ex_disp_plots
Disp plots: colour maps, bar and spline histograms, line plots, image and time-frequency maps.
Libraries: Visualization
Build and run
cmake --build build/developer-dynamic --target ex_disp_plots
./out/Release/examples/ex_disp_plots
Used in the API reference
Source
src/examples/ex_disp_plots/main.cpp
//=============================================================================================================
/**
* SPDX-License-Identifier: BSD-3-Clause
* Copyright (c) 2026 MNE-CPP Authors
*
* @file main.cpp
* @author Christoph Dinh <christoph.dinh@mne-cpp.org>
* @since 2.4.0
* @date October 2026
* @brief Disp plots: colour maps, bar and spline histograms, line plots, image and time-frequency maps.
*
* Colour maps are compared with matplotlib 3.10; every widget is rendered
* offscreen with QWidget::grab() and its pixels are checked against the data
* it was given. Exits non-zero on any mismatch.
*/
//=============================================================================================================
// INCLUDES
//=============================================================================================================
#include <disp/plots/bar.h>
#include <disp/plots/graph.h>
#include <disp/plots/helpers/colormap.h>
#include <disp/plots/imagesc.h>
#include <disp/plots/lineplot.h>
#include <disp/plots/plot.h>
#include <disp/plots/spline.h>
#include <disp/plots/tfplot.h>
//=============================================================================================================
// QT INCLUDES
//=============================================================================================================
#include <QApplication>
#include <QDebug>
#include <QImage>
#include <QVector3D>
//=============================================================================================================
// STL INCLUDES
//=============================================================================================================
#include <cmath>
#include <cstdlib>
#include <cstdio>
//=============================================================================================================
// USED NAMESPACES
//=============================================================================================================
using namespace DISPLIB;
using namespace Eigen;
//=============================================================================================================
// DEFINE GLOBAL METHODS
//=============================================================================================================
namespace
{
bool expect(bool condition, const QString& what)
{
qInfo().noquote() << (condition ? " ok " : " FAIL") << what;
return condition;
}
bool close(QRgb a, QRgb b, int tol)
{
return std::abs(qRed(a) - qRed(b)) <= tol && std::abs(qGreen(a) - qGreen(b)) <= tol && std::abs(qBlue(a) - qBlue(b)) <= tol;
}
/** Number of pixels in @p image within @p tol of @p color. */
int countColor(const QImage& image, QRgb color, int tol)
{
int n = 0;
for (int y = 0; y < image.height(); ++y) {
for (int x = 0; x < image.width(); ++x) {
n += close(image.pixel(x, y), color, tol);
}
}
return n;
}
/** Renders @p widget at 400 x 300 without showing it on screen. */
QImage render(QWidget& widget)
{
widget.resize(400, 300);
widget.setAttribute(Qt::WA_DontShowOnScreen);
widget.show();
QApplication::processEvents();
return widget.grab().toImage();
}
} // namespace
//=============================================================================================================
// MAIN
//=============================================================================================================
int main(int argc, char* argv[])
{
QApplication app(argc, argv);
bool ok = true;
const QRgb steelBlue = qRgb(70, 130, 180);
//! [colormap_usage]
const QRgb low = ColorMap::valueToColor(0.0, "Viridis"); // v in [0, 1]
const QRgb mid = ColorMap::valueToColor(0.5, "Viridis");
const QRgb jet = ColorMap::valueToJet(0.25);
//! [colormap_usage]
