A plugin providing the basic tools for painting functionality.
The Basic Tools example is a static plugin for the Plug & Paint example. It provides a set of basic brushes, shapes, and filters. Through the Basic Tools example, we will review the four steps involved in writing a Qt plugin:
.pro
file.#include <interfaces.h> #include <QRect> #include <QObject> #include <QtPlugin> #include <QStringList> #include <QPainterPath> #include <QImage> class BasicToolsPlugin : public QObject, public BrushInterface, public ShapeInterface, public FilterInterface { Q_OBJECT Q_PLUGIN_METADATA(IID "org.qt-project.Qt.Examples.PlugAndPaint.BrushInterface" FILE "basictools.json") Q_INTERFACES(BrushInterface ShapeInterface FilterInterface)
We start by including interfaces.h
, which defines the plugin interfaces for the Plug & Paint application. For the #include
to work, we
need to add an INCLUDEPATH
entry to the .pro
file with the path to the header file.
The BasicToolsPlugin
class is a QObject subclass that implements the BrushInterface
, the ShapeInterface
, and the FilterInterface
. This is done
through multiple inheritance. The Q_INTERFACES()
macro is necessary to tell moc, Qt's meta-object compiler, that the base classes are plugin interfaces. Without the
Q_INTERFACES()
macro, we couldn't use qobject_cast() in the Plug & Paint application to detect interfaces.
For an explanation for the Q_PLUGIN_METADATA()
macro see Exporting the Plugin.
public: // BrushInterface QStringList brushes() const override; QRect mousePress(const QString &brush, QPainter &painter, const QPoint &pos) override; QRect mouseMove(const QString &brush, QPainter &painter, const QPoint &oldPos, const QPoint &newPos) override; QRect mouseRelease(const QString &brush, QPainter &painter, const QPoint &pos) override; // ShapeInterface QStringList shapes() const override; QPainterPath generateShape(const QString &shape, QWidget *parent) override; // FilterInterface QStringList filters() const override; QImage filterImage(const QString &filter, const QImage &image, QWidget *parent) override; };
In the public
section of the class, we declare all the functions from the three interfaces.
Let's now review the implementation of the BasicToolsPlugin
member functions inherited from BrushInterface
.
QStringList BasicToolsPlugin::brushes() const { return {tr("Pencil"), tr("Air Brush"), tr("Random Letters")}; }
The brushes()
function returns a list of brushes provided by this plugin. We provide three brushes: Pencil, Air Brush, and Random Letters.
QRect BasicToolsPlugin::mousePress(const QString &brush, QPainter &painter, const QPoint &pos) { return mouseMove(brush, painter, pos, pos); }
On a mouse press event, we just call mouseMove()
to draw the spot where the event occurred.
QRect BasicToolsPlugin::mouseMove(const QString &brush, QPainter &painter, const QPoint &oldPos, const QPoint &newPos) { painter.save(); int rad = painter.pen().width() / 2; QRect boundingRect = QRect(oldPos, newPos).normalized() .adjusted(-rad, -rad, +rad, +rad); QColor color = painter.pen().color(); int thickness = painter.pen().width(); QColor transparentColor(color.red(), color.green(), color.blue(), 0);
In mouseMove()
, we start by saving the state of the QPainter and we compute a few variables that we'll need later.
if (brush == tr("Pencil")) { painter.drawLine(oldPos, newPos); } else if (brush == tr("Air Brush")) { int numSteps = 2 + (newPos - oldPos).manhattanLength() / 2; painter.setBrush(QBrush(color, Qt::Dense6Pattern)); painter.setPen(Qt::NoPen); for (int i = 0; i < numSteps; ++i) { int x = oldPos.x() + i * (newPos.x() - oldPos.x()) / (numSteps - 1); int y = oldPos.y() + i * (newPos.y() - oldPos.y()) / (numSteps - 1); painter.drawEllipse(x - (thickness / 2), y - (thickness / 2), thickness, thickness); } } else if (brush == tr("Random Letters")) { QChar ch(QRandomGenerator::global()->bounded('A', 'Z' + 1)); QFont biggerFont = painter.font(); biggerFont.setBold(true); biggerFont.setPointSize(biggerFont.pointSize() + thickness); painter.setFont(biggerFont); painter.drawText(newPos, QString(ch)); QFontMetrics metrics(painter.font()); boundingRect = metrics.boundingRect(ch); boundingRect.translate(newPos); boundingRect.adjust(-10, -10, +10, +10); } painter.restore(); return boundingRect; }
Then comes the brush-dependent part of the code:
At the end, we restore the painter state to what it was upon entering the function and we return the bounding rectangle.
QRect BasicToolsPlugin::mouseRelease(const QString & /* brush */, QPainter & /* painter */, const QPoint & /* pos */) { return QRect(0, 0, 0, 0); }
When the user releases the mouse, we do nothing and return an empty QRect.
