Drawing Image Fragments in Qt GUI: Exploring QPainter::drawPixmapFragments()

Purpose

• It offers more control over the placement and scaling of the pixmap fragments compared to QPainter::drawPixmap().
• This function is used to draw one or more sub-rectangles (fragments) from a pixmap onto a Qt widget or other painting device.

Breakdown

1. • You'll have a QPainter object instantiated, which serves as the "drawing context" for your GUI elements.

2. QPixmapFragment Array
   

   • You need to create an array of QPixmapFragment objects. Each QPixmapFragment defines:
     • sourceRect: The sub-rectangle within the pixmap to draw (specified by sourceLeft, sourceTop, width, height).
     • targetRect: The rectangle on the widget where the fragment will be drawn (can be scaled or transformed).
     • scale (optional): A scaling factor applied to the fragment during drawing (defaults to 1.0 for no scaling).

3. drawPixmapFragments() Function Call
   

   • Pass the following arguments:
     • fragments: Pointer to the QPixmapFragment array you created.
     • fragmentCount: The number of fragments in the array.
     • pixmap: The QPixmap object containing the source image data.
     • hints (optional): A combination of QPainter::PixmapFragmentHint flags that control rendering behavior (e.g., QPainter::OpaqueHint for efficient handling of opaque fragments).

Example

#include <QApplication>
#include <QWidget>
#include <QPainter>
#include <QPixmap>

class MyWidget : public QWidget {
public:
    MyWidget() {
        // Load the pixmap
        pixmap = new QPixmap("image.png");
    }

protected:
    void paintEvent(QPaintEvent *event) override {
        QPainter painter(this);

        // Define fragment rectangles
        QRectF sourceRect1(0, 0, 100, 50);  // Top-left corner of pixmap
        QRectF sourceRect2(150, 75, 75, 100); // Bottom-right corner of pixmap
        QRectF targetRect1(20, 20, 200, 100);
        QRectF targetRect2(100, 150, 150, 200);

        // Create fragment objects (scaling targetRect1 by 2x)
        QPainter::PixmapFragment fragments[2] = {
            QPainter::PixmapFragment::create(targetRect1.center(), sourceRect1, targetRect1.width() / sourceRect1.width(), 1.0),
            QPainter::PixmapFragment::create(targetRect2.center(), sourceRect2, 1.0, targetRect2.height() / sourceRect2.height())
        };

        // Draw the fragments
        painter.drawPixmapFragments(fragments, 2, *pixmap, QPainter::OpaqueHint);
    }

private:
    QPixmap *pixmap;
};

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);
    MyWidget widget;
    widget.show();
    return app.exec();
}

In this example, the code draws two scaled fragments from the image.png pixmap onto the widget.

Key Points

• The QPixmapFragmentHint flags provide options for optimizing drawing performance based on the transparency of your fragments.
• For simpler scenarios where you want to draw the entire pixmap at its original size, use QPainter::drawPixmap().
• Use QPainter::drawPixmapFragments() when you need to draw specific portions of a pixmap or apply custom scaling/transformation.

---

Drawing with Rotation

#include <QtGlobal>

// ... (other includes)

void drawRotatedFragment(QPainter *painter, const QPixmap& pixmap, const QRectF& sourceRect,
                         const QPointF& targetCenter, qreal rotationAngle) {
  QTransform transform;
  transform.translate(targetCenter);
  transform.rotate(rotationAngle);
  transform.translate(-sourceRect.center());

  QPainter::PixmapFragment fragment = QPainter::PixmapFragment::create(
      transform.mapRect(sourceRect), sourceRect, 1.0, 1.0);
  painter->drawPixmapFragments(&fragment, 1, pixmap, QPainter::SmoothHint);
}

// ... (usage in paintEvent)

drawRotatedFragment(painter, myPixmap, sourceRect, targetCenter, 45.0);

This code defines a function drawRotatedFragment that takes a QPixmap, a sourceRect within the pixmap, a targetCenter for placement, and a rotationAngle. It creates a transformation that rotates the fragment around its center and then draws it using drawPixmapFragments with QPainter::SmoothHint for anti-aliasing during rotation.

Drawing Tiled Fragments

#include <cmath>

// ... (other includes)

void drawTiledFragments(QPainter *painter, const QPixmap& tilePixmap, const QRect& targetRect) {
  int tileWidth = tilePixmap.width();
  int tileHeight = tilePixmap.height();
  int numTilesX = std::ceil(targetRect.width() / static_cast<double>(tileWidth));
  int numTilesY = std::ceil(targetRect.height() / static_cast<double>(tileHeight));

  for (int y = 0; y < numTilesY; ++y) {
    for (int x = 0; x < numTilesX; ++x) {
      QRectF sourceRect(0, 0, tileWidth, tileHeight);
      QRectF targetRectPart(targetRect.x() + x * tileWidth, targetRect.y() + y * tileHeight,
                            tileWidth, tileHeight);
      if (targetRectPart.intersects(targetRect)) {
        // Clip the source rect if necessary to fit within target area
        sourceRect &= QRectF(0, 0, tileWidth, tileHeight);
        painter->drawPixmapFragments(
            &QPainter::PixmapFragment::create(targetRectPart.center(), sourceRect, 1.0, 1.0),
            1, tilePixmap, QPainter::OpaqueHint);
      }
    }
  }
}

// ... (usage in paintEvent)

drawTiledFragments(painter, myTilePixmap, targetRect);

This code implements drawTiledFragments that takes a tilePixmap, and a targetRect to be filled with the tiles. It calculates the number of tiles needed horizontally and vertically to cover the target area. It iterates through each tile position, checks if it falls within the target rectangle, and then draws the appropriate clipped sourceRect from the tilePixmap using drawPixmapFragments.

---

QPainter::drawPixmap() with Clipping

• Set a clipping region on the QPainter object using setClipRect() before calling drawPixmap(). This restricts the drawing to the defined rectangle, effectively achieving a similar result to drawing a fragment.
• If you only need to draw a specific rectangular area of the pixmap, you can use QPainter::drawPixmap() with clipping.

QRect targetRect(20, 20, 200, 100); // Area to draw
painter.setClipRect(targetRect);
painter.drawPixmap(0, 0, *pixmap); // Draw entire pixmap with clipping applied

Subclassing QLabel or QWidget

• Within paintEvent(), you have full control over the drawing process. You can use QPainter methods like drawImage() or drawPart() to draw specific portions of the pixmap based on your requirements.
• For more complex scenarios or custom drawing behavior, consider subclassing QLabel or QWidget and overriding the paintEvent() method.

Third-Party Image Processing Libraries

• These libraries provide functions for cropping, scaling, rotating, and applying various effects to images, which can then be drawn onto your Qt widgets using QPainter.
• Qt offers built-in image manipulation capabilities through the QImage class.

• For complex image processing tasks, consider using third-party libraries that specialize in image manipulation.
• If you require more control over drawing or transformations, subclassing QLabel or QWidget and using paintEvent() provides more flexibility.
• For simple cases where you just need to draw a rectangular portion of an image, QPainter::drawPixmap() with clipping might suffice.