cv::inRange

// 简单实现
 cv::namedWindow("Example 2-3", cv::WINDOW_AUTOSIZE);

    cv::VideoCapture cap;

    cap.open(0);
    cout << "Opened file: " << argv[1] << endl;

    cv::Mat frame, frame_threshold;
    int low_r = 30, low_g = 30, low_b = 30;
    int high_r = 100, high_g = 100, high_b = 100;

    for (;;) {

        cap >> frame;

        if (frame.empty()) break; // Ran out of film

        inRange(frame, cv::Scalar(low_b, low_g, low_r), cv::Scalar(high_b, high_g, high_r), frame_threshold);
        //-- Show the frames
        imshow("Video Capture", frame);
        imshow("Object Detection", frame_threshold);

        if ((char)cv::waitKey(33) >= 0) break;

    }

    return 0;

　　

// 完整实现
#include "opencv2/imgproc.hpp"
#include "opencv2/highgui.hpp"
#include <iostream>
#include <stdlib.h>
using namespace std;
using namespace cv;
void on_low_r_thresh_trackbar(int, void*);
void on_high_r_thresh_trackbar(int, void*);
void on_low_g_thresh_trackbar(int, void*);
void on_high_g_thresh_trackbar(int, void*);
void on_low_b_thresh_trackbar(int, void*);
void on_high_b_thresh_trackbar(int, void*);
int low_r = 30, low_g = 30, low_b = 30;
int high_r = 100, high_g = 100, high_b = 100;
int main()
{
    Mat frame, frame_threshold;
    VideoCapture cap(0);
    namedWindow("Video Capture", WINDOW_NORMAL);
    namedWindow("Object Detection", WINDOW_NORMAL);
    //-- Trackbars to set thresholds for RGB values
    createTrackbar("Low R", "Object Detection", &low_r, 255, on_low_r_thresh_trackbar);
    createTrackbar("High R", "Object Detection", &high_r, 255, on_high_r_thresh_trackbar);
    createTrackbar("Low G", "Object Detection", &low_g, 255, on_low_g_thresh_trackbar);
    createTrackbar("High G", "Object Detection", &high_g, 255, on_high_g_thresh_trackbar);
    createTrackbar("Low B", "Object Detection", &low_b, 255, on_low_b_thresh_trackbar);
    createTrackbar("High B", "Object Detection", &high_b, 255, on_high_b_thresh_trackbar);
    while ((char)waitKey(1) != 'q') {
        cap >> frame;
        if (frame.empty())
            break;
        //-- Detect the object based on RGB Range Values
        inRange(frame, Scalar(low_b, low_g, low_r), Scalar(high_b, high_g, high_r), frame_threshold);
        //-- Show the frames
        imshow("Video Capture", frame);
        imshow("Object Detection", frame_threshold);
    }
    return 0;
}
void on_low_r_thresh_trackbar(int, void*)
{
    low_r = min(high_r - 1, low_r);
    setTrackbarPos("Low R", "Object Detection", low_r);
}
void on_high_r_thresh_trackbar(int, void*)
{
    high_r = max(high_r, low_r + 1);
    setTrackbarPos("High R", "Object Detection", high_r);
}
void on_low_g_thresh_trackbar(int, void*)
{
    low_g = min(high_g - 1, low_g);
    setTrackbarPos("Low G", "Object Detection", low_g);
}
void on_high_g_thresh_trackbar(int, void*)
{
    high_g = max(high_g, low_g + 1);
    setTrackbarPos("High G", "Object Detection", high_g);
}
void on_low_b_thresh_trackbar(int, void*)
{
    low_b = min(high_b - 1, low_b);
    setTrackbarPos("Low B", "Object Detection", low_b);
}
void on_high_b_thresh_trackbar(int, void*)
{
    high_b = max(high_b, low_b + 1);
    setTrackbarPos("High B", "Object Detection", high_b);
}

　　参考：https://www.w3cschool.cn/opencv/opencv-k1vh2cod.html