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Conversion from RGB to HSI

asked 2015-05-22 10:20:47 -0500

LFDC89 gravatar image

Hi to all! I need a color conversion from RGB (or BGR) color space into HSI color space. But openCV implements only RGBtoHSV or RGBtoHLS color conversion. Is there a function, a plugin or something else that could make this conversion??

Thanks everybody for your answers!

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answered 2015-05-22 17:19:05 -0500

theodore gravatar image

updated 2015-05-22 17:25:33 -0500

Apparently it does not, however if you follow the equations to convert RGB/BGR values to HSI values, it should not be that hard. If we suppose R, G, and B are the red, green, and blue values of a color and H,S, and I the hue, saturation and intensity respectively then:

image description

which can be translated into:

#include <iostream>
#include <opencv2/opencv.hpp>

using namespace std;
using namespace cv;

int main()
{

  Mat src = imread("baboon.jpg", 1);

  if(src.empty())
    cerr << "Error: Loading image" << endl;
  Mat hsi(src.rows, src.cols, src.type());

  float r, g, b, h, s, in;

  for(int i = 0; i < src.rows; i++)
    {
      for(int j = 0; j < src.cols; j++)
        {
          b = src.at<Vec3b>(i, j)[0];
          g = src.at<Vec3b>(i, j)[1];
          r = src.at<Vec3b>(i, j)[2];

          in = (b + g + r) / 3;

          int min_val = 0;
          min_val = std::min(r, std::min(b,g));

          s = 1 - 3*(min_val/(b + g + r));
          if(s < 0.00001)
            {
                  s = 0;
            }else if(s > 0.99999){
                  s = 1;
            }

          if(s != 0)
            {
              h = 0.5 * ((r - g) + (r - b)) / sqrt(((r - g)*(r - g)) + ((r - b)*(g - b)));
              h = acos(h);

              if(b <= g)
                {
                  h = h;
                } else{
                  h = ((360 * 3.14159265) / 180.0) - h;
                }
            }

          hsi.at<Vec3b>(i, j)[0] = (h * 180) / 3.14159265;
          hsi.at<Vec3b>(i, j)[1] = s*100;
          hsi.at<Vec3b>(i, j)[2] = in;
        }
    }

  namedWindow("RGB image", CV_WINDOW_AUTOSIZE);
  namedWindow("HSI image", CV_WINDOW_AUTOSIZE);

  imshow("RGB image", src);
  imshow("HSI image", hsi);

  waitKey(0);
  return 0;
}

image description

image description

it is an old code of mine, but I think it will serve you purposes.

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Comments

@theodore: Just curious but what crime did HSI model do to not be included in heavenly Opencv library?

saurabheights gravatar imagesaurabheights ( 2016-06-17 07:50:37 -0500 )edit
1

to be honest I do not know the answer to this, but I guess it just didn't happen someone to make a PR with it upstream. I am sure that if someone does it the developers would be glad to add it in the library.

theodore gravatar imagetheodore ( 2016-06-17 15:27:01 -0500 )edit
1

Though I am under a lot of work, I will try to submit a PR. Thank you.

saurabheights gravatar imagesaurabheights ( 2016-06-25 10:53:39 -0500 )edit

that would be perfect. Go for it ;-)...

theodore gravatar imagetheodore ( 2016-06-25 15:03:08 -0500 )edit

Great work. But I know that rgb to hsi conversation means h channel takes places of the r channel, s channel takes places of the g channel and i channel takes places of the b channel. In opencv first channel of a pixel of a rgb formatted image is b. The second one is g and the last one is r. I think at the last of your code should be like this:

      hsi.at<Vec3b>(i, j)[2] = (h * 180) / 3.14159265;
      hsi.at<Vec3b>(i, j)[1] = s*100;
      hsi.at<Vec3b>(i, j)[0] = in;

Maybe I thing wrong. Thanks.

ahmetkorkmaz gravatar imageahmetkorkmaz ( 2017-01-13 21:39:14 -0500 )edit

Can anyone explain why there is a multiplication for s with factor 100?

Wurstpelle gravatar imageWurstpelle ( 2017-04-11 02:39:06 -0500 )edit
0

answered 2015-05-24 13:22:53 -0500

LFDC89 gravatar image

Thanks so much! It works! :)

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Asked: 2015-05-22 10:20:47 -0500

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Last updated: May 24 '15