multiply & divide operation in openCV

Question

I'm a new comer in OpenCV, currently a strange issue in OpenCV make me in the trouble: Core function implement the formula：(base_color)/(blend_color)*255 which just want to eliminate the background color, code is likes this:

void clearBackground(Mat& input, Mat& output){
    Mat gauss;
    Mat dst = input.clone();
    cv::GaussianBlur(input,gauss,Size(101,101),0);

    vector<Mat> rgbChannels;
    vector<Mat> gaussChannels;
    vector<Mat> resultChannels;
    split(input, rgbChannels);
    split(gauss, gaussChannels);

    for(int i =0; i<rgbChannels.size();i++){
        Mat tempBase, tempMix,temp;
        temp =  rgbChannels[i]/gaussChannels[i]*255;
        resultChannels.push_back(temp);
    }
    merge(resultChannels, output);
} 

Code works as my expectation, however, when I just change the formula from

temp =  rgbChannels[i]/gaussChannels[i]*255

into two steps just like:

temp =  rgbChannels[i]/gaussChannels[I]
temp =  temp * 255;

OR like this:

divide(rgbChannels[i], gaussChannels[i], temp);
multiply(temp,255, temp);

Then the result changed and would not as my expectation, in one word , I can not divide the formula: rgbChannels[i]/gaussChannels[i]*255 into two step as I expectation, no matter use the math operator or OpenCV built functions .

So the question is how can I divided the formula rgbChannels[i]/gaussChannels[i]*255 into two steps and do not impact the result .

Answer 1

Thanks @Ziri and @Andrey for the reminder, I just fix this issue and code just as below:

void clearBackground(Mat& input, Mat& output){ Mat gauss,temp;

input.convertTo(input,CV_32F);
cv::GaussianBlur(input,gauss,Size(101,101),0);
divide(input, gauss, temp);
multiply(temp,255, output);

output.convertTo(output,CV_8U);

}