Access to each separate channel in OpenCV

I have an image with 3 channels (img) and another one with a single channel (ch1).

    Mat img(5,5,CV_64FC3);
    Mat ch1 (5,5,CV_64FC1);

Is there any efficient w开发者_如何学JAVAay (not using for loop) to copy the first channel of img to ch1?

In fact, if you just want to copy one of the channels or split the color image in 3 different channels, CvSplit() is more appropriate (I mean simple to use).

Mat img(5,5,CV_64FC3);
Mat ch1, ch2, ch3;
// "channels" is a vector of 3 Mat arrays:
vector<Mat> channels(3);
// split img:
split(img, channels);
// get the channels (dont forget they follow BGR order in OpenCV)
ch1 = channels[0];
ch2 = channels[1];
ch3 = channels[2];

There is a function called cvMixChannels. You'll need to see implementation in the source code, but I bet it is well optimized.

You can use split function and then put zeros to the channels u want to ignore. This will result dispalying one channels out of three. See below..

For example:

Mat img, chans[3]; 
img = imread(.....);  //make sure its loaded with an image

//split the channels in order to manipulate them
split(img, chans);

//by default opencv put channels in BGR order , so in your situation you want to copy the first channel which is blue. Set green and red channels elements to zero.
chans[1]=Mat::zeros(img.rows, img.cols, CV_8UC1); // green channel is set to 0
chans[2]=Mat::zeros(img.rows, img.cols, CV_8UC1);// red channel is set to 0

//then merge them back
merge(chans, 3, img);

//display 
imshow("BLUE CHAN", img);
cvWaitKey();

You can access a specific channel, it works faster than the split operation

Mat img(5,5,CV_64FC3);
Mat ch1;
int channelIdx = 0;
extractChannel(img, ch1, channelIdx); // extract specific channel

// or extract them all
vector<Mat> channels(3);
split(img, channels);
cout << channels[0].size() << endl;

A simpler one if you have a RGB with 3 channels is cvSplit() if i'm not wrong, you have less to configure... (and i think it is also well optimized).

I would use cvMixChannel() for "harder" tasks... :p (i know i am lazy).

here is the documentation for cvSplit()