OpenCV: transforming 3 channel image into 4 channel - c++

Question

I am trying to change 3-channel image into 4-channel like this:
cv::VideoCapture video;
video.open("sample.avi");
cv::Mat source;
cv::Mat newSrc;
int from_to = { 0,0, 1,1, 2,2, 3,3 };
for ( int i = 0; i < 1000; i ++ )
{
video >> source;
cv::mixChannels ( source, 2, newSrc, 1, from_to, 4 );
}
Then I got
too many input arguments in function call
for the 'mixChannels' line. Besides, I am not sure whether I am giving the arguments correctly for my goal. Can someone help me? Thank you.

Answer 1

You can convert 3 channel image to 4 channel as follows:
cv::Mat source = cv::imread(path);
cv::Mat newSrc(source.size(), CV_MAKE_TYPE(source.depth(), 4));
int from_to[] = { 0,0, 1,1, 2,2, 2,3 };
cv::mixChannels(&source,1,&newSrc,1,from_to,4);
This way channel 4 will be a duplicate of channel 3. By using a negative number in the from_to list, the output channel is zero filled. eg:
int from_to[] = { 0,0, 1,1, 2,2, -1,3 };

Answer 2

What is the 4th channel supposed to contain?
How about:
VideoCapture cap(0);
Mat frame;
cap >> frame;
Mat RGBA(frame.size(), CV_8UC4, camData);
cv::cvtColor(frame, RGBA, CV_BGR2RGBA, 4);

Answer 3

I think it should be like this:
cv::Mat source = cv::imread(path);
cv::Mat newSrc = cv::Mat(source.rows,source.cols,CV_8UC4);
int from_to[] = { 0,0, 1,1, 2,2, 3,3 };
cv::mixChannels(&source,1,&newSrc,1,from_to, source.channels());
In C++11 you can use initializer lists to provide multiple matrices for batch conversion inline:
cv::mixChannels({{source}}, {{newSrc}}, from_to, source.channels());
We set 3 pairs to be copied, so this leaves the 4 channel empty in newsrc. And 1 in the second and forth parameter means that the pointers source and newSrc point to one element to be processed. The last parameter gives the length of from_to.