Drawing QR code with Qt in native C/C++

Question

I am successfully drawing and displaying the QrCode on a QLabel, but it's not recognised when I scan it. Here is the code I used - I made a small class with a static function:

void QrCodeDrawer::paintQR(QPainter &painter, const QSize sz, const QString &data, QColor fg)
{
    char *str=data.toUtf8().data();
    // NOTE: At this point you will use the API to get the encoding and format you want, instead of my hardcoded stuff:
    QrCode qr = QrCode::encodeText(str, QrCode::Ecc::HIGH);
    const int s=qr.size>0?qr.size:1;
    const double w=sz.width();
    const double h=sz.height();
    const double aspect=w/h;
    const double size=((aspect>1.0)?h:w);
    const double scale=size/(s+2);
    // NOTE: For performance reasons my implementation only draws the foreground parts in supplied color.
    // It expects background to be prepared already (in white or whatever is preferred).
    painter.setPen(Qt::NoPen);
    painter.setBrush(fg);
    for(int y=0; y<s; y++) {
        for(int x=0; x<s; x++) {
            const int color=qr.getModule(x, y);  // 0 for white, 1 for black
            if(0x0!=color) {
                const double rx1=(x+1)*scale, ry1=(y+1)*scale;
                QRectF r(rx1, ry1, scale, scale);
                painter.drawRects(&r,1);
            }
        }
    }
}

And called it here:

QPixmap map(400,400);
QPainter painter(&map);
QrCodeDrawer::paintQR(painter,QSize(400,400),"Hello World", QColor("white"));
ui.qrCode->setPixmap(map);

I gave "Hello World" as input string and here is the code I get:

I got the source code from here.

Answer 1

I finally solved it, by drawing the background first(white or any) and then draw the qr code on top of it with a different enough color...

void paintQR(QPainter &painter, const QSize sz, const QString &data, QColor fg, QColor bg)
{
    char *str=data.toUtf8().data();
    // NOTE: At this point you will use the API to get the encoding and format you want, instead of my hardcoded stuff:
    QrCode qr = QrCode::encodeText(str, QrCode::Ecc::HIGH);
    const int s=qr.size>0?qr.size:1;
    const double w=sz.width();
    const double h=sz.height();
    const double aspect=w/h;
    const double size=((aspect>1.0)?h:w);
    const double scale=size/(s+2);

    painter.setPen(Qt::NoPen);
    // Setting the background color....
    painter.setBrush(bg);
    for(int y=0; y<400; y++) {
        for(int x=0; x<400; x++) {
            const double rx1=(x+1)*scale, ry1=(y+1)*scale;
            QRectF r(x, y, 1, 1);
            painter.drawRects(&r,1);
        }
    }
    //Drawing the Code....
    painter.setBrush(fg);
    for(int y=0; y<s; y++) {
        for(int x=0; x<s; x++) {
            const int color = qr.getModule(x, y);  // 0 for white, 1 for black
            if(0x0!=color) {
                const double rx1=(x+1)*scale, ry1=(y+1)*scale;
                QRectF r(rx1, ry1, scale, scale);
                painter.drawRects(&r,1);
            }
        }
    }
}

Answer 2

I used the same sample but had problems with garbage QR codes until I realised str can be a dangling pointer. I changed my code to this and it worked fine:

void QrCodeDrawer::paintQR(QPainter &painter, const QSize sz, const QString &data, QColor fg)
{
    // NOTE: At this point you will use the API to get the encoding and format you want, instead of my hardcoded stuff:
    QrCode qr = QrCode::encodeText(data.toUtf8().constData(), QrCode::Ecc::HIGH);