I tried to get horizontal projection using countNonZero() function as below.
Mat src = imread(INPUT_FILE, CV_LOAD_IMAGE_COLOR);
Mat binaryImage = src.clone();
cvtColor(src, src, CV_BGR2GRAY);
Mat horizontal = Mat::zeros(1,binaryImage.cols, CV_8UC1);
for (int i = 0; i<binaryImage.cols; i++)
{
Mat roi = binaryImage(Rect(0, 0, 1, binaryImage.rows));
horizontal.at<int>(0,i) = countNonZero(roi);
cout << "Col no:" << i << " >>" << horizontal.at<int>(0, i);
}
But an error is occured in the line of calling countonZero() function. Error is as follows.
OpenCV Error: Assertion failed (src.channels() == 1 && func != 0) in cv::countNo
nZero, file C:\builds\2_4_PackSlave-win32-vc12-shared\opencv\modules\core\src\st
at.cpp, line 549
Can somebody please point out the mistake?
Assertion src.channels() == 1 means that image should have 1 channel, i.e. it has to be gray, not colored. You are calling countNonZero on roi, which is a subimage of binaryImage, which is a clone of src, which is originally colored.
I suppose you wanted to write cvtColor(binaryImage, binaryImage, CV_BGR2GRAY);. In this case it makes sense. However, I do not see you using src anywhere again, so perhaps you do not need this intermediate image. In case you do, do not call "binary", since "binary" in computer vision usually stands for black-or-white image, only two colors. Your image is "gray", since it has all shades of black and white.
Concerning your original task, Miki is right, you should use cv::reduce for it. He already gave you an example on how to use it.
BTW, you can compute horizontal projection using reduce giving as argument CV_REDUCE_SUM.
A minimal example:
Mat1b mat(4, 4, uchar(0));
mat(0,0) = uchar(1);
mat(0,1) = uchar(1);
mat(1,1) = uchar(1);
// mat is:
//
// 1100
// 0100
// 0000
// 0000
// Horizontal projection, result would be a column matrix
Mat1i reducedHor;
cv::reduce(mat, reducedHor, 1, CV_REDUCE_SUM);
// reducedHor is:
//
// 2
// 1
// 0
// 0
// Vertical projection, result would be a row matrix
Mat1i reducedVer;
cv::reduce(mat, reducedVer, 0, CV_REDUCE_SUM);
// reducedVer is:
//
// 1200
// Summary
//
// 1100 > 2
// 0100 > 1
// 0000 > 0
// 0000 > 0
//
// vvvv
// 1200
You can use this with your images like this:
// RGB image
Mat3b img = imread("path_to_image");
// Gray image, contains values in [0,255]
Mat1b gray;
cvtColor(img, gray, CV_BGR2GRAY);
// Binary image, contains only 0,1 values
// The sum of pixel values will equal the count of non-zero pixels
Mat1b binary;
threshold(gray, binary, 1, 1, THRESH_BINARY);
// Horizontal projection
Mat1i reducedHor;
cv::reduce(binary, reducedHor, 1, CV_REDUCE_SUM);
// Vertical projection
Mat1i reducedVer;
cv::reduce(binary, reducedVer, 0, CV_REDUCE_SUM);
Related
Problem : Watershed algorithm
I started app project, for image processing, using OpenCv 4.5.3 and Swift ( with C++ ). I'm fighting with watershaded alg. for a really long time... And i have no clue what did i do wrong. Just don't know...
Error :
libc++abi.dylib: terminating with uncaught exception of type cv::Exception: OpenCV(4.5.3)
/Volumes/build-storage/build/master_iOS-mac/opencv/modules/imgproc/src/segmentation.cpp:161:
error: (-215:Assertion failed) src.type()
== CV_8UC3 && dst.type() == CV_32SC1 in function 'watershed'
terminating with uncaught exception of type cv::Exception: OpenCV(4.5.3)
/Volumes/build-storage/build/master_iOS-mac/opencv/modules/imgproc/src/segmentation.cpp:161: error:
(-215:Assertion failed) src.type()
== CV_8UC3 && dst.type() == CV_32SC1 in function 'watershed'
In the definition of openCv's watershed we can find :
#param image Input 8-bit 3-channel image.
#param markers Input/output 32-bit single-channel image (map) of markers. It should have the same size as image .
