Assign HSV values to specific pixels? - c++

Question

I'm looking for an efficient way of assigning values to an element of a 3-channel matrix. In particular, I need to assign HSV values to elements of a 2D cv::Mat which is initialized as follows:
cv::Mat clusterImage(height,width,CV_8UC3,cv::Scalar(0,0,0));
For this matrix, how do I set the pixel in row i and column j to an HSV value (H=59, S=255, V=255), as efficiently as possible?
My current method (complete code) is below. My fear is that splitting a matrix into channels, editing those channels and then merging them back together is not very efficient - especially since I need to do it in a loop, preferably at 30Hz and above. Does a more efficient method exist?
#include <vector>
#include <stdlib.h>
#include <iostream>
#include <opencv/cv.h>
#include <opencv/highgui.h>
using namespace std;
int main() {
int height = 480;
int width = 640;
cv::Mat clusterImage(height,width,CV_8UC3,cv::Scalar(0,0,0));
vector<cv::Mat> channels(3);
// split the channels
split(clusterImage, channels);
// modify the channels
vector<int> i;
vector<int> j;
int numberOfDots = 1000;
for (int k=0; k<numberOfDots; k++) {
i.push_back(rand() % height + 1);
j.push_back(rand() % width + 1);
}
for (int k=0; k<numberOfDots; k++) {
channels[0].at<unsigned char>(i[k],j[k]) = 59;
channels[1].at<unsigned char>(i[k],j[k]) = 255;
channels[2].at<unsigned char>(i[k],j[k]) = 255;
}
// merge channels
merge(channels, clusterImage);
// convert to RGB and draw
cv::cvtColor(clusterImage, clusterImage, CV_HSV2BGR);
imshow("test_window", clusterImage);
cv::waitKey(0);
return 0;
}

Answer 1

This code would be my choice:
int height = 480;
int width = 640;
cv::Mat clusterImage(height,width,CV_8UC3,cv::Scalar(0,0,0));
int numberOfDots = 1000;
int i , j;
for (int k=0; k<numberOfDots; k++)
{
i = rand() % height ; j = rand() % width ;
clusterImage.at<Vec3b>(i , j )[0] = 59;
clusterImage.at<Vec3b>(i , j )[1] = 255;
clusterImage.at<Vec3b>(i , j )[2] = 255;
}
// convert to RGB and draw
cv::cvtColor(clusterImage, clusterImage, CV_HSV2BGR);
imshow("test_window", clusterImage);
cv::waitKey(0);

Answer 2

Yes, you can make this a lot more efficient.
You can assign to a CV::Mat more or less directly. Assuming your system is underlying RGB, simply set up a CV::Mat of width and hight and with three or four channels (often a dummy alpha makes things a bit faster). Then look up the rgb values for HSV 59, 255, 255 - there are plenty of formulae - and set them directly. I think you can use the "at" member function but that's based on a casual glance at the CV::Mat interface.
Finally, you can get rid of the vectors i and j of the dot x, y co-cordinates, assuming you don't need them later on. Just loop on numberOfDots and generatate two temporary random numbers