Drawing Line in grid with snap Functionality - c++

Question

I want to draw a line in grid and implement snap functionality to it. I have drawn grid and line but snap functionality is not implemented. How can I implement that functionality in it. I have drawn grid as follows:
void CadGraphicsScene::drawBackground(QPainter *painter, const QRectF &rect)
{
if (isGridVisible)
{
const int gridSize = 50;
const int realLeft = static_cast<int>(std::floor(rect.left()));
const int realRight = static_cast<int>(std::ceil(rect.right()));
const int realTop = static_cast<int>(std::floor(rect.top()));
const int realBottom = static_cast<int>(std::ceil(rect.bottom()));
// Draw grid.
const int firstLeftGridLine = realLeft - (realLeft % gridSize);
const int firstTopGridLine = realTop - (realTop % gridSize);
QVarLengthArray<QLine, 100> lines;
for (qreal x = firstLeftGridLine; x <= realRight; x += gridSize)
lines.append(QLine(x, realTop, x, realBottom));
for (qreal y = firstTopGridLine; y <= realBottom; y += gridSize)
lines.append(QLine(realLeft, y, realRight, y));
painter->setPen(QPen(QColor(220, 220, 220), 0.0));
painter->drawLines(lines.data(), lines.size());
// Draw axes.
painter->setPen(QPen(Qt::lightGray, 0.0));
painter->drawLine(0, realTop, 0, realBottom);
painter->drawLine(realLeft, 0, realRight, 0);
}
}
My Line class is as follows:
#include "line.h"
Line::Line(int i, QPointF p1, QPointF p2)
{
// assigns id
id = i;
// set values of start point and end point of line
startP = p1;
endP = p2;
}
int Line::type() const
{
// Enable the use of qgraphicsitem_cast with line item.
return Type;
}
QRectF Line::boundingRect() const
{
qreal extra = 1.0;
// bounding rectangle for line
return QRectF(line().p1(), QSizeF(line().p2().x() - line().p1().x(),
line().p2().y() - line().p1().y()))
.normalized()
.adjusted(-extra, -extra, extra, extra);
}
void Line::paint(QPainter *painter, const QStyleOptionGraphicsItem *option,
QWidget *widget)
{
// draws/paints the path of line
QPen paintpen;
painter->setRenderHint(QPainter::Antialiasing);
paintpen.setWidth(1);
if (isSelected())
{
// sets brush for end points
painter->setBrush(Qt::SolidPattern);
paintpen.setColor(Qt::red);
painter->setPen(paintpen);
painter->drawEllipse(startP, 2, 2);
painter->drawEllipse(endP, 2, 2);
// sets pen for line path
paintpen.setStyle(Qt::DashLine);
paintpen.setColor(Qt::black);
painter->setPen(paintpen);
painter->drawLine(startP, endP);
}
else
{
painter->setBrush(Qt::SolidPattern);
paintpen.setColor(Qt::black);
painter->setPen(paintpen);
painter->drawEllipse(startP, 2, 2);
painter->drawEllipse(endP, 2, 2);
painter->drawLine(startP, endP);
}
}
snap.cpp
#include "snap.h"
#include <QApplication>
Snap::Snap(const QRect& rect, QGraphicsItem* parent,
QGraphicsScene* scene):
QGraphicsRectItem(QRectF())
{
setFlags(QGraphicsItem::ItemIsSelectable |
QGraphicsItem::ItemIsMovable |
QGraphicsItem::ItemSendsGeometryChanges);
}
void Snap::mousePressEvent(QGraphicsSceneMouseEvent *event){
offset = pos() - computeTopLeftGridPoint(pos());
QGraphicsRectItem::mousePressEvent(event);
}
QVariant Snap::itemChange(GraphicsItemChange change,
const QVariant &value)
{
if (change == ItemPositionChange && scene()) {
QPointF newPos = value.toPointF();
if(QApplication::mouseButtons() == Qt::LeftButton &&
qobject_cast<CadGraphicsScene*> (scene())){
QPointF closestPoint = computeTopLeftGridPoint(newPos);
return closestPoint+=offset;
}
else
return newPos;
}
else
return QGraphicsItem::itemChange(change, value);
}
QPointF Snap::computeTopLeftGridPoint(const QPointF& pointP){
CadGraphicsScene* customScene = qobject_cast<CadGraphicsScene*> (scene());
int gridSize = customScene->getGridSize();
qreal xV = floor(pointP.x()/gridSize)*gridSize;
qreal yV = floor(pointP.y()/gridSize)*gridSize;
return QPointF(xV, yV);
}
Can anyone please help me to solve this?

Answer 1

In the MouseMoveEvent of the Item you want to snap:
Check if the distance between the Item and the Next GridLine is below a certain distance (the distance from which the item would snap).
Then change the Position such that your Item moves to the next GridLine.
In Pseudo:
if( distance(Item.Position, Grid.NextLine) < SnapDistance)
line.Position = Grid.MextLine.Position
You have to seperate X and Y though to make this work properly.