# Euclidean Distance between Matrix Channels

I am working on background subtraction in OpenCV Android/Java. I need to find the Euclidean distance between two RGB images. First, I subtract the two images from each other, element-by-element.

Then I use this function:

```
private Mat euclidChannels(Mat in){
Core.pow(in, 2, in);
int rows = in.rows();
Mat out = new Mat(new Size(in.cols(), in.rows()), CvType.CV_32F);
Core.reduce(in.reshape(1,in.rows() * in.cols()), out, 1, Core.REDUCE_SUM);
Core.sqrt(out.reshape(1, rows), out);
return out;
}
```

based on the following technique: how to sum a 3 channel matrix to a one channel matrix?

I get the error:

11-06 22:45:02.094: E/cv::error()(10655): OpenCV Error: Unsupported format or combination of formats (Unsupported combination of input and output array formats) in void cv::reduce(cv::InputArray, cv::OutputArray, int, int, int), file /home/reports/ci/slave_desktop/50-SDK/opencv/modules/core/src/matrix.cpp, line 2365

So I chased it down in the c++ source:

```
1921 if(op == CV_REDUCE_SUM)
1922 {
1923 if(sdepth == CV_8U && ddepth == CV_32S)
1924 func = reduceC_<uchar,int,OpAdd<int> >;
1925 if(sdepth == CV_8U && ddepth == CV_32F)
1926 func = reduceC_<uchar,float,OpAdd<int> >;
1927 if(sdepth == CV_8U && ddepth == CV_64F)
1928 func = reduceC_<uchar,double,OpAdd<int> >;
1929 if(sdepth == CV_16U && ddepth == CV_32F)
1930 func = reduceC_<ushort,float,OpAdd<float> >;
1931 if(sdepth == CV_16U && ddepth == CV_64F)
1932 func = reduceC_<ushort,double,OpAdd<double> >;
1933 if(sdepth == CV_16S && ddepth == CV_32F)
1934 func = reduceC_<short,float,OpAdd<float> >;
1935 if(sdepth == CV_16S && ddepth == CV_64F)
1936 func = reduceC_<short,double,OpAdd<double> >;
1937 if(sdepth == CV_32F && ddepth == CV_32F)
1938 func = reduceC_<float,float,OpAdd<float> >;
1939 if(sdepth == CV_32F && ddepth == CV_64F)
1940 func = reduceC_<float,double,OpAdd<double> >;
1941 if(sdepth == CV_64F && ddepth == CV_64F)
1942 func = reduceC_<double,double,OpAdd<double> >;
1943 }
...
1964 if( !func )
1965 CV_Error( CV_StsUnsupportedFormat,
1966 "Unsupported combination of input and output array formats" );
```

I think this means that none of those if statements are hitting. But, I dumped the depths of the source and destination matrices and I get 0 & 5 respectively, which should correspond to CV_8U & CV_32F (checks out in both Java and C++ source code) - and hit on line 1925 of the source.

Any ideas as to what I'm doing wrong?

(I know my listing line numbers don't match the line numbers in the error message, but the listing is all I could find on the web: matrix.cpp

stop coding for a moment (it's all broken in the most horrible way), and have a look at the builtin http://docs.opencv.org/java/org/opencv/video/BackgroundSubtractorMOG2.html

Thanks for the link - while that's what I'll probably end up using, I'd like to learn more about what's going on under the hood.