个出去的就算已经出去了
    tmax = min(min(max(t1, t2), max(t2, t3), max(t3, t4)));

    // if camera is insi
}

hit fixed_step() {
    // get cur voxel id
    vec3 cur = vec3(posWS.x/voxelSize, ...);
    vec3 dir = normalize(viewDirMS);

    float stepX = dir.x > 0 ? 1 : -1;
    // same for stepY and stepZ

    // find the next voxel
    float next_voxel_x = (cur.x + stepX) * voxelSize;
    // same for next_voxel_y and z

    // compute tmaxX, Y and Z
    // 他们表示从当前的voxel pos到边界，要多少t

    // compute the deltaX, Y and Z
    // 他们按照当前的dir分量，走一个voxel size需要多少t

    for() {
        // follow the minimum tmax
        // every loop, step forward along the dir of minimum tmax
        // tmax += tDelta
    }
}