OpenCV 3.0 printing Mat - opencv

Question

I am a newbie to OpenCV, so pls bear with me.. I am trying to dump the histogram Mat object for the given image.. It fails with the below error - Any help appreciated...
The first cout in the below program i.e of the loaded image prints successfully - While the second cout of the hist of the image fails with the below error
OpenCV Error: Assertion failed (m.dims <= 2) in FormattedImpl, file /mycode/ws/opencv/opencv-3.0.0-beta/modules/core/src/out.cpp, line 86
libc++abi.dylib: terminating with uncaught exception of type cv::Exception: /mycode/ws/opencv/opencv-3.0.0-beta/modules/core/src/out.cpp:86: error: (-215) m.dims <= 2 in function FormattedImpl
Here is the complete code
#include <stdio.h>
#include <string>
#include <opencv2/opencv.hpp>
using namespace std;
using namespace cv;
int main(int argc, char** argv) {
if (argc != 2) {
printf("usage: opencv.out <Image_Path>\n");
return -1;
}
string imagePath = (argv[1]);
cout << "loading image..." << imagePath << endl;
Mat image = imread(imagePath, 1);
Mat hist;
int imgCount = 1;
int dims = 3;
const int histSizes[] = {4, 4, 4};
const int channels[] = {0, 1, 2};
float rRange[] = {0, 256};
float gRange[] = {0, 256};
float bRange[] = {0, 256};
const float *ranges[] = {rRange, gRange, bRange};
Mat mask = Mat();
calcHist(&image, imgCount, channels, mask, hist, dims, histSizes, ranges);
cout << image << "Loaded image..." << endl;
cout << "Hist of image..." << hist;
return 0;
}

Answer 1

Based on the OpenCV 2.4.9 source code:
static inline std::ostream& operator << (std::ostream& out, const Mat& mtx)
{
Formatter::get()->write(out, mtx);
return out;
}
Is the function you are calling when using << operator. Formatter::get() returns appropriate
formatter class based on the programming language you are using.
write() function basicly calls:
static void writeMat(std::ostream& out, const Mat& m, char rowsep, char elembrace, bool singleLine)
{
CV_Assert(m.dims <= 2);
int type = m.type();
char crowbrace = getCloseBrace(rowsep);
char orowbrace = crowbrace ? rowsep : '\0';
if( orowbrace || isspace(rowsep) )
rowsep = '\0';
for( int i = 0; i < m.rows; i++ )
{
if(orowbrace)
out << orowbrace;
if( m.data )
writeElems(out, m.ptr(i), m.cols, type, elembrace);
if(orowbrace)
out << crowbrace << (i+1 < m.rows ? ", " : "");
if(i+1 < m.rows)
{
if(rowsep)
out << rowsep << (singleLine ? " " : "");
if(!singleLine)
out << "\n ";
}
}
}
As you can see if your Mat dimensionality is greater than 2 assertion will be thrown like in your code (CV_Assert(m.dims<=2)).
calcHist() with the parameters you gave produces 3-dimentional Mat and thus it cannot be displayed using << operator
By calling calcHist() function that way you are getting 3-dimentional histogram and I don't see a simple solution to visualize that in OpenCV (which doesn't mean it can't be done). If it's something you must do I would suggest to look into OpenGL for 3D data visualization. If not you could simply call this function for each channel seperatly - you will get 3 one-dimenational histograms which you can print using << operator.