Plot Export - QwtPlotRenderer¶

QwtPlotRenderer provides functionality to export plots to various file formats, supporting image files (PNG, JPG, etc.), PDF, SVG vector formats, and print output.

Main Features¶

Export to PNG, JPG, BMP, PDF, SVG, PS, and other formats
Configurable DPI resolution for exported images
Custom export dimensions, including millimeter units
Titles, legends, and other components optionally ignored during export

Usage¶

1. Export to Image File¶

All file exports go through QwtPlotRenderer::renderDocument(): you pass the plot, the target filename (whose extension selects the format), the output size in millimeters (QSizeF), and the resolution in DPI (default 85). For image formats the pixel dimensions are derived as mm / 25.4 * DPI. The example below writes the same plot as PNG, JPG, and BMP:

#include <QwtPlotRenderer>

QwtPlotRenderer renderer;

// Export to PNG: 200 x 150 mm at 300 DPI (about 2362 x 1772 pixels)
renderer.renderDocument(plot, "plot.png", QSizeF(200, 150), 300);

// Export to JPG: same physical size at 150 DPI (about 1181 x 886 pixels)
renderer.renderDocument(plot, "plot.jpg", QSizeF(200, 150), 150);

// Export to BMP with the default resolution of 85 DPI
renderer.renderDocument(plot, "plot.bmp", QSizeF(200, 150));

// File format is automatically determined by extension

2. Export to PDF¶

PDF output uses the same renderDocument() entry point; just give the file a .pdf extension. The result is a vector document that stays sharp at any zoom level, ideal for printing and document embedding:

// Export to PDF
renderer.renderDocument(plot, "plot.pdf", QSizeF(200, 150), 300);  // 200x150 mm, 300 DPI

// PDF supports vector graphics, suitable for high-resolution printing

3. Export to SVG¶

With a .svg filename, renderDocument() produces a scalable vector graphic (Qwt must be built with SVG support, QWT_CONFIG_QWTSVG). SVG output remains editable and can be embedded directly into web pages:

// Export to SVG vector format
renderer.renderDocument(plot, "plot.svg", QSizeF(200, 150));

// SVG advantages:
// - Vector graphics, lossless scaling
// - Smaller file size
// - Can be viewed in browsers

4. Export to QImage/QPixmap¶

Instead of writing a file, the renderer can also draw into an in-memory paint device: the renderTo(QwtPlot*, QPaintDevice&) overload accepts any QImage or QPixmap that you create yourself at the desired pixel size, which is handy for embedding the result in an application or post-processing it:

// Export as QImage: create the image, then render into it
QSize size(800, 600);
QImage image(size, QImage::Format_ARGB32);
image.fill(Qt::white);
renderer.renderTo(plot, image);
image.save("plot.png");  // optional: write it to a file

// Export as QPixmap
QPixmap pixmap(size);
pixmap.fill(Qt::white);
renderer.renderTo(plot, pixmap);

// Can be used for in-application display or further processing

5. Export Options¶

The DiscardFlag enum strips selected components from the output: DiscardBackground, DiscardTitle, DiscardLegend, DiscardCanvasBackground, DiscardFooter, and DiscardCanvasFrame. Single flags are toggled with setDiscardFlag(), while setDiscardFlags() assigns the whole set at once:

// Set export layout options
renderer.setDiscardFlag(QwtPlotRenderer::DiscardTitle, true);     // Ignore title
renderer.setDiscardFlag(QwtPlotRenderer::DiscardLegend, true);    // Ignore legend
renderer.setDiscardFlag(QwtPlotRenderer::DiscardFooter, true);    // Ignore footer
renderer.setDiscardFlag(QwtPlotRenderer::DiscardCanvasBackground, true);  // Ignore canvas background

// Set frame options
renderer.setDiscardFlag(QwtPlotRenderer::DiscardCanvasFrame, true);  // Ignore the canvas frame

// Restore defaults: assign the empty flag set (render everything)
renderer.setDiscardFlags(QwtPlotRenderer::DiscardNone);

6. Print Function¶

For printing, a configured QPrinter is handed to the renderer (renderTo(QwtPlot*, QPrinter&)), which lays the plot out across the page. The printer itself decides the target: a physical device or, as below, a PDF file:

#include <QPrinter>

// Print to a printer (or to a PDF file via setOutputFileName)
QPrinter printer(QPrinter::HighResolution);
printer.setOutputFileName("print.pdf");  // Leave unset to print on a physical printer

renderer.renderTo(plot, printer);

// Alternatively, exportTo() opens a file dialog and renders to the chosen file
renderer.exportTo(plot, "plot.pdf");

7. Using Millimeter Units for Size¶

The size argument of renderDocument() is a QSizeF interpreted in millimeters, so the exported document has well-defined physical dimensions; the resolution argument (default 85) sets the dots per inch used when rasterizing content:

// Specify size in millimeter units
QSizeF sizeMm(200, 150);  // 200mm x 150mm
renderer.renderDocument(plot, "plot.pdf", sizeMm, 300);  // 300 DPI

Export Format Comparison¶

Format	Advantages	Disadvantages	Use Cases
PNG	Lossless compression, transparency support	Larger file size	Web sharing, screenshots
JPG	Small file size, widely supported	Lossy compression, no transparency	Photo-like charts
PDF	Vector, high resolution	Requires PDF reader	Printing, documents
SVG	Vector, editable	Complex graphics render slowly	Web pages, editing
BMP	Lossless, simple	Very large file size	Specific software

Export Option Enum¶

Option	Description
`DiscardBackground`	Ignore the plot background
`DiscardTitle`	Ignore title
`DiscardLegend`	Ignore legend
`DiscardFooter`	Ignore footer
`DiscardCanvasBackground`	Ignore canvas background
`DiscardCanvasFrame`	Ignore the canvas frame (no effect with style sheets, where the frame is part of the background)

Core Method Summary¶

Method	Description
`renderDocument()`	Export to a file (format chosen by extension or passed explicitly)
`renderTo()`	Render into a `QPaintDevice` (`QImage`/`QPixmap`), a `QPrinter` or a `QSvgGenerator`
`exportTo()`	Open a file dialog and export to the selected file
`setDiscardFlag()` / `setDiscardFlags()`	Set/reset discard options (`DiscardNone` renders everything)

Tip — Export Recommendations

Use PDF or high-resolution PNG (300 DPI) for printing

Use SVG or PNG for web pages

Use PDF or SVG for document embedding

Use PNG or JPG for screenshot sharing

Example — Related Examples

Export functionality demo: examples/2D/barchart