CODEWITHSUNDEEP
c++opencv
Missy Zewdie
Missy Zewdie

Reputation: 231

Splitting a color image into three channels using Open CV and C++?

I am new to Open CV, so please forgive me if my question sounds stupid. So, i was studying about this new concept of splitting a BGR channel to individual channels using the split function. I was reading this article(http://answers.opencv.org/question/37132/i-want-to-split-and-show-r-b-g-pictures-why-does-it-not-work/) and i could not understand the code. So, please can anyone explain me the following line of code as i am really wanted to understand the concept.

I did not understand the create blue channel part at all. Please can anyone explain me a bit?

src = imread("pic.png");
vector<mat> spl(3);
split(src,spl);

Mat empty_image =  Mat::zeros(src.rows, src.cols, CV_8UC1);


Mat result_blue(src.rows, src.cols, CV_8UC3); // notice the 3 channels here!

// Create blue channel
Mat in1[] = { spl[0], empty_image, empty_image };
int from_to1[] = { 0,0, 1,1, 2,2 };
mixChannels( in1, 3, &result_blue, 1, from_to1, 3 );

imshow("blue image", result_blue);

Upvotes: 1

Views: 3063

Answers (2)

kcc__
kcc__

Reputation: 1648

Th OpenCV Split Function:

cv:split(src, spl)

Takes a 3 channel 24bit (8 bit for each channel) of type CV_8UC3 and split it into type CV_8UC1 (GrayScale), that is a single channel image or type R, G and B separately.

If you take as an example a Green channel image from spl1 vector and do cv::imshow, you should notice that the green color from the original RGB image will appear as high intensity value in only Green channel image. Likewise for any other channel.

mixChannels( in1, 3, &result_blue, 1, from_to1, 3 );

mixChannel is basically, a function that copies some channel of source image to the new destination image. When using the function you need to specify the the number of channel you need to mix which in your case is 3. 

cv::Mat result_blue(src.rows, src.cols, CV_8UC3);

Note that this variable of type cv::Mat is memory to hold the output image where the channels from src are mixed to the destination image.

1 specifies the number of matrix or image you want in the final output image, that is in cv::Mat result_blue.

int from_to1[] = { 0,0, 1,1, 2,2 };

This array specifies which channel from src needs to be mixed with which channel in the destination. That is 0 will be copied to 0 and so on.

The last param 3 in mixChannel specifies the number of channel pairs in the destination image.

You may also look at this function of merging channels.

The cv::merge() function on the other than takes multiple single channel image and merge it to produce a high level. Note that cv::merge also takes a int, the second param which specifies the number of channels you need to merge.

Lastly, I would suggest that you play with the function to understand them well.

Upvotes: 1

Photon
Photon

Reputation: 3222

What the code does is split the color image into 3 grayscale images, holding the intensities of the red, green and blue.

Then the code takes the blue channel and constructs a color image with a zero red and green, so that when you show it, it will show as bluish, and not just grayscale.

Upvotes: 1

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