Re-coloring image preserving luminance - opencv

Question

Given an input image, I was thinking about how the image could be re-colored to a single new color keeping the luminance of the image similar to what it was earlier.
So I wrote a naive code:
#include <opencv2/imgproc/imgproc.hpp>
#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <bits/stdc++.h>
using namespace cv;
using namespace std;
int main() {
Mat img = imread("test2.png", 1);
Mat hsv; cvtColor(img, hsv, CV_BGR2HSV);
vector<Mat > channels;split(hsv, channels);
Mat luminance; channels[2].copyTo(luminance);
Mat res; img.copyTo(res);
channels.clear(); split(res, channels);
for (int i = 0; i<res.rows; i++) {
for (int j = 0; j<res.cols; j++) {
channels[0].at<uchar>(i, j) = 0;
channels[1].at<uchar>(i, j) = 0;
channels[2].at<uchar>(i, j) = 255;
}
}
merge(channels, res);
cvtColor(res, hsv, CV_BGR2HSV);
channels.clear(); split(hsv, channels);
luminance.copyTo(channels[2]);
merge(channels, res);
cvtColor(res, res, CV_HSV2BGR);
imwrite("result.png", res);
return 0;
}
What I actually did is just extracted the luminance map of the original image, then created an image with the color I want it to be in, and replaced the luminance map of this output image with the luminance map of input image.
But resultant image seems to be darker in shade. Is there any better way to do this?
Input image:
Resulting image:

Answer 1

I think you are looking for "tinting". I don't have any references for how you to do it with OpenCV but there is a description in Anthony Thyssen's excellent ImageMagick notes here - search for the word "somehow". Maybe you can adapt it to OpenCV, if the effect is what you seek.
At the command-line, with ImageMagick, I did this:
convert drop.png -fill red -tint 50% result.jpg

Answer 2

Here is another way in Imagemagick.
convert \( input.png -colorspace gray \) \( -clone 0 -fill red -colorize 100 \) \( -clone 0 \) -compose colorize -composite result1.png
convert \( input.png -colorspace lab -channel red -separate \) \( -clone 0 -fill red -colorize 100 \) \( -clone 0 \) -compose colorize -composite result2.png
convert \( input.png -colorspace hsi -channel blue -separate \) \( -clone 0 -fill red -colorize 100 \) \( -clone 0 \) -compose colorize -composite result3.png
Choose what colorspace represents the intensity/luminance you want to use. See my script, color2gray at http://www.fmwconcepts.com/imagemagick/color2gray/index.php to see what different colorspace intensity/luminance show as gray.