QStringList BasicToolsPlugin::shapes() const { return {tr("Circle"), tr("Star"), tr("Text...")}; }
The plugin provides three shapes: Circle, Star, and Text.... The three dots after Text are there because the shape pops up a dialog asking for more information. We know that the shape names will end up in a menu, so we include the three dots in the shape name.
A cleaner but more complicated design would have been to distinguish between the internal shape name and the name used in the user interface.
QPainterPath BasicToolsPlugin::generateShape(const QString &shape, QWidget *parent) { QPainterPath path; if (shape == tr("Circle")) { path.addEllipse(0, 0, 50, 50); } else if (shape == tr("Star")) { path.moveTo(90, 50); for (int i = 1; i < 5; ++i) { path.lineTo(50 + 40 * std::cos(0.8 * i * M_PI), 50 + 40 * std::sin(0.8 * i * M_PI)); } path.closeSubpath(); } else if (shape == tr("Text...")) { QString text = QInputDialog::getText(parent, tr("Text Shape"), tr("Enter text:"), QLineEdit::Normal, tr("Qt")); if (!text.isEmpty()) { QFont timesFont("Times", 50); timesFont.setStyleStrategy(QFont::ForceOutline); path.addText(0, 0, timesFont, text); } } return path; }
The generateShape()
creates a QPainterPath for the specified shape. If the shape is Text, we pop up a QInputDialog to let the user
enter some text.
QStringList BasicToolsPlugin::filters() const { return {tr("Invert Pixels"), tr("Swap RGB"), tr("Grayscale")}; }
The plugin provides three filters: Invert Pixels, Swap RGB, and Grayscale.
QImage BasicToolsPlugin::filterImage(const QString &filter, const QImage &image, QWidget * /* parent */) { QImage result = image.convertToFormat(QImage::Format_RGB32); if (filter == tr("Invert Pixels")) { result.invertPixels(); } else if (filter == tr("Swap RGB")) { result = result.rgbSwapped(); } else if (filter == tr("Grayscale")) { for (int y = 0; y < result.height(); ++y) { for (int x = 0; x < result.width(); ++x) { QRgb pixel = result.pixel(x, y); int gray = qGray(pixel); int alpha = qAlpha(pixel); result.setPixel(x, y, qRgba(gray, gray, gray, alpha)); } } } return result; }
The filterImage()
function takes a filter name and a QImage as parameters and returns an altered QImage. The first thing we do is to convert the image to
a 32-bit RGB format, to ensure that the algorithms will work as expected. For example, QImage::invertPixels(), which is used to implement the Invert Pixels filter, gives
counterintuitive results for 8-bit images, because they invert the indices into the color table instead of inverting the color table's entries.
To finally export your plugin you just have to add the Q_PLUGIN_METADATA()
macro right next to the Q_OBJECT()
macro into the header file of the plugin. It must contain the plugins IID and optionally a
filename pointing to a json file containing the metadata for the plugin.
Q_PLUGIN_METADATA(IID "org.qt-project.Qt.Examples.PlugAndPaint.BrushInterface" FILE "basictools.json")
Within this example the json file does not need to export any metadata, so it just contains an empty json object.
{}
Here's the project file for building the Basic Tools plugin:
TEMPLATE = lib CONFIG += plugin static QT += widgets INCLUDEPATH += ../../app HEADERS = basictoolsplugin.h SOURCES = basictoolsplugin.cpp TARGET = $$qtLibraryTarget(pnp_basictools) DESTDIR = ../../plugins
The .pro
file differs from typical .pro
files in many respects. First, it starts with a TEMPLATE
entry specifying lib
. (The default template is app
.) It also
adds plugin
to the CONFIG
variable. This is necessary on some platforms to avoid generating symbolic links with version numbers in the file name, which is appropriate for most dynamic libraries but not
for plugins.
To make the plugin a static plugin, all that is required is to specify static
in addition to plugin
. The Extra Filters
plugin, which is compiled as a dynamic plugin, doesn't specify static
in its .pro
file.
The INCLUDEPATH
variable sets the search paths for global headers (i.e., header files included using #include <...>
). We add ../../app
to the list, so that we can include
<interfaces.h>
.
The TARGET
variable specifies which name we want to give the target library. We use pnp_
as the prefix to show that the plugin is designed to work with Plug & Paint. On Unix, lib
is also
prepended to that name. On all platforms, a platform-specific suffix is appended (e.g., .dll
on Windows, .a
on Linux).
The CONFIG()
code at the end is necessary for this example because the example is part of the Qt distribution and Qt can be configured to be built simultaneously in debug and in release modes. You don't need to
for your own plugins.
Files: