Render well a scene

[AlvaroS]

Hello everyone:

I have an scene where I load a .jpeg in it.
And I add some lines in it as well.
Now I would like to create a new .jpeg of the scene (with the new lines added)

I do this:

void MainWindow::on_line_to_map_Button_clicked()
{

    QList<QGraphicsItem*>a;
    a = scene->items(); //Get items

    QPainter painter(&img);
    painter.setRenderHint(QPainter::NonCosmeticDefaultPen);
    
    painter.setPen(Qt::black);

    for (int i=0; i<a.size();i++){ //for each item:
        double XStart = line_at->line().x1();
        double XEnd = line_at->line().x2();
        double YStart = line_at->line().y1();
        double YEnd = line_at->line().y2();

        painter.drawLine(XStart, YStart, XEnd, YEnd);
}

The problem is that when I see the image created some pixels around the line are in grey... see this http://oi63.tinypic.com/jjpeza.jpg

So how can I fix that?
Maybe with another render way?

[VRonin]

This looks a lot like it's a lossy compression problem of jpg rather than a problem of the painter. see https://www.youtube.com/watch?v=yBX8GFqt6GA

[AlvaroS]

@VRonin And do you know how can I fix that problem?

Thanks a lot

[VRonin]

Try saving it in png and see if the problem solves itself

[kshegunov]

@AlvaroS
As @VRonin hinted this is a color artifact produced by the way jpeg encodes (and compresses) the image data. To not have that, you need to choose a better suited file format, I suggest png here, which supports compression without loss.

Kind regards.

[AlvaroS]

@VRonin said:

Try saving it in png and see if the problem solves itself

@kshegunov said:

@AlvaroS
As @VRonin hinted this is a color artifact produced by the way jpeg encodes (and compresses) the image data. To not have that, you need to choose a better suited file format, I suggest png here, which supports compression without loss.

Kind regards.

Thanks a lot, png does not have that problem :)