关于opencv回调函数形参的问题

Question

#include<iostream>
#include<opencv2/opencv.hpp>
#include<stdlib.h>

using namespace cv;
using namespace std;

// 定义全局变量
cv::Mat img_Original;
cv::Mat img_Ranged;
cv::Mat img_Median;

// 定义回调函数

void on_low_height_thresh_trackbar(int, void*);
void on_high_height_thresh_trackbar(int, void*);

int low_height = 50, high_height = 85;

int main()
{
    img_Original = cv::imread("ROI1.jpg", 0);
    cv::medianBlur(img_Original, img_Median, 5);



    // 创建窗口
    cv::namedWindow("original image");
    cv::namedWindow("selected image");
    // 创建回调函数进度条
    cv::createTrackbar("Low height", "selected image", &low_height, 255, on_low_height_thresh_trackbar);
    cv::createTrackbar("High height", "selected image", &high_height, 255, on_high_height_thresh_trackbar);

    // 显示图像
    cv::imshow("original image", img_Original);

    on_low_height_thresh_trackbar(low_height, 0);
    on_high_height_thresh_trackbar(high_height, 0);

    cv::waitKey(0);
    return 0;
}

// 定义回调函数
void on_low_height_thresh_trackbar(int, void*)
{
    low_height = cv::min(high_height - 1, low_height);
    cv::setTrackbarPos("Low height", "selected image", low_height);
    // 选取阈值
    cv::inRange(img_Median, low_height, high_height, img_Ranged);
    // 显示图像
    cv::imshow("selected image", img_Ranged);
}

void on_high_height_thresh_trackbar(int, void*)
{
    high_height = cv::max(high_height, low_height + 1);
    cv::setTrackbarPos("High height", "selected image", high_height);
    // 选取阈值
    cv::inRange(img_Median, low_height, high_height, img_Ranged);
    // 显示图像
    cv::imshow("selected image", img_Ranged);
}

void on_low_height_thresh_trackbar(int, void*)
void on_high_height_thresh_trackbar(int, void*)
在这两个函数中int，void*这两个形参都没有出现，那么设置这两个形参是为什么呢？