C++ 如何知道图像在 OpenCV 中是否为 RGB？
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How can I know if a image is RGB in OpenCV?
提问by Graver
I've made a program in c++ using OpenCV library. The program record video from webcam and then split it in frames. I want to know if the frames are in RGB beacuse i want to access the RGB properties of every pixel. The codec for capture is CV_FOURCC('M','J','P','G'). How can i get the frames in RGB colorspace?
我已经使用 OpenCV 库在 C++ 中制作了一个程序。该程序从网络摄像头录制视频，然后将其分割成帧。我想知道帧是否在 RGB 中，因为我想访问每个像素的 RGB 属性。捕获的编解码器是 CV_FOURCC('M','J','P','G')。如何获得 RGB 色彩空间中的帧？
int main() {
Mat image;
VideoCapture cap(0);
cap.set(CV_CAP_PROP_FPS, 10);
if ( !cap.isOpened() ) {
cout << "ERROR : Cannot open the video file"<<endl;
return -1;
}
namedWindow("MyWindow", CV_WINDOW_AUTOSIZE);
double dWidth = cap.get(CV_CAP_PROP_FRAME_WIDTH);
double dHeight = cap.get(CV_CAP_PROP_FRAME_HEIGHT);
cout << "Frame size :" << dWidth << "x" << dHeight << endl;
Size frameSize(static_cast<int>(dWidth), static_cast<int>(dHeight));
VideoWriter oVideoWriter("E:/myVideo.avi", CV_FOURCC('M', 'J', 'P', 'G'), 10, frameSize, true);
if (!oVideoWriter.isOpened()) {
cout << "ERROR : Failed to write the video"<<endl;
return - 1;
}
while (1) {
Mat image;
bool bSuccess = cap.read(image);
if (!bSuccess) {
cout << "ERROR : Cannot read a frame from video file" << endl;
break;
}
oVideoWriter.write(image);
imshow("MyWindow", image);
if (waitKey(10) == 27) {
saveImages();
cout << "ESC key is pressed by user" << endl;
break
}
}
return 0;
}
int saveImages() {
CvCapture *capture = cvCaptureFromFile("E:/myVideo.avi");
if(!capture)
{
cout<<"!!! cvCaptureFromAVI failed (file not found?)"<<endl;
return -1;
}
int fps = (int) cvGetCaptureProperty(capture, CV_CAP_PROP_FPS);
IplImage* frame = NULL;
int frame_number = 0;
char key = 0;
while (key != 'q')
{
frame = cvQueryFrame(capture);
if (!frame)
{
cout<<"!!! cvQueryFrame failed: no frame"<<endl;
break;
}
char filename[100];
strcpy(filename, "frame_");
char frame_id[30];
_itoa(frame_number, frame_id, 10);
strcat(filename, frame_id);
strcat(filename, ".jpg");
printf("* Saving: %s\n", filename);
if (!cvSaveImage(filename, frame))
{
cout<<"!!! cvSaveImage failed"<<endl;
break;
}
frame_number++;
key = cvWaitKey(1000 / fps);
}
cvReleaseCapture(&capture);
return 0;
}
回答by karlphillip
When OpenCV loads colored images (i.e. 3 channel) from the disk, camera, or a video file, the image data will be stored in the BGRformat. This is a simple test that you can do:
当 OpenCV 从磁盘、相机或视频文件加载彩色图像（即 3 通道）时，图像数据将以BGR格式存储。这是您可以执行的简单测试：
/* Code using the C++ API */
cv::VideoCapture cap(0);
if (!cap.isOpened()) {
std::cout << "!!! Failed to open webcam" << std::endl;
return -1;
}
if (!cap.read(frame)) {
std::cout << "!!! Failed to read a frame from the camera" << std::endl;
return -1;
}
bool is_colored = false;
if (frame.channels() == 3) {
is_colored = true;
}
// Do something with is_colored
// ...
Unless you have a weird camera, the frames will always be colored (and as result, stored as BGR).
除非你有一个奇怪的相机，否则帧将始终是彩色的（因此，存储为 BGR）。
When cv::imwrite()(C++ API) or cvSaveImage()(C API) are called, OpenCV does the proper magic tricks to ensure the data is saved in a compatible way with requested output format (JPG, PNG, AVI, etc) and during this process it automatically converts the data to RGB if it needs to.
当cv::imwrite()(C++ API) 或cvSaveImage()(C API) 被调用时，OpenCV 会执行适当的魔术来确保数据以与请求的输出格式（JPG、PNG、AVI 等）兼容的方式保存，并在此过程中自动转换如果需要，将数据转换为 RGB。
Nevertheless, if for some reason you need to convert the image to RGB you can call:
尽管如此，如果由于某种原因您需要将图像转换为 RGB，您可以调用：
cv::Mat img_rgb;
cv::cvtColor(frame, img_rgb, CV_BGR2RGB);
Please note that OpenCV has a C APIand also a C++ API, and they shouldn't be mixed:
请注意，OpenCV 有一个C API和一个C++ API，它们不应该混合使用：
If you use
IplImagethen stick with the rest of the C API.If you decide to go with
cv::Mat, then keep using the C++ API.
如果您使用，
IplImage则坚持使用 C API 的其余部分。如果您决定使用
cv::Mat，请继续使用 C++ API。
There are different ways to access the pixels of a cv::Mat, here is one of them:
有多种方法可以访问 a 的像素，以下cv::Mat是其中一种：
unsigned char* pixels = (unsigned char*)(frame.data);
for (int i = 0; i < frame.rows; i++)
{
for (int j = 0; j < frame.cols; j++)
{
char b = pixels[frame.step * j + i] ;
char g = pixels[frame.step * j + i + 1];
char r = pixels[frame.step * j + i + 2];
}
}
