CODEWITHSUNDEEP
c++qt
Henrik Brinch
Henrik Brinch

Reputation: 161

Drawing line / erase part of it (Qt / C++)

I try to accomplish a simple erase functionality when drawing lines in Qt (not bitmaps but true lines).

I'm e.g. drawing a black line in one "layer" and a read line in another "layer". Then I want to erase some of the red line, so I paint a white line. However I want to be able to see some of the black line where the intersect.

This is my situation:

enter image description here

I want to accomplish something like this:

enter image description here

I've been playing around with creating a customline class that inherits from a QGraphicsLine and implements the paint event to be able to control the composition modes - but I haven't yet found the right solution.

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    _scene = new QGraphicsScene(this);
    ui->graphicsView->setScene(_scene);

    _blackPen = QPen(Qt::black);
    _blackPen.setWidth(40);

    _redPen = QPen(Qt::red);;
    _redPen.setWidth(40);

    _eraserPen = QPen(Qt::white);
    _eraserPen.setWidth(10);

    _scene->addItem(new CustomLine(0,0,100,100, _blackPen, QPainter::CompositionMode_Source));
    _scene->addItem(new CustomLine(0,100,100,100, _redPen, QPainter::CompositionMode_Source));
    _scene->addItem(new CustomLine(0,100,100,100, _eraserPen, QPainter::CompositionMode_Source));
}


    #include "customline.h"
    #include <QPainter>

    CustomLine::CustomLine(qreal x, qreal y, qreal x2, qreal y2, QPen &pen,     QPainter::CompositionMode mode)
    {
    _x = x;
    _y = y;
    _x2 = x2;
    _y2 = y2;
    _pen = pen;
    _mode = mode;
}

    void CustomLine::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget)
    {
        painter->setPen(_pen);
        painter->setCompositionMode(_mode);
        painter->drawLine(_x,_y, _x2, _y2);
    }

Any suggestions?

Upvotes: 0

Views: 828

Answers (1)

goug
goug

Reputation: 2444

There may be a way to do it with compositing, but you might also try using an outline of the red line, and to get that, use QPainterPathStroker. Create a path containing the line, and then use the stroker to create an outline around that. The code will be something like:

QPainterPath path;
path.lineTo (...);
QPainterPathStroker stroker;
QPainterPath outline = stroker.createStroke (path).simplified ());
painter.drawPath (outline);

You'll probably need to play with this to get what you want. When I first started using QPainterPathStroker, I didn't find it to be terribly intuitive.

Upvotes: 1

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