// matplotlib.colormaps["viridis"](0, 0.5, 1) and ["jet"](0.25, 0.5), ["cool"](0.5); bone is MNE-C's piecewise ramp, not matplotlib's
ok &= expect(close(low, qRgb(68, 1, 84), 1) && close(mid, qRgb(33, 145, 140), 1) && close(ColorMap::valueToColor(1.0, "Viridis"), qRgb(253, 231, 37), 1) && close(jet, qRgb(0, 128, 255), 1) && close(ColorMap::valueToJet(0.5), qRgb(125, 255, 122), 5) && close(ColorMap::valueToCool(0.5), qRgb(128, 127, 255), 1),
"ColorMap viridis, jet and cool match matplotlib to 1-5 levels");
//! [bar_usage]
Bar bar("Amplitude histogram");
VectorXd classLimits(5); // class edges
classLimits << 0.0, 1.0, 2.0, 3.0, 4.0;
VectorXi counts(4); // counts per class
counts << 5, 20, 10, 15;
bar.setData(classLimits, counts, 2); // 2 digits on the axis labels
//! [bar_usage]
const QImage barImage = render(bar);
// Plot height 300 - 40 - 100 = 160 px and the tallest bar (20 counts) fills it, so heights follow the counts
QList<int> heights;
for (int b = 0; b < 4; ++b) {
int h = 0;
for (int py = 0; py < barImage.height(); ++py) {
h += close(barImage.pixel(60 + 80 * b + 40, py), steelBlue, 2);
}
heights.append(h);
}
ok &= expect(std::abs(heights[1] - 160) <= 2 && std::abs(heights[0] - 40) <= 2 && std::abs(heights[2] - 80) <= 2 && std::abs(heights[3] - 120) <= 2,
QString("Bar heights %1, %2, %3, %4 px for counts 5, 20, 10, 15").arg(heights[0]).arg(heights[1]).arg(heights[2]).arg(heights[3]));
//! [spline_usage]
Spline spline(nullptr, "Amplitude spline");
spline.setData(classLimits, counts);
spline.setThreshold(QVector3D(1.0f, 2.0f, 3.0f)); // three vertical marker lines
//! [spline_usage]
ok &= expect(countColor(render(spline), steelBlue, 30) > 50, "Spline draws its curve");
//! [line_plot_usage]
QVector<double> x;
QVector<double> y;
for (int i = 0; i <= 100; ++i) {
x.append(i / 100.0);
y.append(std::sin(2.0 * M_PI * x.last()));
}
LinePlot line(x, y, "sin(2 pi t)");
line.setXLabel("t [s]");
line.setYLabel("amplitude");
//! [line_plot_usage]
const QImage lineImage = render(line);
int top = lineImage.height();
int bottom = 0;
for (int py = 0; py < lineImage.height(); ++py) {
for (int px = 0; px < lineImage.width(); ++px) {
if (close(lineImage.pixel(px, py), steelBlue, 40)) {
top = std::min(top, py);
bottom = std::max(bottom, py);
}
}
}
ok &= expect(bottom > top + 100, QString("LinePlot spans %1 px vertically for a full sine period").arg(bottom - top));
//! [image_sc_usage]
MatrixXd ramp(4, 8);
for (int r = 0; r < ramp.rows(); ++r) {
for (int c = 0; c < ramp.cols(); ++c) {
ramp(r, c) = c; // rises left to right
}
}
ImageSc image(ramp); // min..max mapped to 0..1, "Hot" by default
image.setColorMap("Viridis");
image.setTitle("ramp");
//! [image_sc_usage]
const QImage scImage = render(image);
// Data area: x in [60, 340); first and last columns are the colour map ends
ok &= expect(close(scImage.pixel(70, 150), qRgb(68, 1, 84), 4) && close(scImage.pixel(330, 150), qRgb(253, 231, 37), 4),
"ImageSc maps the minimum and maximum columns to the ends of viridis");
//! [plot_usage]
VectorXd samples = VectorXd::LinSpaced(50, -1.0, 1.0);
Plot plot(samples); // Plot is a Graph with title and axis labels
plot.setTitle("ramp");
plot.setXLabel("sample");
plot.setYLabel("value");
//! [plot_usage]
const QImage plotImage = render(plot);
ok &= expect(countColor(plotImage, qRgb(255, 255, 255), 0) < plotImage.width() * plotImage.height(), "Plot renders axes and data");
//! [tf_plot_usage]
MatrixXd tf = MatrixXd::Zero(50, 40); // frequency rows x time columns
tf.middleRows(20, 10).setConstant(1.0); // 20-30 Hz band at 100 Hz sampling
TFplot tfPlot(tf, 100.0, 0.0, 50.0, Jet);
//! [tf_plot_usage]
const QImage tfImage = render(tfPlot);
ok &= expect(countColor(tfImage, ColorMap::valueToJet(1.0), 10) > 100 && countColor(tfImage, ColorMap::valueToJet(0.0), 10) > countColor(tfImage, ColorMap::valueToJet(1.0), 10),
"TFplot draws the 20-30 Hz band in the top colour over a bottom-colour background");
qInfo().noquote() << (ok ? "All disp plot checks passed." : "disp plot checks FAILED.");
return ok ? EXIT_SUCCESS : EXIT_FAILURE;
}