将gstreamer YUV4:2:0 (I420)原始帧转换为OpenCV cv::Mat

#include <gst/gst.h>
#include <opencv2/opencv.hpp>

gst_init(NULL, NULL);

GstElement *pipeline = gst_pipeline_new("pipeline");
GstElement *source = gst_element_factory_make("appsrc", "source");
GstElement *filter = gst_element_factory_make("capsfilter", "filter");
GstElement *sink = gst_element_factory_make("appsink", "sink");

g_object_set(G_OBJECT(filter), "caps", gst_caps_new_simple("video/x-raw",
    "format", G_TYPE_STRING, "I420", NULL), NULL);

gst_bin_add_many(GST_BIN(pipeline), source, filter, sink, NULL);

gst_element_link_many(source, filter, sink, NULL);

cv::Mat frame;

GstFlowReturn new_sample(GstElement *sink, cv::Mat *frame) {
    GstSample *sample = gst_app_sink_pull_sample(GST_APP_SINK(sink));
    GstBuffer *buffer = gst_sample_get_buffer(sample);
    GstMapInfo map;
    gst_buffer_map(buffer, &map, GST_MAP_READ);

    // 将YUV4:2:0帧数据转换为OpenCV的cv::Mat对象
    cv::Mat yuv(map.height * 3 / 2, map.width, CV_8UC1, map.data);
    cv::cvtColor(yuv, *frame, cv::COLOR_YUV2BGR_I420);

    gst_buffer_unmap(buffer, &map);
    gst_sample_unref(sample);

    return GST_FLOW_OK;
}

g_signal_connect(sink, "new-sample", G_CALLBACK(new_sample), &frame);

gst_element_set_state(pipeline, GST_STATE_PLAYING);

while (true) {
    // 在这里可以对frame进行进一步的处理，如图像处理、分析等

    // 检查是否有按键输入，如果有则退出循环
    if (cv::waitKey(1) != -1)
        break;
}

12. 停止管道并释放资源：
```cpp
gst_element_set_state(pipeline, GST_STATE_NULL);
gst_object_unref(GST_OBJECT(pipeline));