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3D Bar Chart - Qwt3DBar

Qwt3DBar is a 3D plot item for drawing bar charts in 3D space. Each sample becomes an axis-aligned cuboid ("bar") whose height encodes a scalar value. It supports both 1D series of bars and 2D grids (3D histograms / "bar3"), with per-bar colormap coloring, lit flat shading, and wireframe/edge overlays.

For an overview of the 3D module see 3D Plot Introduction. Qwt3DBar is a Qwt3DPlotItem and follows the same attach/draw/hull contract as Qwt3DSurface.

Key Features

  • Two data shapes: 1D series (bars placed freely on the xy-plane) and 2D grid (3D histogram over an x/y domain)

  • Lit solid geometry: per-bar cuboids with flat per-face normals, responding to Qwt3DPlot::enableLighting() (Blinn-Phong)

  • Three styles: Filled, FilledMesh (filled + edge lines), Wireframe

  • Per-bar color: driven by a Qwt3DColor functor (typically by height z), with colormap presets available through Qwt3DColorMapColor

  • Configurable geometry: baseline and auto footprint (bar width/depth = 80% of spacing by default)

  • Theme integration: picked up automatically by Qwt3DTheme::applyToItem()

Basic Concepts

Data Shapes

Shape setSamples overload Use case
1D series setSamples(QVector<QwtPoint3D>) or (x, heights) Bars along an axis / scattered bars
2D grid setSamples(double** z, cols, rows, minX, maxX, minY, maxY) or (Qwt3DFunctionData) 3D histogram (bar3)

For the 1D series, (x, y) is the bar footprint center and z is the height. For the 2D grid, z[i][j] is the height at grid node (xi, yj).

Rendering Styles

Style Description
Qwt3DBar::Filled Filled bars, no edges
Qwt3DBar::FilledMesh Filled bars + separately colored edge lines (default)
Qwt3DBar::Wireframe Edge lines only

Architecture


classDiagram

    class Qwt3DPlotItem {

        <<abstract>>

        +attach(plot)

        +draw()*

        +hull()*

        +itemChanged()

    }

    class Qwt3DBar {

        +setSamples()

        +setBarStyle()

        +setDataColor()

        +setBarWidth()/setBarDepth()

        +setBaseline()

        +invalidateColors()

    }

    Qwt3DPlotItem <|-- Qwt3DBar

Qwt3DBar reuses the lit surface shader (:/shaders/surface.vert/frag); each bar is emitted as 6 flat-normal faces (24 vertices, 36 triangle indices). It owns its VBO/VAO/EBO and a Qwt3DColor functor, exactly like Qwt3DSurface.

Usage

The 3D bar chart example is located at: examples/3D/bar3D.

1. 2D Grid Bar Chart (3D histogram)

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#include <qwt3d_plot.h>

#include <qwt3d_bar.h>

#include <qwt3d_colormap_color.h>



Qwt3DPlot* plot = new Qwt3DPlot();

plot->setTitle("Gaussian peak as 3D bars");



Qwt3DBar* bars = new Qwt3DBar();

bars->attach(plot);



// 9x9 grid of bar heights over [-2,2] x [-2,2]

const int cols = 9, rows = 9;

const double minX = -2, maxX = 2, minY = -2, maxY = 2;

QVector<QVector<double>> z(cols, QVector<double>(rows));

for (int i = 0; i < cols; ++i)

    for (int j = 0; j < rows; ++j) {

        const double x = minX + (maxX - minX) * i / (cols - 1);

        const double y = minY + (maxY - minY) * j / (rows - 1);

        z[i][j] = std::exp(-(x * x + y * y) / 1.5);

    }

std::vector<double*> ptrs(cols);

for (int i = 0; i < cols; ++i)

    ptrs[i] = z[i].data();

bars->setSamples(ptrs.data(), cols, rows, minX, maxX, minY, maxY);



bars->setBarStyle(Qwt3DBar::FilledMesh);

bars->setDataColor(new Qwt3DColorMapColor("viridis"));

bars->setBaseline(0.0);



plot->enableLighting(true);

plot->setRotation(35, 0, 25);

plot->show();

2. 1D Series of Bars

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// Bars along the x axis, y = 0, height = h[i]

QVector<double> x = {0, 1, 2, 3, 4};

QVector<double> h = {1.2, 2.3, 0.8, 3.1, 1.7};

bars->setSamples(x, h);



// Or place bars freely in the xy-plane (x,y = footprint center, z = height)

QVector<QwtPoint3D> pts;

pts << QwtPoint3D(0, 0, 1.2) << QwtPoint3D(1, 1, 2.3) << QwtPoint3D(2, 0, 0.8);

bars->setSamples(pts);

3. Bar Geometry: Footprint and Baseline

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// Explicit bar footprint (world units). <= 0 means auto = 80% of spacing.

bars->setBarWidth(0.7);

bars->setBarDepth(0.7);



// Baseline: the z where bars start (default 0). Negative heights make a

// bar extend downward from the baseline.

bars->setBaseline(0.0);

4. Coloring

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// Per-bar colormap by height (default behavior). Reuse any of the 22 presets.

bars->setDataColor(new Qwt3DColorMapColor("plasma"));



// Mutating an attached functor in place (e.g. alpha) requires a rebuild:

auto* cm = new Qwt3DColorMapColor("viridis");

bars->setDataColor(cm);

cm->setAlpha(0.8);          // silent mutator

bars->invalidateColors();   // trigger VBO color rebuild

5. Switching Styles

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bars->setBarStyle(Qwt3DBar::Filled);      // solid only

bars->setBarStyle(Qwt3DBar::FilledMesh);  // solid + edge lines (default)

bars->setBarStyle(Qwt3DBar::Wireframe);   // edges only

bars->setMeshColor(RGBA(0.1, 0.1, 0.1, 0.4));

bars->setMeshLineWidth(1.0);

6. Theme & Coordinate System

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// Apply a theme β€” Qwt3DBar is handled by Qwt3DTheme::applyToItem().

plot->applyTheme(Qwt3DTheme::Scientific);



// Tune axis tick length (per-axis, anisotropy-proof):

plot->coordinates()->setTicLengthScale(0.02);

Core Methods Summary

Method Description
setSamples(...) Load bar data (1D or 2D grid, see Data Shapes)
setBarWidth(w) / setBarDepth(d) Bar footprint (<= 0 = auto, 80% of spacing)
setBaseline(z) Bar bottom z (default 0)
setBarStyle(BarStyle) Filled / FilledMesh / Wireframe
setDataColor(Qwt3DColor*) Per-bar color functor (takes ownership)
dataColor() Get the color functor
setMeshColor(RGBA) / setMeshLineWidth(double) Edge line appearance
invalidateColors() Rebuild VBO colors after mutating the functor in place
hull() Bounding box (used by the plot to frame the coordinate system)
draw() / populateLegendColors() Qwt3DPlotItem interface

Tip

  • Keep grids modest (a few thousand bars); each bar emits 24 vertices.

  • Use FilledMesh with a semi-transparent mesh color for visible edges without clutter.

  • enableLighting(true) makes flat-shaded bars easier to read in 3D.

  • For anisotropic data, call plot->coordinates()->setTicLengthScale(...) so axis ticks stay proportional.

Example β€” Related Examples

  • 3D bar chart: examples/3D/bar3D

  • Live stacked traces (spectra) instead of bars: 3D Waterfall, examples/3D/waterfall3D