Code
+(UIImage *) watershed:(UIImage *)src{
cv::Mat img, mask;
UIImageToMat(src, img);
// Change the background from white to black, since that will help later to extract
// better results during the use of Distance Transform
cv::inRange(img, cv::Scalar(255,255,255), cv::Scalar(255,255,255), mask);
img.setTo(cv::Scalar(0,0,0), mask);
// Create a kernel that we will use to sharpen our image
// an approximation of second derivative, a quite strong kernel
cv::Mat kernel = (cv::Mat_<float>(3,3) <<
1, 1, 1,
1, -8, 1,
1, 1, 1);
// do the laplacian filtering as it is
// well, we need to convert everything in something more deeper then CV_8U
// because the kernel has some negative values,
// and we can expect in general to have a Laplacian image with negative values
// BUT a 8bits unsigned int (the one we are working with) can contain values from 0 to 255
// so the possible negative number will be truncated
cv::Mat lapl;
cv::filter2D(img, lapl, CV_32F, kernel);
cv::Mat sharp;
img.convertTo(sharp, CV_32F);
cv::Mat result = sharp - lapl;
// convert back to 8bits gray scale
result.convertTo(result, CV_8UC3);
lapl.convertTo(lapl, CV_8UC3);
cv::Mat bw;
cv::cvtColor(result, bw, cv::COLOR_BGR2GRAY);
cv::threshold(bw, bw, 40, 255, cv::THRESH_BINARY | cv::THRESH_OTSU);
// Perform the distance transform algorithm
cv::Mat dist;
cv::distanceTransform(bw, dist, cv::DIST_L2, cv::DIST_MASK_3);
// Normalize the distance image for range = {0.0, 1.0}
// so we can visualize and threshold it
cv::normalize(dist, dist, 0, 1.0, cv::NORM_MINMAX);
// Threshold to obtain the peaks
// This will be the markers for the foreground objects
cv::threshold(dist, dist, 0.4, 1.0, cv::THRESH_BINARY);
// Dilate a bit the dist image
cv::Mat kernel1 = cv::Mat::ones(3, 3, CV_8U);
dilate(dist, dist, kernel1);
// Create the CV_8U version of the distance image
// It is needed for findContours()
cv::Mat dist_8u;
dist.convertTo(dist_8u, CV_8U);
// Find total markers
std::vector<std::vector<cv::Point> > contours;
findContours(dist_8u, contours, cv::RETR_EXTERNAL, cv::CHAIN_APPROX_SIMPLE);
// Create the marker image for the watershed algorithm
cv::Mat markers = cv::Mat::zeros(dist.size(), CV_32S);
// Draw the foreground markers
for (size_t i = 0; i < contours.size(); i++)
{
drawContours(markers, contours, static_cast<int>(i), cv::Scalar(static_cast<int>(i)+1), -1);
}
// Draw the background marker
circle(markers, cv::Point(5,5), 3, cv::Scalar(255), -1);
cv::Mat markers8u;
markers.convertTo(markers8u, CV_8U, 10);
// Perform the watershed algorithm
watershed(result, markers);
return MatToUIImage(result);
}
You can clearly see, that variables has proper type, as in descr. of function:
result.convertTo(result, CV_8UC3);
cv::Mat markers = cv::Mat::zeros(dist.size(), CV_32S);
The convertTo can not add channels as well can not reduce/convert image to image with smaller amount of channels.
The key in this case is to use :
cvtColor(src, src, COLOR_BGRA2BGR); // change 4 to 3 channels
Firstly I integrate OpenCV framework to XCode and All the OpenCV code is on ObjectiveC and I am using in Swift Using bridging header. I am new to OpenCV Framework and trying to achieve count of vertical lines from the image.
