Get list of points affected by drawLine() or QLineF

Question

How do I get a list of points/coordinates that the drawLine() function affects? For example if I have

QLineF line(10.0, 80.0, 90.0, 20.0);
QPainter(this);
painter.drawLine(line);

I want to get the list of QPoint coordinates that the drawLine() will draw. I'm aware of Bresenham's line algorithm, however I need this to work in a more general case where I can set different QPen and QBrush:

painter.setPen(QPen(myColor, myWidth, Qt::SolidLine, Qt::RoundCap));

I also need it to be consistent with other programs that might draw lines with same end points and parameters, but without Qt, so I want to end up with the same result, and without off-by-one differences.

Note: This is different from this question, because I need ALL pixels that are going to be affected by the drawing function, not just the vertices of polygon/path.

Update: I think it is possible to subclass QImage or QPainterDevice and QPaintEngine to catch all "setPixel()" operations. Could anyone prove otherwise or confirm and maybe even write a short code to do that?

mzelina mzelina · Accepted Answer · 2012-09-11T03:46:47

Not sure of your performance requirements but you could render the line off screen then look at the image to see what pixels were painted. Here's an example that prints the pixels painted between a 3 pixel line on a 400x400 image from 190,190 to 210,210.

When you work with QImage, be sure to use the scanLine() function for performance reasons.

#include <QCoreApplication>
#include <QImage>
#include <QPainter>
#include <QDebug>

int main(int argc, char *argv[])
{
    QImage image(400,400,QImage::Format_ARGB32);
    QPainter painter;
    painter.begin(&image);
    painter.fillRect(0,0,400,400,QBrush(Qt::white));
    QBrush brush(Qt::black);
    QPen pen(brush, 3.0);
    painter.setPen(pen);
    painter.drawLine(190,190,210,210);
    painter.end();

    for (int i=0;i<image.height();i++) {
        const uchar *scanLine = image.scanLine(i);
        for (int j=0;j<image.width()*4;j+=4) {
            if ( scanLine[j] == 0 ) {
                qDebug() << "pixel" << i << j/4 << scanLine[j];
            }
        }
    }

    return (0);
}

Get list of points affected by drawLine() or QLineF

2 Answers