Zoomed images now stay centered during resize

Author: svetter

src/viewer/scalable_image_label.cpp

@@ -45,10 +45,11 @@ ScalableImageLabel::ScalableImageLabel(QScrollArea* parent) : QLabel(parent), pa
 void ScalableImageLabel::setImage(const QImage& image)
 {
 	fullSizePixmap = QPixmap::fromImage(image);
+	imageLoaded = true;
+	setPixmap(fullSizePixmap);
 	fillMode = true;
 	setBarsEnabled(false);
-	setPixmap(fullSizePixmap);
-	imageLoaded = true;
+	imageCenter = QPoint();
 	setHandCursor(true);
 }
 
@@ -60,6 +61,7 @@ void ScalableImageLabel::clearImage()
 	fullSizePixmap = QPixmap();
 	setPixmap(QPixmap());
 	imageLoaded = false;
+	imageCenter = QPoint();
 	setNormalCursor();
 }
 
@@ -135,6 +137,12 @@ void ScalableImageLabel::wheelEvent(QWheelEvent* event)
 		int newMaxScrollY = newImageHeight - availableArea.height();
 		setMaxScroll(newMaxScrollX, newMaxScrollY);
 		setScroll(newScrollX, newScrollY);
+		
+		// Calculate and save new center position
+		saveNewImageCenter();
+	}
+	else {
+		imageCenter = QPoint();
 	}
 
 	event->accept();
@@ -156,7 +164,7 @@ void ScalableImageLabel::paintEvent(QPaintEvent* event)
 	if (!imageLoaded) return;
 	QSize availableArea = parent->maximumViewportSize();
 	bool resize = false;
-
+	
 	// Resize if max-zoomed image is smaller than the available area
 	resize |= MAX_ZOOM_RATIO * fullSizePixmap.width() < availableArea.width() && MAX_ZOOM_RATIO * fullSizePixmap.height() < availableArea.height();
 	// Resize if fill mode is active but the image is bigger than the available area
@@ -172,6 +180,13 @@ void ScalableImageLabel::paintEvent(QPaintEvent* event)
 		}
 	}
 
+	if (!fillMode) {
+		// Make sure image stays centered
+		int moveX = getScrollX() + availableArea.width()  / 2 - imageCenter.x();
+		int moveY = getScrollY() + availableArea.height() / 2 - imageCenter.y();
+		if (moveX || moveY) scrollRelative(QPoint(moveX, moveY));
+	}
+	
 	QLabel::paintEvent(event);
 }
 
@@ -207,6 +222,7 @@ void ScalableImageLabel::mouseMoveEvent(QMouseEvent* event)
 	if (!imageLoaded || fillMode || mousePressedAt.isNull()) return;
 	QPoint mouseMove = event->globalPosition().toPoint() - mousePressedAt;
 	scrollRelative(mouseMove);
+	saveNewImageCenter();
 	mousePressedAt = event->globalPosition().toPoint();
 }
 
@@ -229,6 +245,15 @@ void ScalableImageLabel::mouseReleaseEvent(QMouseEvent* event)
 
 
 
+void ScalableImageLabel::saveNewImageCenter()
+{
+	QSize availableArea = parent->maximumViewportSize();
+	int centerX = getScrollX() + availableArea.width () / 2;
+	int centerY = getScrollY() + availableArea.height() / 2;
+	imageCenter = QPoint(centerX, centerY);
+}
+
+
 /**
  * Sets the enabled status of the parent QScrollArea's scroll bars.
  * 

src/viewer/scalable_image_label.h

@@ -43,17 +43,19 @@ class ScalableImageLabel : public QLabel
 
 	/** The parent QScrollArea. */
 	QScrollArea* parent;
-	/** The image to display in full available resolution. */
-	QPixmap fullSizePixmap;
 	/** Indicates whether an image is currently loaded and displayed. */
 	bool imageLoaded;
+	/** The image to display in full available resolution. */
+	QPixmap fullSizePixmap;
 	/**
 	 * Indicates whether the image should be scaled to fill the available space.
 	 * This mode is enabled by default, whenever the full-size image is smaller than the available
 	 * space and when the user has zoomed all the way back out.
 	 */
 	bool fillMode;
 
+	/** When fill mode is inactive, indicates where in the current size of the image the center of the displayed area is. */
+	QPoint imageCenter;
 	/** The location where the mouse was last registered after being pressed over the image. Used for dragging. */
 	QPoint mousePressedAt;
 
@@ -78,6 +80,8 @@ private slots:
 	void mouseReleaseEvent(QMouseEvent* event) override;
 
 private:
+	void saveNewImageCenter();
+	
 	void setBarsEnabled(bool enabled) const;
 	int getScrollX() const;
 	int getScrollY() const;