Here is my code:
First I am converting the image to GrayScale
+ (UIImage *)convertToGrayscale:(UIImage *)image {
cv::Mat mat;
UIImageToMat(image, mat);
cv::Mat gray;
cv::cvtColor(mat, gray, CV_RGB2GRAY);
UIImage *grayscale = MatToUIImage(gray);
return grayscale;
}
Then, I am detecting edges so I can find the line of gray color
+ (UIImage *)detectEdgesInRGBImage:(UIImage *)image {
cv::Mat mat;
UIImageToMat(image, mat);
//Prepare the image for findContours
cv::threshold(mat, mat, 128, 255, CV_THRESH_BINARY);
//Find the contours. Use the contourOutput Mat so the original image doesn't get overwritten
std::vector<std::vector<cv::Point> > contours;
cv::Mat contourOutput = mat.clone();
cv::findContours( contourOutput, contours, CV_RETR_EXTERNAL, CV_CHAIN_APPROX_SIMPLE );
NSLog(#"Count =>%lu", contours.size());
//For Blue
/*cv::GaussianBlur(mat, gray, cv::Size(11, 11), 0); */
UIImage *grayscale = MatToUIImage(mat);
return grayscale;
}
This both Function is written on Objective C
Here, I am calling both function Swift
override func viewDidLoad() {
super.viewDidLoad()
let img = UIImage(named: "imagenamed")
let img1 = Wrapper.convert(toGrayscale: img)
self.capturedImageView.image = Wrapper.detectEdges(inRGBImage: img1)
}
I was doing this for some days and finding some useful documents(Reference Link)
OpenCV - how to count objects in photo?
How to count number of lines (Hough Trasnform) in OpenCV
OPENCV Documents
https://docs.opencv.org/2.4/modules/imgproc/doc/structural_analysis_and_shape_descriptors.html?#findcontours
Basically, I understand the first we need to convert this image to black and white, and then using cvtColor, threshold and findContours we can find the colors or lines.
I am attaching the image that vertical Lines I want to get.
Original Image
Output Image that I am getting
I got number of lines count =>10
I am not able to get accurate count here.
Please guide me on this. Thank You!
Since you want to detect the number of the vertical lines, there is a very simple approach I can suggest for you. You already got a clear output and I used this output in my code. Here are the steps before the code:
Preprocess the input image to get the lines clearly
Check each row and check until get a pixel whose value is higher than 100(threshold value I chose)
Then increase the line counter for that row
Continue on that line until get a pixel whose value is lower than 100
Restart from step 3 and finish the image for each row
At the end, check the most repeated element in the array which you assigned line numbers for each row. This number will be the number of vertical lines.
Note: If the steps are difficult to understand, think like this way:
" I am checking the first row, I found a pixel which is higher than
100, now this is a line edge starting, increase the counter for this
row. Search on this row until get a pixel smaller than 100, and then
research a pixel bigger than 100. when row is finished, assign the
line number for this row to a big array. Do this for all image. At the
end, since some lines looks like two lines at the top and also some
noises can occur, you should take the most repeated element in the big
array as the number of lines."
Here is the code part in C++:
#include <vector>
#include <iostream>
#include <opencv2/opencv.hpp>
#include <opencv2/highgui/highgui.hpp>
int main()
{
cv::Mat img = cv::imread("/ur/img/dir/img.jpg",cv::IMREAD_GRAYSCALE);
std::vector<int> numberOfVerticalLinesForEachRow;
cv::Rect r(0,0,img.cols-10,200);
img = img(r);
bool blackCheck = 1;
for(int i=0; i<img.rows; i++)
{
int numberOfLines = 0;
for(int j=0; j<img.cols; j++)
{
if((int)img.at<uchar>(cv::Point(j,i))>100 && blackCheck)
{
numberOfLines++;
blackCheck = 0;
}
if((int)img.at<uchar>(cv::Point(j,i))<100)
blackCheck = 1;
}
numberOfVerticalLinesForEachRow.push_back(numberOfLines);
}
// In this part you need a simple algorithm to check the most repeated element
for(int k:numberOfVerticalLinesForEachRow)
std::cout<<k<<std::endl;
cv::namedWindow("WinWin",0);
cv::imshow("WinWin",img);
cv::waitKey(0);
}
Here's another possible approach. It relies mainly on the cv::thinning function from the extended image processing module to reduce the lines at a width of 1 pixel. We can crop a ROI from this image and count the number of transitions from 255 (white) to 0 (black). These are the steps:
Threshold the image using Otsu's method
Apply some morphology to clean up the binary image
Get the skeleton of the image
Crop a ROI from the center of the image
Count the number of jumps from 255 to 0
This is the code, be sure to include the extended image processing module (ximgproc) and also link it before compiling it:
