# How to rotate a triangle towards the point in Qt?

I have an issue with items rotating.

I have a point and a triangle which needs to be rotated with its apex point towards the taregt point. The right side of the picture represents how it is should to be, and the left side represents how it works. Red dashed arrows represent motion, the triangle moves along its arrow. Green dashed arrow represent rotation, the triangle should rotates along its arrow. How do I do calculations:

calculating desired velocity aka direction

velocity(direction) = Vec2DNormalize(targetPoint - locationPoint) * maxVelocity;

calculating angles for target point and location point

float angleLoc = atan2(rect->location.y, rect->location.x);

float angleTarg = atan2(rect->target.y, rect->target.x);

rotating after subtracting angleLoc - angleTarg

rotate((angleLoc - angleTarg) * 100);

Here it is the source code.

ster.cpp

#include "steer.h"
#include <QPointF>
#include <QBrush>
#include <QPen>
#include <vector2d.h>
#include <QGraphicsPolygonItem>
#include <QPolygonF>
#include <QPointF>
#include <QGraphicsItem>
#include <QDebug>
#include <cmath>
#include <vector>
#include <QtWidgets>

void Steer::seek()
{
//calculating desired velocity aka direction
rect->desired = Vec2DNormalize(rect->target - rect->location) * rect->maxspeed;

//calculating steering force
rect->steer = rect->desired - rect->velocity;

//if the steer force is bgger than maxforce
rect->steer.Truncate(rect->maxforce);

rect->acceleration += rect->steer;

//add to velocity acceleration which has steering force only
rect->velocity += rect->acceleration;

//if the velocity is bgger than maxspeed
rect->velocity.Truncate(rect->maxspeed);

//changing our position
rect->location += rect->velocity;

//reset the acceleration
rect->acceleration *= 0;

viewport()->repaint();
}

Steer::Steer(QGraphicsView *parent)
: QGraphicsView(parent)
{
scene = new QGraphicsScene;
rect = new Vehicle;

scene->setSceneRect(0, 0, 500, 500);

polygon <<  QPointF(5.0, 0.0) << QPointF(-5.0, 0.0) <<  QPointF(0.0, 20.0);

this->setScene(scene);

timer = new QTimer(this);
QObject::connect(timer, SIGNAL(timeout()), this, SLOT(seek()));
timer->start();

this->show();
}

void Steer::paintEvent(QPaintEvent *)
{
QPainter painter(viewport());

painter.setBrush(QBrush(Qt::green));
painter.setPen(QPen(Qt::black));

painter.save();

//moving to position
painter.translate(rect->location.x, rect->location.y);

//calculating angles for target point and location point
float angleLoc = atan2(rect->location.y, rect->location.x);
float angleTarg = atan2(rect->target.y, rect->target.x);

//rotating after substracting angleLoc - angleTarg
painter.rotate((angleLoc - angleTarg) * 100);

painter.drawPolygon(polygon);

painter.restore();

for(int i = 0; i < vec.size(); i++)
painter.drawEllipse(vec[i].x() - 1, vec[i].y() - 1, 1 * 2.0, 1 * 2.0);
}

void Steer::mousePressEvent(QMouseEvent * click)
{
point = mapToScene(click->pos());

vec.push_back(point);

rect->target.x = point.x();
rect->target.y = point.y();
}


Here the whole project.