【发布时间】：2020-11-10 06:49:30
【问题描述】：
我的应用程序要求将 OpenCV VideoCapture 对象用作成员变量。没有办法绕过这个要求。
当使用cv::VideoCapture 作为用户定义类的成员时，我遇到了奇怪的行为。我已经运行了以下代码：
#define int64 opencv_broken_int
#include <opencv2/opencv.hpp>
#undef int64
class Foo {
public:
Foo() = default;
void run(void) {
cv::VideoCapture* cap = new cv::VideoCapture();
cv::VideoCapture g_cap = *cap;
if (not g_cap.open(0)) {
std::cerr << "Cannot open camera 0" << std::endl;
exit(-1);
}
for (int i = 0; i < 10; i++) {
if (not g_cap.isOpened())
std::cerr << "Video capture is not open here!" << std::endl;
cv::Mat f;
g_cap >> f;
if (f.empty()) std::cerr << "Frame was empty" << std::endl; break;
std::string filename = "Foo_test_" + std::to_string(i) + ".tiff";
bool b = cv::imwrite(filename, f);
if (not b) std::cerr << "Error in writing image" << std::endl;
//cv::waitKey(25);
}
};
};
int main(void) {
cv::VideoCapture cap;
if (not cap.open(0)) {
std::cerr << "Cannot open camera 0" << std::endl;
return -1;
}
for (int i =0; i < 10; i++) {
cv::Mat frame;
cap >> frame;
if (frame.empty()) {
std::cerr << "frame is empty" << std::endl;
break;
}
// Else write the image to a file
std::string filename = "test_" + std::to_string(i) + ".tiff";
bool res = cv::imwrite(filename, frame);
if (not res)
std::cerr << "Error in writing image" << std::endl;
}
cap.release();
Foo f;
f.run();
return 0;
}
只为 N = [0,9] 生成 test_N.tiff，即，生成的唯一图像来自 main( ) 中的 cv::VideoCapture 对象，不是来自类 @987654327 @。
我尝试实例化cv::VideoCapture 类型的全局变量g_cap，但我仍然只能从主函数中的对象读取/写入图像。如上面的代码所示，我还尝试将 VideoCapture 对象实例化为指针（如果你愿意的话，这是一个冰雹玛丽），但这也不起作用。但是请注意，将main( ) 中声明的cap 设置为对g_cap 的引用（显然当g_cap 在全局范围内时）给出了相同的输出——从main( ) 中获取图像，但不是从Foo::run( ) 中获取图像根据需要。
请注意另一个奇怪的行为是控制台中没有出现错误消息。这表明Foo 的VideoCapture 类型的成员实际上是打开的，并且将图像帧加载到cv::Mat 类型的f 中不会返回空帧。同样，imwrite 函数不返回 false 表示操作成功。但是，如前所述，没有生成名称为 Foo_test_N.tiff 的文件。
如何解释这种行为？是否有一些要求cv::VideoCapture 可能在不同的范围内？如果图片没有保存或者视频流没有正确打开，会不会像上面的代码那样产生错误信息？
编辑 20201110_1： 回应 Viktor Latypov 在下面的评论： 我已经实施了建议的更改，并且仍在观察相同的行为。编辑：
class Foo {
public:
Foo() = default;
void run(void) {
cv::VideoCapture* cap = new cv::VideoCapture();
//cv::VideoCapture g_cap = *cap;
if (not cap -> open(0)) {
std::cerr << "Cannot open camera 0" << std::endl;
exit(-1);
}
for (int i = 0; i < 10; i++) {
if (not cap -> isOpened())
std::cerr << "Video capture is not open here!" << std::endl;
cv::Mat f;
*cap >> f;
if (f.empty()) std::cerr << "Frame was empty" << std::endl; break;
std::string filename = "Foo_test_" + std::to_string(i) + ".tiff";
bool b = cv::imwrite(filename, f);
if (not b) std::cerr << "Error in writing image" << std::endl;
//cv::waitKey(25);
}
cap -> release();
};
};
编辑 20201110_2： OpenCV 版本是 4.2.0
我尝试从 (a) 函数中捕获图像帧，并 (b) 将在 main( ) 中实例化的 OpenCV VideoCapture 对象作为副本传递给参数化构造函数，并以相同的结果传递指针。完整代码如下：
#define int64 opencv_broken_int
#include <opencv2/opencv.hpp>
#undef int64
class Foo {
public:
Foo() = default;
void run(void) {
cv::VideoCapture* cap = new cv::VideoCapture();
//cv::VideoCapture g_cap = *cap;
if (not cap -> open(0)) {
std::cerr << "Cannot open camera 0" << std::endl;
exit(-1);
}
for (int i = 0; i < 10; i++) {
if (not cap -> isOpened())
std::cerr << "Video capture is not open here!" << std::endl;
cv::Mat f;
*cap >> f;
if (f.empty()) std::cerr << "Frame was empty" << std::endl; break;
std::string filename = "Foo_test_" + std::to_string(i) + ".tiff";
bool b = cv::imwrite(filename, f);
if (not b) std::cerr << "Error in writing image" << std::endl;
//cv::waitKey(25);
}
cap -> release();
};
};
class Bar {
public:
Bar(cv::VideoCapture* c) : cap(c) {};
void run(void) {
//cv::VideoCapture* cap = new cv::VideoCapture();
//cv::VideoCapture g_cap = *cap;
if (not cap -> open(0)) {
std::cerr << "Cannot open camera 0" << std::endl;
exit(-1);
}
for (int i = 0; i < 10; i++) {
if (not cap -> isOpened())
std::cerr << "Video capture is not open here!" << std::endl;
cv::Mat f;
*cap >> f;
if (f.empty()) std::cerr << "Frame was empty" << std::endl; break;
std::string filename = "Bar_test_" + std::to_string(i) + ".tiff";
bool b = cv::imwrite(filename, f);
if (not b) std::cerr << "Error in writing image" << std::endl;
//cv::waitKey(25);
}
cap -> release();
};
cv::VideoCapture* cap;
};
void test(void) {
cv::VideoCapture cap(0);
if (not cap.open(0)) {
std::cerr << "Cannot open camera 0" << std::endl;
exit(-1);
}
for (int i =0; i < 10; i++) {
cv::Mat frame;
cap >> frame;
if (frame.empty()) {
std::cerr << "frame is empty" << std::endl;
break;
}
// Else write the image to a file
std::string filename = "testFunc_test_" + std::to_string(i) + ".tiff";
bool res = cv::imwrite(filename, frame);
if (not res)
std::cerr << "Error in writing image" << std::endl;
}
cap.release();
}
int main(void) {
cv::VideoCapture cap;
if (not cap.open(0)) {
std::cerr << "Cannot open camera 0" << std::endl;
return -1;
}
for (int i =0; i < 10; i++) {
cv::Mat frame;
cap >> frame;
if (frame.empty()) {
std::cerr << "frame is empty" << std::endl;
break;
}
// Else write the image to a file
std::string filename = "main_test_" + std::to_string(i) + ".tiff";
bool res = cv::imwrite(filename, frame);
if (not res)
std::cerr << "Error in writing image" << std::endl;
}
cap.release();
Bar b(&cap); b.run();
Foo f; f.run();
test();
return 0;
}
【问题讨论】：
标签： c++ opencv video-capture