#include <iostream>
#include <opencv2/opencv.hpp>
#include <opencv2/ximgproc.hpp> // The extended image processing module
// Read Image:
std::string imagePath = "D://opencvImages//";
cv::Mat inputImage = cv::imread( imagePath+"IN2Xh.png" );
// Convert BGR to Grayscale:
cv::cvtColor( inputImage, inputImage, cv::COLOR_BGR2GRAY );
// Get binary image via Otsu:
cv::threshold( inputImage, inputImage, 0, 255, cv::THRESH_OTSU );
The above snippet produces the following image:
Note that there's a little bit of noise due to the thresholding, let's try to remove those isolated blobs of white pixels by applying some morphology. Maybe an opening, which is an erosion followed by dilation. The structuring elements and iterations, though, are not the same, and these where found by experimentation. I wanted to remove the majority of the isolated blobs without modifying too much the original image:
// Apply Morphology. Erosion + Dilation:
// Set rectangular structuring element of size 3 x 3:
cv::Mat SE = cv::getStructuringElement( cv::MORPH_RECT, cv::Size(3, 3) );
// Set the iterations:
int morphoIterations = 1;
cv::morphologyEx( inputImage, inputImage, cv::MORPH_ERODE, SE, cv::Point(-1,-1), morphoIterations);
// Set rectangular structuring element of size 5 x 5:
SE = cv::getStructuringElement( cv::MORPH_RECT, cv::Size(5, 5) );
// Set the iterations:
morphoIterations = 2;
cv::morphologyEx( inputImage, inputImage, cv::MORPH_DILATE, SE, cv::Point(-1,-1), morphoIterations);
This combination of structuring elements and iterations yield the following filtered image:
Its looking alright. Now comes the main idea of the algorithm. If we compute the skeleton of this image, we would "normalize" all the lines to a width of 1 pixel, which is very handy, because we could reduce the image to a 1 x 1 (row) matrix and count the number of jumps. Since the lines are "normalized" we could get rid of possible overlaps between lines. Now, skeletonized images sometimes produce artifacts near the borders of the image. These artifacts resemble thickened anchors at the first and last row of the image. To prevent these artifacts we can extend borders prior to computing the skeleton:
// Extend borders to avoid skeleton artifacts, extend 5 pixels in all directions:
cv::copyMakeBorder( inputImage, inputImage, 5, 5, 5, 5, cv::BORDER_CONSTANT, 0 );
// Get the skeleton:
cv::Mat imageSkelton;
cv::ximgproc::thinning( inputImage, imageSkelton );
This is the skeleton obtained:
Nice. Before we count jumps, though, we must observe that the lines are skewed. If we reduce this image directly to a one row, some overlapping could indeed happen between to lines that are too skewed. To prevent this, I crop a middle section of the skeleton image and count transitions there. Let's crop the image:
// Crop middle ROI:
cv::Rect linesRoi;
linesRoi.x = 0;
linesRoi.y = 0.5 * imageSkelton.rows;
linesRoi.width = imageSkelton.cols;
linesRoi.height = 1;
cv::Mat imageROI = imageSkelton( linesRoi );
This would be the new ROI, which is just the middle row of the skeleton image:
Let me prepare a BGR copy of this just to draw some results:
// BGR version of the Grayscale ROI:
cv::Mat colorROI;
cv::cvtColor( imageROI, colorROI, cv::COLOR_GRAY2BGR );
Ok, let's loop through the image and count the transitions between 255 and 0. That happens when we look at the value of the current pixel and compare it with the value obtained an iteration earlier. The current pixel must be 0 and the past pixel 255. There's more than a way to loop through a cv::Mat in C++. I prefer to use cv::MatIterator_s and pointer arithmetic:
// Set the loop variables:
cv::MatIterator_<cv::Vec3b> it, end;
uchar pastPixel = 0;
int jumpsCounter = 0;
int i = 0;
// Loop thru image ROI and count 255-0 jumps:
for (it = imageROI.begin<cv::Vec3b>(), end = imageROI.end<cv::Vec3b>(); it != end; ++it) {
// Get current pixel
uchar ¤tPixel = (*it)[0];
// Compare it with past pixel:
if ( (currentPixel == 0) && (pastPixel == 255) ){
// We have a jump:
jumpsCounter++;
// Draw the point on the BGR version of the image:
cv::line( colorROI, cv::Point(i, 0), cv::Point(i, 0), cv::Scalar(0, 0, 255), 1 );
}
// current pixel is now past pixel:
pastPixel = currentPixel;
i++;
}
// Show image and print number of jumps found:
cv::namedWindow( "Jumps Found", CV_WINDOW_NORMAL );
cv::imshow( "Jumps Found", colorROI );
cv::waitKey( 0 );
std::cout<<"Jumps Found: "<<jumpsCounter<<std::endl;
The points where the jumps were found are drawn in red, and the number of total jumps printed is:
Jumps Found: 9
I want to calculate numbers of all white pixels within every polygon area efficiently.
