How to rotate a triangle (apex point) towards the point in Qt?

Question

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);

    //adding to acceleration steering force
    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);
    rect->triangle = scene->addPolygon(polygon);

    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.

It is easier to to form a vector from the midpoint of the base in the triangle to the point to point at. Then you can just normalize that vector and multiply with the height of the triangle to calculate the vector from the midpoint to the tip of the triangle. — user2672165
How to get the position of the midpoint in Qt? I was thinking about solution you proposed but how to get the exact position? — Mikhail

xtofl xtofl · Accepted Answer · 2016-01-03T13:42:50

The problem is in your conversion from radians to degrees, plus: you need an offset of 90 degrees or draw your triangle in the 0 degrees direction (right):

// initially point right
polygon <<  QPointF(20, 0) << QPointF(0, -5) <<  QPointF(0, 5);

// angle -> degrees conversion
const float angle = atan2(vehicle->velocity.y, vehicle->velocity.x);
vehicle->triangle->setRotation(
     angle * 180./3.14);
// but in qt 5 they have this qRadiansToDegrees in <QtMath>

How to rotate a triangle (apex point) towards the point in Qt?

1 Answers