qcam: Support scaling of the viewfinder

The viewfinder is drawn using a QLabel. This could support scaling
through QLabel::setScaledContents(), but in a very inefficient way. To
maintain reasonable efficiency, turn the viewfinder into a QWidget and
draw the image directly using a QPainter.

No performance change was noticed running on a fast x86 machine, and
performance was 60% higher when scaling up to full screen compared to
QLabel.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

src/qcam/main_window.cpp

@@ -33,7 +33,6 @@ MainWindow::MainWindow(CameraManager *cm, const OptionsParser::Options &options)
 
 	viewfinder_ = new ViewFinder(this);
 	setCentralWidget(viewfinder_);
-	viewfinder_->setFixedSize(500, 500);
 	adjustSize();
 
 	ret = openCamera(cm);

src/qcam/viewfinder.cpp

@@ -6,13 +6,13 @@
  */
 
 #include <QImage>
-#include <QPixmap>
+#include <QPainter>
 
 #include "format_converter.h"
 #include "viewfinder.h"
 
 ViewFinder::ViewFinder(QWidget *parent)
-	: QLabel(parent), format_(0), width_(0), height_(0), image_(nullptr)
+	: QWidget(parent), format_(0), width_(0), height_(0), image_(nullptr)
 {
 }
 
@@ -24,9 +24,7 @@ ViewFinder::~ViewFinder()
 void ViewFinder::display(const unsigned char *raw, size_t size)
 {
 	converter_.convert(raw, size, image_);
-
-	QPixmap pixmap = QPixmap::fromImage(*image_);
-	setPixmap(pixmap);
+	update();
 }
 
 int ViewFinder::setFormat(unsigned int format, unsigned int width,
@@ -42,10 +40,20 @@ int ViewFinder::setFormat(unsigned int format, unsigned int width,
 	width_ = width;
 	height_ = height;
 
-	setFixedSize(width, height);
-
 	delete image_;
 	image_ = new QImage(width, height, QImage::Format_RGB32);
 
+	updateGeometry();
 	return 0;
 }
+
+void ViewFinder::paintEvent(QPaintEvent *)
+{
+	QPainter painter(this);
+	painter.drawImage(rect(), *image_, image_->rect());
+}
+
+QSize ViewFinder::sizeHint() const
+{
+	return image_ ? image_->size() : QSize(640, 480);
+}

src/qcam/viewfinder.h

@@ -7,13 +7,13 @@
 #ifndef __QCAM_VIEWFINDER_H__
 #define __QCAM_VIEWFINDER_H__
 
-#include <QLabel>
+#include <QWidget>
 
 #include "format_converter.h"
 
 class QImage;
 
-class ViewFinder : public QLabel
+class ViewFinder : public QWidget
 {
 public:
 	ViewFinder(QWidget *parent);
@@ -23,6 +23,10 @@ public:
 		      unsigned int height);
 	void display(const unsigned char *rgb, size_t size);
 
+protected:
+	void paintEvent(QPaintEvent *) override;
+	QSize sizeHint() const override;
+
 private:
 	unsigned int format_;
 	unsigned int width_;