Algorithm for drawing a straight line with Shift button pressing - c++

Question

I have line painter widget in my project. So, I have to develope algorithm, which allows to correct line to straight, when user presser Shift key. i have seen similar in Photoshop and Paint.
But my own method doesn't work. I'm working with Qt libs.
Code here. drawedPoints_ is QList of QPointF
void GesturePaintSurface::mousePressEvent(QMouseEvent *event)
{
if (event->button() == Qt::LeftButton)
{
isDrawingFinished_ = false;
drawedPoints_.clear();
firstPoint_ = event->pos();
drawedPoints_.append(firstPoint_);
}
else
{
event->ignore();
isDrawingFinished_ = true;
}
}
void GesturePaintSurface::mouseMoveEvent(QMouseEvent *event)
{
if (!isDrawingFinished_)
{
if (isShiftPressed_)
{
QPoint newPoint;
if (abs(drawedPoints_.last().rx() - event->pos().rx()) < 5)
{
newPoint.rx() = firstPoint_.x();
newPoint.ry() = event->pos().y();
}
else
{
newPoint.rx() = event->pos().x();
newPoint.ry() = firstPoint_.y();
}
drawedPoints_.append(newPoint);
}
else
drawedPoints_.append(event->pos());
repaint();
}
}
Sorry for my language. thanks in advance!

Answer 1

First problem is that you are appending point every time mouse moves! Second problem is that this condition to make vertical or horizontal line is wrong.
It should be like that:
void GesturePaintSurface::mouseMoveEvent(QMouseEvent *event)
{
if (!isDrawingFinished_)
{
QPoint newPoint;
if (event->modifiers().test(Qt::ShiftModifier)) {
newPoint = drawedPoints_.last();
if (qAbs(newPoint.x()-event->x())<qAbs(newPoint.y()-event->y())) {
newPoint.setY(event->y());
} else {
newPoint.setX(event->X());
}
} else {
newPoint = event->pos();
}
if (drawedPoints_.count()>1) {
drawedPoints_.last() = newPoint;
} else {
drawedPoints_.append(newPoint);
}
update(); // update is better then repaint!
}
}