Image Composition Example¶
Shows how composition modes work in QPainter
.
The Image Composition example lets the user combine images together using any composition mode supported by QPainter
, described in detail in Composition Modes
.
Setting Up The Resource File¶
The Image Composition example requires two source images, butterfly.png and checker.png that are embedded within imagecomposition.qrc. The file contains the following code:
<Code snippet "/tmp/snapshot-qt5full-6.0/qt5/qtbase/painting/imagecomposition/imagecomposition.qrc" not found>
For more information on resource files, see The Qt Resource System .
ImageComposer Class Definition¶
The ImageComposer
class is a subclass of QWidget that implements three private slots, chooseSource()
, chooseDestination()
, and recalculateResult()
.
class ImageComposer : public QWidget { Q_OBJECT public: ImageComposer(); private slots: void chooseSource(); void chooseDestination(); void recalculateResult();
In addition, ImageComposer
consists of five private functions, addOp()
, chooseImage()
, loadImage()
, currentMode()
, and imagePos()
, as well as private instances of QToolButton
, QComboBox , QLabel
, and QImage
.
private: void addOp(QPainter::CompositionMode mode, const QString &name); void chooseImage(const QString &title, QImage *image, QToolButton *button); void loadImage(const QString &fileName, QImage *image, QToolButton *button); QPainter::CompositionMode currentMode() const; QPoint imagePos(const QImage &image) const; QToolButton *sourceButton; QToolButton *destinationButton; QComboBox *operatorComboBox; QLabel *equalLabel; QLabel *resultLabel; QImage sourceImage; QImage destinationImage; QImage resultImage; };
ImageComposer Class Implementation¶
We declare a QSize
object, resultSize
, as a static constant with width and height equal to 200.
static const QSize resultSize(200, 200);
Within the constructor, we instantiate a QToolButton
object, sourceButton
and set its iconSize
property to resultSize
. The operatorComboBox
is instantiated and then populated using the addOp()
function. This function accepts a CompositionMode
, mode
, and a QString
, name
, representing the name of the composition mode.
ImageComposer::ImageComposer() { sourceButton = new QToolButton; sourceButton->setIconSize(resultSize); operatorComboBox = new QComboBox; addOp(QPainter::CompositionMode_SourceOver, tr("SourceOver")); addOp(QPainter::CompositionMode_DestinationOver, tr("DestinationOver")); addOp(QPainter::CompositionMode_Clear, tr("Clear")); addOp(QPainter::CompositionMode_Source, tr("Source")); addOp(QPainter::CompositionMode_Destination, tr("Destination")); addOp(QPainter::CompositionMode_SourceIn, tr("SourceIn")); addOp(QPainter::CompositionMode_DestinationIn, tr("DestinationIn")); addOp(QPainter::CompositionMode_SourceOut, tr("SourceOut")); addOp(QPainter::CompositionMode_DestinationOut, tr("DestinationOut")); addOp(QPainter::CompositionMode_SourceAtop, tr("SourceAtop")); addOp(QPainter::CompositionMode_DestinationAtop, tr("DestinationAtop")); addOp(QPainter::CompositionMode_Xor, tr("Xor")); addOp(QPainter::CompositionMode_Plus, tr("Plus")); addOp(QPainter::CompositionMode_Multiply, tr("Multiply")); addOp(QPainter::CompositionMode_Screen, tr("Screen")); addOp(QPainter::CompositionMode_Overlay, tr("Overlay")); addOp(QPainter::CompositionMode_Darken, tr("Darken")); addOp(QPainter::CompositionMode_Lighten, tr("Lighten")); addOp(QPainter::CompositionMode_ColorDodge, tr("ColorDodge")); addOp(QPainter::CompositionMode_ColorBurn, tr("ColorBurn")); addOp(QPainter::CompositionMode_HardLight, tr("HardLight")); addOp(QPainter::CompositionMode_SoftLight, tr("SoftLight")); addOp(QPainter::CompositionMode_Difference, tr("Difference")); addOp(QPainter::CompositionMode_Exclusion, tr("Exclusion"));
The destinationButton
is instantiated and its iconSize
property is set to resultSize
as well. The QLabel
s equalLabel
and resultLabel
are created and resultLabel
’s minimumWidth
is set.
destinationButton = new QToolButton; destinationButton->setIconSize(resultSize); equalLabel = new QLabel(tr("=")); resultLabel = new QLabel; resultLabel->setMinimumWidth(resultSize.width());
We connect the following signals to their corresponding slots:
sourceButton
’s clicked() signal is connected tochooseSource()
,
operatorComboBox
’s activated() signal is connected torecalculateResult()
, and
destinationButton
’s clicked() signal is connected tochooseDestination()
.connect(sourceButton, &QAbstractButton::clicked, this, &ImageComposer::chooseSource); connect(operatorComboBox, &QComboBox::activated, this, &ImageComposer::recalculateResult); connect(destinationButton, &QAbstractButton::clicked, this, &ImageComposer::chooseDestination);
A QGridLayout
, mainLayout
, is used to place all the widgets. Note that mainLayout
’s sizeConstraint
property is set to SetFixedSize
, which means that ImageComposer
’s size cannot be resized at all.
