计算出的 fps 如何大于相机声明的 fps？

Question

我正在尝试测量处理来自相机的帧时的每秒帧数。计算没什么特别的，可以在这个问题中找到：我的相机很旧，制造商声明 FPS 不超过 30，分辨率为 640x480。但是，当我运行那些计算时，它显示我在直播中是 40-50。怎么会这样？

更新：代码：

#include <chrono>
#include <iostream>

using std::cerr;
using std::cout;
using std::endl;

#include <string>
#include <numeric>

#include <opencv2/highgui/highgui.hpp>
#include <opencv2/videoio.hpp>
#include <opencv2/imgproc.hpp>

using cv::waitKey;
using cv::Mat;

using time_type = decltype(std::chrono::high_resolution_clock::now());

void showFPS(Mat* frame, const time_type &startTime);

int main(int argc, char** argv) {

    cv::VideoCapture capture;
    std::string videoDevicePath = "/dev/video0";

    if (!capture.open(videoDevicePath)) {
        std::cerr << "Unable to open video capture.";
        return 1;
    }
    //TODO normally through cmd or from cameraParameters.xml
    bool result;
    result = capture.set(CV_CAP_PROP_FOURCC, CV_FOURCC('M', 'J', 'P', 'G'));
    if (result) {
        std::cout << "Camera: PROP_FOURCC: MJPG option set.";
    } else {
        std::cerr << "Camera: PROP_FOURCC: MJPG option was not set.";
    }
    result = capture.set(CV_CAP_PROP_FRAME_WIDTH, 640);
    if (result) {
        std::cout << "Camera: PROP_FRAME_WIDTH option set.";
    } else {
        std::cerr << "Camera: PROP_FRAME_WIDTH option was not set.";
    }
    result = capture.set(CV_CAP_PROP_FRAME_HEIGHT, 480);
    if (result) {
        std::cout << "Camera: PROP_FRAME_HEIGHT option set.";
    } else {
        std::cerr << "Camera: PROP_FRAME_HEIGHT option was not set.";
    }
    result = capture.set(CV_CAP_PROP_FPS, 30);
    if (result) {
        std::cout << "Camera: PROP_FPS option set.";
    } else {
        std::cerr << "Camera: PROP_FPS option was not set.";
    }

    Mat frame, raw;
    while (cv::waitKey(5) != 'q') {
        auto start = std::chrono::high_resolution_clock::now();
        capture >> raw;

        if (raw.empty()) {
            return 1;
        }
        if (raw.channels() > 1) {
            cv::cvtColor(raw, frame, CV_BGR2GRAY);
        } else {
            frame = raw;
        }
        showFPS(&raw1, start);
    }
    return 0;
}

void showFPS(Mat* frame, const time_type &startTime) {
    typedef std::chrono::duration<float> fsec_t;

    auto stopTime = std::chrono::high_resolution_clock::now();
    fsec_t duration = stopTime - startTime;

    double sec = duration.count();
    double fps = (1.0 / sec);
    std::stringstream s;
    s << "FPS: " << fps;
    cv::putText(*frame, s.str(), Point2f(20, 20), constants::font,
                constants::fontScale, constants::color::green);
}

Answer 1

Camera's FPS 是相机每秒可以提供的帧数。这意味着相机每 33 毫秒提供一个新帧。

另一方面，您测量的不是 FPS。您正在测量新帧检索和颜色转换功能的倒数时间。根据您的结果，这次是 20-25 毫秒。

这不是测量 FPS 的正确方法，至少因为您不能保证这两个进程的同步。

如果你想正确测量FPS，你可以测量显示最后N帧的时间。

伪代码：

counter = 0;
start = getTime();
N = 100;

while (true) {
  captureFrame();
  convertColor();
  counter++;

  if (counter == N) {
    fps = N / (getTime() - start);
    printFPS(fps);

    counter = 0; 
    start = getTime();
  }
}

Answer 2

你中间有一个cvtColor，所以它会影响你的时间计算，因为cvtColor的过程时间在每个循环中可能会有所不同（可能是因为windows的其他过程).

Consider this example:

You get the first frame with capture at moment 0, then do a cvtColor and that takes e.g. 10 ms, then you make a stopTime at moment 10 ms. 23 ms later (33-10) you capture the second frame. But this time cvtColor takes 5 ms (It could happen) and you make the second stopTime at moment 38 (33+5), so the first tick was at moment 10 and the second tick is at moment 38. Now your fps becomes

1000/(38-10) = 35.7

Answer 3

Aleksey Petrov 的回答还不错，但是虽然对最后 N 帧进行平均会给出更平滑的值，但无需平均也可以相对准确地测量帧速率。这里修改了问题的代码来做到这一点：

    // see question for earlier code
    Mat frame, raw;
    time_type prevTimePoint;   // default-initialized to epoch value
    while (waitKey(1) != 'q') {
        capture >> raw;
        auto timePoint = std::chrono::high_resolution_clock::now();

        if (raw.empty()) {
            return 1;
        }
        if (raw.channels() > 1) {
            cv::cvtColor(raw, frame, CV_BGR2GRAY);
        } else {
            frame = raw;
        }

        showFPS(&frame, prevTimePoint, timePoint);

        cv::imshow("frame", frame);
    }
    return 0;
}

void showFPS(Mat* frame, time_type &prevTimePoint, const time_type &timePoint) {
    if (prevTimePoint.time_since_epoch().count()) {
        std::chrono::duration<float> duration = timePoint - prevTimePoint;
        cv::putText(*frame, "FPS: " + std::to_string(1/duration.count()),
            cv::Point2f(20, 40), 2, 2, cv::Scalar(0,255,0));
    }
    prevTimePoint = timePoint;
}

请注意，这会测量 capture >> raw returns 之后的时间点，这（不影响 OpenCV）是相机发送帧时最接近的时间点，并且时间每个循环仅测量一次并与之前的测量结果进行比较，这给出了非常精确的当前帧速率。当然，如果处理时间超过1/(帧率)，测量将关闭。

问题的代码给出过高帧率的原因实际上是两次时间测量之间的代码：showFPS() 中的 now() 和 [=15 中的 now() =] 循环。我的预感是这个代码包含 cv::imshow()，它不在问题中，并且与 cv::waitKey(5) 和 cv::putText() 一起可能负责帧速率计算中的大部分 "missing time" （导致帧率过高）。

计算出的 fps 如何大于相机声明的 fps？

How calculated fps can be greater than camera's declared fps?

c++

camera

opencv

frame-rate