The QWT Library gives us the ability to create graphs, scale axes, insert legend and do a whole lot of graphing stuff, in a very easy manner. I wanted to show how easy it was to use so in this tutorial I plot an openCV histogram using QWT.
The set-up for this tutorial is similar to the one in my previous tutorial, only this time I also include the OpenCV library. So we can dive straight into the code.
Code
#include <QApplication> #include <qwt_plot.h> #include <qwt_plot_curve.h> #include <qwt_plot_grid.h> #include <qwt_symbol.h> #include <qwt_legend.h> #include <opencv2/core/core.hpp> #include <opencv2/imgproc/imgproc.hpp> #include <opencv2/highgui/highgui.hpp> int main(int argc, char *argv[]) { QApplication a(argc, argv); if (argc < 2) return 1; //Read input image cv::Mat img = cv::imread(argv[1]); //Convert to grayscale if (img.data && img.channels() == 3) cv::cvtColor(img, img, CV_BGR2GRAY); else return 1; int histSize[1] = {256}; // number of bins float hranges[2] = {0.0, 255.0}; // min and max pixel value const float* ranges[1] = {hranges}; int channels[1] = {0}; // only 1 channel used cv::MatND hist; // Compute histogram cv::calcHist(&img, 1, channels, cv::Mat(), hist, 1, histSize,ranges); double minVal, maxVal; cv::minMaxLoc(hist, &minVal, &maxVal);//Locate max and min values QwtPlot plot; //Create plot widget plot.setTitle( "Plot Demo" ); //Name the plot plot.setCanvasBackground( Qt::black ); //Set the Background colour plot.setAxisScale( QwtPlot::yLeft, minVal, maxVal ); //Scale the y-axis plot.setAxisScale(QwtPlot::xBottom,0,255); //Scale the x-axis plot.insertLegend(new QwtLegend()); //Insert a legend QwtPlotCurve *curve = new QwtPlotCurve(); // Create a curve curve->setTitle("Count"); //Name the curve curve->setPen( Qt::white, 2);//Set colour and thickness for drawing the curve //Use Antialiasing to improve plot render quality curve->setRenderHint( QwtPlotItem::RenderAntialiased, true ); /*Insert the points that should be plotted on the graph in a Vector of QPoints or a QPolgonF */ QPolygonF points; for( int h = 0; h < histSize[0]; ++h) { float bin_value = hist.at<float>(h); points << QPointF((float)h, bin_value); } curve->setSamples( points ); //pass points to be drawn on the curve curve->attach( &plot ); // Attach curve to the plot plot.resize( 600, 400 ); //Resize the plot plot.show(); //Show plot return a.exec(); }
The code is well commented and is therefore self explanatory - no need for extra explanations. The result can be seen in the image below.
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