# Extracting the Percentage of color (Red,blue,green,yellow,orange) in an image in Opencv?

I have to differentiate between 5 types of images which could have mostly either red,green, blue, orange or yellow color with some white or black color. I have to find which color is prominent in the image.

The source of images is Webcam, so the actual color also depends upon the illumination and distance of the image from webcam.

I am trying to calculate the percentage based upon "Hue" values. I am specifying some range for each color. My ranges are:

Red: 0-10
Green: 50-65

Yellow: 18-21

Blue: 100-115

PROBLEM: Even though the image displayed is not Red still i am getting high % for red color.

My code is following:

int findRect::checkByHSV(int svmResult, Mat detectedSquare)
{

Mat hsv_img;
cvtColor(detectedSquare,hsv_img,CV_BGR2HSV);

Vec3b pixel;
float totalPixel=0; // to count the total number of pixels in an image---to get the Percentage later
float totalClass[6];// because we want to test for 5 classes + a garbage class.{{ Class-0 -> Garbage, Class-1->Orange, Class-2->Green, Class-3->Red,
//  Class-4->Blue, Class-5->Yellow }}

for(int i=0; i<hsv_img.rows; i++)
{
for (int j=0; j<hsv_img.cols; j++)
{

totalPixel++;
pixel= hsv_img.at<Vec3b>(i,j);

if(  pixel[0]>0 &&  pixel[0]<1  )           totalClass[1]++;    // Class-1->Orange
else if (  pixel[0]>50 &&  pixel[0]<65  )   totalClass[2]++;    // To check Green class-2 //svmResult==2 &&
else if (  pixel[0]>0 &&  pixel[0]<10  )        totalClass[3]++;    // Class-3->Red
else if (  pixel[0]>100 &&  pixel[0]<115  )     totalClass[4]++;    // Class-4->Blue
else if (  pixel[0]>18 &&  pixel[0]<21  )       totalClass[5]++;    // Class-5->Yellow
}

}

float percentage[5];
totalClass[0]=0; //Putting zero to the Garbage class

for (int i=0; i<=5; i++)
{
percentage[i] = (totalClass[i] / totalPixel )*100;
}

cout<<"\n Organge: "<<percentage[1]<<"  Green: "<<percentage[2]<<"  Red: "<<percentage[3]<<"  Blue: "<<percentage[4]<<"  Yellow: "<<percentage[5]<<"\n \n";

return svmResult;
}

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You can use the function countNonZero and divide by the total number of pixels of image. A example of use:

vector<Mat> channels;
split(hsv_img,channels);

Mat red, blue, green;
inRange(channels[0], Scalar(0), Scalar(10), red); // red
// ... do the same for blue, green, etc only changing the Scalar values and the Mat

double image_size = hsv_img.cols*hsv_img.rows;
double red_percent = ((double) cv::countNonZero(red))/image_size;


But it could be not optimal depending of the application (for example, if you need to scan lots of image). Anyway, you can use it to compare the values.

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