有一段代码不是很理解，有哪位大佬帮我解下惑

[yanchujian]

void UltraFace::generateBBox(std::vector &bbox_collection, MNN::Tensor *scores, MNN::Tensor *boxes) {
for (int i = 0; i < num_anchors; i++) {
if (scores->host()[i * 2 + 1] > score_threshold) {
FaceInfo rects;
float x_center = boxes->host()[i * 4] * center_variance * priors[i][2] + priors[i][0];
float y_center = boxes->host()[i * 4 + 1] * center_variance * priors[i][3] + priors[i][1];
float w = exp(boxes->host()[i * 4 + 2] * size_variance) * priors[i][2];
float h = exp(boxes->host()[i * 4 + 3] * size_variance) * priors[i][3];

        rects.x1 = clip(x_center - w / 2.0, 1) * image_w;
        rects.y1 = clip(y_center - h / 2.0, 1) * image_h;
        rects.x2 = clip(x_center + w / 2.0, 1) * image_w;
        rects.y2 = clip(y_center + h / 2.0, 1) * image_h;
        rects.score = clip(scores->host<float>()[i * 2 + 1], 1);
        bbox_collection.push_back(rects);
    }
}

}
就是这段代码中 ，x_center y_center w h这几个参数为什么以这样的方式生成，推理过程中为什么先有先验框的信息，这是rfb这个网络独有的吗

[yanchujian]

经查看python代码 大致理解。