QGridLayout *mainLayout = new QGridLayout; mainLayout->addWidget(sourceButton, 0, 0, 3, 1); mainLayout->addWidget(operatorComboBox, 1, 1); mainLayout->addWidget(destinationButton, 0, 2, 3, 1); mainLayout->addWidget(equalLabel, 1, 3); mainLayout->addWidget(resultLabel, 0, 4, 3, 1); mainLayout->setSizeConstraint(QLayout::SetFixedSize); setLayout(mainLayout);
We create a QImage
, resultImage
, and we invoke loadImage()
twice to load both the image files in our imagecomposition.qrc file. Then, we set the windowTitle
property to “Image Composition”.
resultImage = QImage(resultSize, QImage::Format_ARGB32_Premultiplied); loadImage(":/images/butterfly.png", &sourceImage, sourceButton); loadImage(":/images/checker.png", &destinationImage, destinationButton); setWindowTitle(tr("Image Composition")); }
The chooseSource()
and chooseDestination()
functions are convenience functions that invoke chooseImage()
with specific parameters.
void ImageComposer::chooseSource() { chooseImage(tr("Choose Source Image"), &sourceImage, sourceButton); } void ImageComposer::chooseDestination() { chooseImage(tr("Choose Destination Image"), &destinationImage, destinationButton); }
The chooseImage()
function loads an image of the user’s choice, depending on the title
, image
, and button
.
void ImageComposer::chooseImage(const QString &title, QImage *image, QToolButton *button) { QString fileName = QFileDialog::getOpenFileName(this, title); if (!fileName.isEmpty()) loadImage(fileName, image, button); }
The recalculateResult()
function is used to calculate amd display the result of combining the two images together with the user’s choice of composition mode.
void ImageComposer::recalculateResult() { QPainter::CompositionMode mode = currentMode(); QPainter painter(&resultImage); painter.setCompositionMode(QPainter::CompositionMode_Source); painter.fillRect(resultImage.rect(), Qt::transparent); painter.setCompositionMode(QPainter::CompositionMode_SourceOver); painter.drawImage(0, 0, destinationImage); painter.setCompositionMode(mode); painter.drawImage(0, 0, sourceImage); painter.setCompositionMode(QPainter::CompositionMode_DestinationOver); painter.fillRect(resultImage.rect(), Qt::white); painter.end(); resultLabel->setPixmap(QPixmap::fromImage(resultImage)); }
The addOp()
function adds an item to the operatorComboBox
using QComboBox ‘s addItem function. This function accepts a CompositionMode
, mode
, and a QString
, name
. The rectangle is filled with Qt::Transparent and both the sourceImage
and destinationImage
are painted, before displaying it on resultLabel
.
void ImageComposer::addOp(QPainter::CompositionMode mode, const QString &name) { operatorComboBox->addItem(name, mode); }
The loadImage()
function paints a transparent background using fillRect()
and draws image
in a centralized position using drawImage()
. This image
is then set as the button
’s icon.
void ImageComposer::loadImage(const QString &fileName, QImage *image, QToolButton *button) { image->load(fileName); // Scale the image to given size *image = image->scaled(resultSize, Qt::KeepAspectRatio); QImage fixedImage(resultSize, QImage::Format_ARGB32_Premultiplied); QPainter painter(&fixedImage); painter.setCompositionMode(QPainter::CompositionMode_Source); painter.fillRect(fixedImage.rect(), Qt::transparent); painter.setCompositionMode(QPainter::CompositionMode_SourceOver); painter.drawImage(imagePos(*image), *image); painter.end(); button->setIcon(QPixmap::fromImage(fixedImage)); *image = fixedImage; recalculateResult(); }
The currentMode()
function returns the composition mode currently selected in operatorComboBox
.
QPainter::CompositionMode ImageComposer::currentMode() const { return (QPainter::CompositionMode) operatorComboBox->itemData(operatorComboBox->currentIndex()).toInt(); }
We use the imagePos()
function to ensure that images loaded onto the QToolButton
objects, sourceButton
and destinationButton
, are centralized.
QPoint ImageComposer::imagePos(const QImage &image) const { return QPoint((resultSize.width() - image.width()) / 2, (resultSize.height() - image.height()) / 2); }
- The
main()
Function¶
The main()
function instantiates QApplication
and ImageComposer
and invokes its show()
function.
int main(int argc, char *argv[]) { Q_INIT_RESOURCE(imagecomposition); QApplication app(argc, argv); ImageComposer composer; composer.show(); return app.exec(); }
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