QGraphicsScene/View Scale Understanding - c++

Question

I'm lost with understanding the scale value of QGraphicsScene/View.
Here is how I'm placing my targets in the scene.
QPointF Mainwindow::pointLocation(double bearing, double range){
int offset = 90; //used to offset Cartesian system
double centerX = baseSceneSize/2;//push my center location out to halfway point
double centerY = baseSceneSize/2;
double newX = centerX + qCos(qDegreesToRadians(bearing - offset)) * range;
double newY = centerY + qSin(qDegreesToRadians(bearing - offset)) * range;
QPointF newPoint = QPointF(newX, newY);
return newPoint;
}
So each target has a bearing and range. As long as I don't scale, or zoom, the scene, these values work sufficiently. My problem is that I need to implement the zooming.
Here's where things go wrong:
I have a target at Bearing 270, Range 10.
When the app runs, and my vertical slider is at a value of zero, I can see this target in my view. I should not. I need for this target to only come into view when the slider has gotten to a value of 10. Just think each position value on the slider equates to 1 nautical mile. So if a target is at 10 NMs it should only be visible once the slider is >= 10.
here is how I'm doing the zooming:
void MainWindow:: on_PlotSlider_sliderMoved(int position){
const qreal factor = 1.01;
viewScaleValue = qPow(factor, -position);//-position to invert the scale
QMatrix matrix;
matrix.scale(viewScaleValue, viewScaleValue);
view->setMatrix(matrix);
}
I've tried making the View bigger, the Scene bigger, but nothing is having the proper effect.
Here is my Scene setup:
view = ui->GraphicsView;
scene = new QGraphicsScene(this);
int baseSize = 355;
scene->setSceneRect(0,0,baseSize,baseSize);
baseSceneSize = scene->sceneRect().width();
view->setScene(scene);
How do I take the range of my target and push it out into the scene so that it lines up with the slider value?