Given some processes:
// some codes for reading gray image
// cv::Mat gray = cv::imread("gray.jpg");
// given polygons
// vector< vector<cv::Point> > polygons;
cv::Mat cropped;
cv::Mat mask = cv::Mat::zeros(gray.size(), CV_8UC1);
cv::fillPoly(mask, polygons, cv::Scalar(255));
cv::bitwise_and(gray, gray, cropped, mask);
cv::Mat binary;
cv::threshold(cropped, binary, 20, 255, CV_THRESH_BINARY);
So until now, we can get a image with multiple polygon areas(say we have 3 areas) which have white( with value 255) pixels. Then after some operations we expect to get a vector like:
// some efficient operations
// ...
vector<int> pixelNums;
The size of pixelNums should be same with polygons which is 3 here. And if we print them we may get some outputs like(the values are basically depended on the pre-processes):
index: 0; value: 120
index: 1; value: 1389
index: 2; value: 0
Here is my thought. Counting every pixels within every polygon area with help of cv::countNonZero, but I need to call it within a loop which I don't think it's a efficient way, isn't it?
vector<int> pixelNums;
for(auto polygon : polygons)
{
vector< vector<cv::Point> > temp_polygons;
temp_polygons.push_back(polygon);
cv::Mat cropped;
cv::Mat mask = cv::Mat::zeros(gray.size(), CV_8UC1);
cv::fillPoly(mask, temp_polygons, cv::Scalar(255));
cv::bitwise_and(gray, gray, cropped, mask);
cv::Mat binary;
cv::threshold(cropped, binary, 20, 255, CV_THRESH_BINARY);
pixelNums.push_back(cv::countNonZero(binary));
}
If you have some better ways, please kindly answer this post. Here I say better way is consuming as little time as you can just in cpu environment.
There are some minor improvements that can be done, but all of them combined should provide a decent speedup.
Compute the threshold only once
Make most operations on smaller images, using the bounding box of your polygon to get the region of interest
Avoid unneeded copies in the for loop, use const auto&
Example code:
#include <vector>
#include <opencv2/opencv.hpp>
int main()
{
// Your image
cv::Mat1b gray = cv::imread("path/to/image", cv::IMREAD_GRAYSCALE);
// Your polygons
std::vector<std::vector<cv::Point>> polygons
{
{ {15,120}, {45,200}, {160,160}, {140, 60} },
{ {10,10}, {15,30}, {50,25}, {40, 15} },
// etc...
};
// Compute the threshold just once
cv::Mat1b thresholded = gray > 20;
std::vector<int> pixelNums;
for (const auto& polygon : polygons)
{
// Get bbox of polygon
cv::Rect bbox = cv::boundingRect(polygon);
// Make a new (small) mask
cv::Mat1b mask(bbox.height, bbox.width, uchar(0));
cv::fillPoly(mask, std::vector<std::vector<cv::Point>>{polygon}, cv::Scalar(255), 8, 0, -bbox.tl());
// Get crop
cv::Mat1b cropped = thresholded(bbox) & mask;
// Compute the number of white pixels only on the crop
pixelNums.push_back(cv::countNonZero(cropped));
}
return 0;
}
I am trying to make an average of two blobs in OpenCV. To achieve that I was planning to use watershed algorithm on the image preprocessed in the following way:
cv::Mat common, diff, processed, result;
cv::bitwise_and(blob1, blob2, common); //calc common area of the two blobs
cv::absdiff(blob1, blob2, diff); //calc area where they differ
cv::distanceTransform(diff, processed, CV_DIST_L2, 3); //idea here is that the highest intensity
//will be in the middle of the differing area
cv::normalize(processed, processed, 0, 255, cv::NORM_MINMAX, CV_8U); //convert floats to bytes
cv::Mat watershedMarkers, watershedOutline;
common.convertTo(watershedMarkers, CV_32S, 1. / 255, 1); //change background to label 1, common area to label 2
watershedMarkers.setTo(0, processed); //set 0 (unknown) for area where blobs differ
cv::cvtColor(processed, processed, CV_GRAY2RGB); //watershed wants 3 channels
