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 full colormap-preset support via 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)¶

#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¶

// 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¶

// 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¶

// 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¶

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¶

// Apply a theme — 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

Tips

Keep grids modest (a few thousand bars) — each bar emits 24 vertices.
Use FilledMesh with a semi-transparent mesh color for crisp edges without clutter.
enableLighting(true) greatly improves the 3D read of flat-shaded bars.
For anisotropic data, call plot->coordinates()->setTicLengthScale(...) so axis ticks stay proportional.

Related Examples

3D bar chart: examples/3D/bar3D