Answer 1

QGraphicsView::fitInView is everything you need to select the displayed range and center the view.
Here's how you might do it. It's a complete example.
// https://github.com/KubaO/stackoverflown/tree/master/questions/scene-radar-40680065
#include <QtWidgets>
#include <random>
First, let's obtain random target positions. The scene is scaled in e.g. Nautical Miles: thus any coordinate in the scene is meant to be in these units. This is only a convention: the scene otherwise doesn't care, nor does the view. The reference point is at 0,0: all ranges/bearings are relative to the origin.
QPointF randomPosition() {
static std::random_device dev;
static std::default_random_engine eng(dev());
static std::uniform_real_distribution<double> posDis(-100., 100.); // NM
return {posDis(eng), posDis(eng)};
}
Then, to aid in turning groups of scene items on and off (e.g. graticules), it helps to have an empty parent item for them:
class EmptyItem : public QGraphicsItem {
public:
QRectF boundingRect() const override { return QRectF(); }
void paint(QPainter *, const QStyleOptionGraphicsItem *, QWidget *) override {}
};
A scene manager sets up the display. The empty items act as item collections and they can be easily made hidden/visible without having to modify child items. They also enforce the relative Z-order of their children.
class SceneManager : public QObject {
Q_OBJECT
Q_PROPERTY(bool microGraticuleVisible READ microGraticuleVisible WRITE setMicroGraticuleVisible)
QGraphicsScene m_scene;
QPen m_targetPen{Qt::green, 1};
EmptyItem m_target, m_center, m_macroGraticule, m_microGraticule;
An event filter can be installed on the view to signal when the view has been resized. This can be used to keep the view centered in spite of resizing:
bool eventFilter(QObject *watched, QEvent *event) override {
if (event->type() == QEvent::Resize
&& qobject_cast<QGraphicsView*>(watched))
emit viewResized();
return QObject::eventFilter(watched, event);
}
Scene has the following Z-order: center cross, macro- and micro-graticule, then the targets are on top.
public:
SceneManager() {
m_scene.addItem(&m_center);
m_scene.addItem(&m_macroGraticule);
m_scene.addItem(&m_microGraticule);
m_scene.addItem(&m_target);
m_targetPen.setCosmetic(true);
addGraticules();
}
We can monitor a graphics view for resizing; we also expose the visibility of the micro graticule.
void monitor(QGraphicsView *view) { view->installEventFilter(this); }
QGraphicsScene * scene() { return &m_scene; }
Q_SLOT void setMicroGraticuleVisible(bool vis) { m_microGraticule.setVisible(vis); }
bool microGraticuleVisible() const { return m_microGraticule.isVisible(); }
Q_SIGNAL void viewResized();
Targets can be randomly generated. A target has a fixed size in view coordinates. Its position, though, is subject to any scene-to-view transformations.
The pens for targets and graticules are cosmetic pens: their width is given in the view device units (pixels), not scene units.
void newTargets(int count = 200) {
qDeleteAll(m_target.childItems());
for (int i = 0; i < count; ++i) {
auto target = new QGraphicsEllipseItem(-1.5, -1.5, 3., 3., &m_target);
target->setPos(randomPosition());
target->setPen(m_targetPen);
target->setBrush(m_targetPen.color());
target->setFlags(QGraphicsItem::ItemIgnoresTransformations);
}
}
The graticules are concentric circles centered at the origin (range reference point) and a cross at the origin. The origin cross has fixed size in view units - this is indicated by the ItemIgnoresTransformations flag.
void addGraticules() {
QPen pen{Qt::white, 1};
pen.setCosmetic(true);
auto center = {QLineF{-5.,0.,5.,0.}, QLineF{0.,-5.,0.,5.}};
for (auto l : center) {
auto c = new QGraphicsLineItem{l, &m_center};
c->setFlags(QGraphicsItem::ItemIgnoresTransformations);
c->setPen(pen);
}
for (auto range = 10.; range < 101.; range += 10.) {
auto circle = new QGraphicsEllipseItem(0.-range, 0.-range, 2.*range, 2.*range, &m_macroGraticule);
circle->setPen(pen);
}
pen = QPen{Qt::white, 1, Qt::DashLine};
pen.setCosmetic(true);
for (auto range = 2.5; range < 9.9; range += 2.5) {
auto circle = new QGraphicsEllipseItem(0.-range, 0.-range, 2.*range, 2.*range, &m_microGraticule);
circle->setPen(pen);
}
}
};
The mapping between the scene units and the view is maintained as follows:
Each time the view range is changed (from e.g. the combo box), the QGraphicsView::fitInView method is called with a rectangle in scene units (of nautical miles). This takes care of all of the scaling, centering, etc.. E.g. to select a range of 10NM, we'd call view.fitInView(QRect{-10.,-10.,20.,20.), Qt::KeepAspectRatio)
The graticule(s) can be disabled/enabled as appropriate for a given range to unclutter the view.
int main(int argc, char ** argv) {
QApplication app{argc, argv};
SceneManager mgr;
mgr.newTargets();
QWidget w;
QGridLayout layout{&w};
QGraphicsView view;
QComboBox combo;
QPushButton newTargets{"New Targets"};
layout.addWidget(&view, 0, 0, 1, 2);
layout.addWidget(&combo, 1, 0);
layout.addWidget(&newTargets, 1, 1);
view.setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
view.setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
view.setBackgroundBrush(Qt::black);
view.setScene(mgr.scene());
view.setRenderHint(QPainter::Antialiasing);
mgr.monitor(&view);
combo.addItems({"10", "25", "50", "100"});
auto const recenterView = [&]{
auto range = combo.currentText().toDouble();
view.fitInView(-range, -range, 2.*range, 2.*range, Qt::KeepAspectRatio);
mgr.setMicroGraticuleVisible(range <= 20.);
};
QObject::connect(&combo, &QComboBox::currentTextChanged, recenterView);
QObject::connect(&mgr, &SceneManager::viewResized, recenterView);
QObject::connect(&newTargets, &QPushButton::clicked, [&]{ mgr.newTargets(); });
w.show();
return app.exec();
}
#include "main.moc"

Answer 2

So as Kuba suggested, I was overcomplicating this a bit. With his help this is what ended up getting me the result I needed. Not 100% sure on some of it, but for now it's working the way I need it to.
view = ui->GraphicsView;
scene = new QGraphicsScene(this);
int baseSize = 1000; // MAGIC value that works, anything other than this, not so much
view->setSceneRect(0,0,baseSize,baseSize);
baseViewSize = view->sceneRect().width();
view->setScene(scene);
My drawPoint method works fine, no changes were needed.
Finally, here is my slider
void MainWindow:: on_PlotSlider_sliderMoved(int position){
const qreal factor = 1.01;
viewScaleValue = qPow(factor, -position);//-position to invert the scale
QMatrix matrix;
// below is the update, again 6 is a MAGIC number, no clue why 6 works...
matrix.scale((baseViewSize/6 / position, baseViewSize/6 / position);
view->setMatrix(matrix);
}
While my problem is solved, I would love some explanation as to my 2 MAGIC numbers.
Why does it all only work is the baseSize is 1000?
Why does it only scale correctly if I divide the BaseViewSize by 6?