cv::watershed(processed, watershedMarkers);
cv::rectangle(watershedMarkers, cv::Rect(0, 0, watershedMarkers.cols, watershedMarkers.rows), 1); //remove the outline
//draw the boundary in red (for debugging)
watershedMarkers.convertTo(watershedOutline, CV_16S);
cv::threshold(watershedOutline, watershedOutline, 0, 255, CV_THRESH_BINARY_INV);
watershedOutline.convertTo(watershedOutline, CV_8U);
processed.setTo(cv::Scalar(CV_RGB(255, 0, 0)), watershedOutline);
//convert computed labels back to mask (blob), less relevant but shows my ultimate goal
watershedMarkers.convertTo(watershedMarkers, CV_8U);
cv::threshold(watershedMarkers, watershedMarkers, 1, 0, CV_THRESH_TOZERO_INV);
cv::bitwise_not(watershedMarkers * 255, result);
My problem with the results is that the calculated boundary is (almost) always adjacent to the area common to both blobs. Here are the pictures:
Input markers (black = 0, gray = 1, white = 2)
Watershed input image (distance transform result) with resulting outline drawn in red:
I would expect the boundary to go along the maximum intensity region of the input (that is, along the middle of the differing area). Instead (as you can see) it mostly goes around the area marked as 2, with a bit shifted to touch the background (marked as 1). Do I do something wrong here, or did I misunderstand how watershed works?
Starting from this image:
You can get the correct result simply passing an all-zero image to watershed algorithm. The "basin" is then equally filled of "water" starting from each "side" (then just remember to remove the outer border which is set by default to -1 by watershed algorithm):
Code:
#include <opencv2\opencv.hpp>
using namespace cv;
using namespace std;
int main()
{
Mat1b img = imread("path_to_image", IMREAD_GRAYSCALE);
Mat1i markers(img.rows, img.cols, int(0));
markers.setTo(1, img == 128);
markers.setTo(2, img == 255);
Mat3b image(markers.rows, markers.cols, Vec3b(0,0,0));
markers.convertTo(markers, CV_32S);
watershed(image, markers);
Mat3b result;
cvtColor(img, result, COLOR_GRAY2BGR);
result.setTo(Scalar(0, 0, 255), markers == -1);
imshow("Result", result);
waitKey();
return(0);
}
I'm new to image processing and development. I need to take the inside triangle pixels of the image. In order to do it I used the following code. Unfortunately I obtain unwanted black pixels. get rid of that problem i tried to remove background[0] pixels by giving alfa value.(tranparent background) But it gives following Error. Any help is appreciated.
My code:
Mat img = cv::imread("/home/fabio/code/lena.jpg", cv::IMREAD_GRAYSCALE);
Mat alpha(img.size(), CV_8UC1, Scalar(0));
//triangle definition (example points)
vector<Point> points;
points.push_back(Point(200, 70));
points.push_back(Point(60, 150));
points.push_back(Point(500, 500));
//apply triangle to mask
fillConvexPoly(alpha, points, Scalar(255));
cv::Mat finalImage = cv::Mat::zeros(img.size(), img.type());
img.copyTo(finalImage, alpha);
imshow("image", finalImage);
Mat dst;
Mat rgb[1];
split(finalImage, rgb);
Mat rgba[2] = { finalImage, alpha };
merge(rgba, 2, dst);
imshow("dst", dst);
Error: OpenCV Error: Bad number of channels (Source image must have 1, 3 or 4 channels) in cvConvertImage, file C:\builds\2_4_PackSlave-win64-vc12-shared\opencv\modules\highgui\src\utils.cpp, line 611
use this instead of your last block:
std::vector<cv::Mat> channels;
cv::split(finalImage,m channels);
if(channels.size() == 0)
{
std::cout << "unexpected error" << std::endl;
return 1;
}
// fill up to reach 3 channels
while(channels,size() < 3)
{
channels.push_back(channels[0]);
}
// add alpha channel
channels.push_back(alpha);
cv::merge(channels, dst);
I didn't test it but this should be what you want?