I found a solution using vector math.(thanks to my colleague Dima Chernikov)
ABCD - our picture.
K' - cursor point.
D2 - the point we are looking for(new position of D)
gif example: https://media.giphy.com/media/uffXKjNNy5ykzpvsR2/giphy.gif
(circlePos_ == eBottomLeft) in code
[image: d835c77d-c279-4bed-8ff5-7fc321c38062.jpg]
code: (I will most likely redo it later using templates. but now it is more clear for understanding)
void MovableCircle::mouseMoveEvent(QGraphicsSceneMouseEvent *event)
{
auto pos = mapToScene(event->pos() + _shiftMouseCoords);
qreal xl = pos.x();
qreal yl = pos.y();
auto rect = parentItem()->boundingRect();
QPointF a(rect.x(), rect.y());
QPointF b(rect.x() + rect.width(), rect.y());
QPointF c(rect.x() + rect.width(), rect.y() + rect.height());
QPointF d(rect.x(), rect.y() + rect.height());
if (circlePos_ == eTopRight) {
Vec2d dc(c.x()-d.x(), c.y()-d.y());
Vec2d cb(b.x()-c.x(), b.y()-c.y());
Vec2d db(b.x()-d.x(), b.y()-d.y());
Vec2d dk(pos.x()-d.x(), pos.y()-d.y());
auto dc_len = dc.length();
auto cb_len = cb.length();
auto db_len = db.length();
auto dk_len = dk.length();
auto dkdb_dot = Vec2d<qreal>::dot(db, dk);
auto cos_kdb = dkdb_dot/(db_len*dk_len);
auto dd2_len = dk_len * cos_kdb;
auto x =(dd2_len * dc_len) / (std::sqrt(cb_len * cb_len + dc_len * dc_len));
auto y = std::sqrt(dd2_len * dd2_len - x * x);
if (x < 10 || y < 10) return;
pos.setX(d.x()+x);
pos.setY(d.y()-y);
}
if (circlePos_ == eBottomRight) {
Vec2d ad(d.x()-a.x(), d.y()-a.y());
Vec2d dc(c.x()-d.x(), c.y()-d.y());
Vec2d ac(c.x()-a.x(), c.y()-a.y());
Vec2d ak(pos.x()-a.x(), pos.y()-a.y());
auto ad_len = ad.length();
auto dc_len = dc.length();
auto ac_len = ac.length();
auto ak_len = ak.length();
auto akac_dot = Vec2d<qreal>::dot(ac, ak);
auto cos_kac = akac_dot/(ac_len*ak_len);
auto ad2_len = ak_len * cos_kac;
auto x =(ad2_len * dc_len) / (std::sqrt(ad_len * ad_len + dc_len * dc_len));
auto y = std::sqrt(ad2_len * ad2_len - x * x);
if (x < 10 || y < 10) return;
pos.setX(a.x()+x);
pos.setY(a.y()+y);
}
if (circlePos_ == eTopLeft) {
qDebug()<<this->parentItem()->boundingRect();
Vec2d cb(b.x()-c.x(), b.y()-c.y());
Vec2d ba(a.x()-b.x(), a.y()-b.y());
Vec2d ca(a.x()-c.x(), a.y()-c.y());
Vec2d ck(pos.x()-c.x(), pos.y()-c.y());
auto cb_len = cb.length();
auto ba_len = ba.length();
auto ca_len = ca.length();
auto ck_len = ck.length();
auto ckca_dot = Vec2d<qreal>::dot(ca, ck);
auto cos_kca = ckca_dot/(ca_len*ck_len);
auto cd2_len = ck_len * cos_kca;
auto y =(cd2_len * cb_len) / (std::sqrt(ba_len * ba_len + cb_len * cb_len));
auto x = std::sqrt(cd2_len * cd2_len - y * y);
if (x < 10 || y < 10) return;
pos.setX(c.x()-x);
pos.setY(c.y()-y);
}
if (circlePos_ == eBottomLeft) {
qDebug()<<this->parentItem()->boundingRect();
Vec2d ba(a.x()-b.x(), a.y()-b.y());
Vec2d ad(d.x()-a.x(), d.y()-a.y());
Vec2d bd(d.x()-b.x(), d.y()-b.y());
Vec2d bk(pos.x()-b.x(), pos.y()-b.y());
auto ba_len = ba.length();
auto ad_len = ad.length();
auto bd_len = bd.length();
auto bk_len = bk.length();
auto bkbd_dot = Vec2d<qreal>::dot(bd, bk);
auto cos_kdb = bkbd_dot/(bd_len*bk_len);
auto bd2_len = bk_len * cos_kdb;
auto x =(bd2_len * ba_len) / (std::sqrt(ad_len * ad_len + ba_len * ba_len));
auto y = std::sqrt(bd2_len * bd2_len - x * x);
if (x < 10 || y < 10) return;
pos.setX(b.x()-x);
pos.setY(b.y()+y);
}
setPos(pos);
emit circleMoved();
}
