Converting an RGB image into an HSI image without using opencv functions - c++

Question

I am sort of a beginner when it comes to c++ and opencv.
I have this assignment where i have to successfully convert an image from RGB to HSI and then split the HSI image into the 3 channels Hue, Saturation and respectively Intensity, without using any library functions when implementing my algorithm, after that the 3 images have to be displayed.
I was able to do most of these but when converting from RGB to HSI i am completely lost. From what i saw on other posts the pixel values should be put in a matrix and then changed based on my algorithms, after that the new values should go into a new matrix (HSI).
My main problem (i think) is that i cannot seem to put the values into the new matrix, i tried different methods but the outcome was the same.
Any input is welcome.
Best Regards
Stefan
#include <opencv\highgui.h>
#include <opencv\cv.h>
#include <iostream>
using namespace cv;
using namespace std;
int main()
{
Mat rgb;
rgb = imread("Flower.jpg", CV_LOAD_IMAGE_COLOR);
unsigned char *input = (unsigned char*)(rgb.data);
Mat hsi = rgb.clone();
double R,G,B,a,H,S,I;
int i,j;
const double PI= 3.14;
for(int i = 0;i < hsi.rows ;i++){
for(int j = 0;j < hsi.cols ;j++){
B = input[hsi.step * j + i ];
G = input[hsi.step * j + i + 1];
R = input[hsi.step * j + i + 2];
}
if (R < G && R < B)
a = R;
if (G < R && G < B)
a = G;
if (B < G && B < R)
a = B;
I = (R+G+B)/3.0;
S = 1 - 3.0/(R+G+B)*a;
if (S == 0.0)
{
H = 0.0;
}
else
{
if (B <= G)
H = acos((((R-G)+(R-B))/2.0)/(sqrt((R-G)*(R-G) + (R-B)*(G-B))));
else
{
if (B > G)
H = 2*PI - acos((((R-G)+(R-B))/2.0)/(sqrt((R-G)*(R-G) + (R-B)*(G-B))));
}
}
}
namedWindow("RGB", CV_WINDOW_AUTOSIZE);
imshow("RGB", rgb);
namedWindow("HSI", CV_WINDOW_AUTOSIZE);
imshow("HSI", hsi);
waitKey(0);
return 0;
}

Answer 1

Your code has two problems.
First, you perform calculation outside the second loop:
for(int i = 0;i < hsi.rows ;i++){
for(int j = 0;j < hsi.cols ;j++){
}
// HSI calculation
}
So, you work only with last pixel in each row.
Second, you don't write results to hsi matrix.
Use this code template:
Mat bgr = ...; // OpenCV uses BGR format
Mat hsi(bgr.size(), CV_8UC3);
for (int i = 0; i < bgr.rows; ++i)
{
const Vec3b* bgr_row = bgr.ptr<Vec3b>();
Vec3b* hsi_row = hsi.ptr<Vec3b>();
for (int j = 0; j < bgr.cols; ++j)
{
double B = bgr_row[j][0];
double G = bgr_row[j][1];
double R = bgr_row[j][2];
double H = ...;
double S = ...;
double I = ...;
hsi_row[j] = Vec3b(H, S, I);
}
}