QGraphicsScene background glitches

[gfgit]

Hi,
I'm new to this forum so please forgive me if this is not the right section.
I'm trying to draw a background grid in which vertical lines represent hours.
On top of this lines I draw the labels for each hour, like: '00:00' '01:00' '02:00' and so on.
I was using a QGraphicsLineItem for each hour line and a QGraphicsSimpleText for its label.
That's not very efficent because when the sceneRect chages I have to manually extend the vertical lines.

Now I'm trying to get the same result reimplementing QGraphicsScene::drawBackground().
When the window containing the QGraphicsView is created everithing is fine but when I start scrolling the scene the hour labels are not drawn.
The vertical lines instead work well.

horizOffset represents Left margin width.
vertOffset represent Top margin height (Left margin rect extends to the top)



_____________________________________
| | ? vert. offset |
| horiz. |____?___________________|
| offset | |
|<-------->| |
|__________|________________________|


The hour lines and labels are like this



________________________________________________
| 00:00 01:00 02:00 03:00
| horiz. | |<-------->| |
| offset | | hour | |
| | | offset | |
|__________|__________|__________|__________|___



Qt version 5.11.3
Compiler: MingW 32 bit
OS: Windows 8.1 64 bit
Here is my reimplemented function of QGraphicsScene:


void ShiftScene::drawBackground(QPainter *painter, const QRectF& rect)
{
//vertOffset: constant of type double representing the vertical space at the top of the graph reservet for hour labels (Top margin)
//hourOffset: constant of type double representig the space (horizontal) between two hour lines (vertical lines).
//horizOffset: constant of type double representing the left margin. The first hour (00:00) should be at x = horizOffset
//following hours should be at x = horizOffset + hourOffset * N (where N is between 0 and 24)

const qreal x1 = rect.left();
const qreal x2 = rect.right();
const qreal t = qMax(rect.top(), vertOffset); //Don't draw lines in vertOffset space, leave it for the labels
const qreal b = rect.bottom();

if(t > b)
return;

qDebug() << x1 << x2;

int f = qFloor((x1 - horizOffset) / hourOffset);
qreal fx = f * hourOffset + horizOffset;

if(f < 0.0)
{
f = 0.0;
fx = horizOffset;
}

if(fx < x1)
{
f += 1;
fx += hourOffset;
}

int l = qFloor((x2 - horizOffset) / hourOffset);
qreal lx = l * hourOffset + horizOffset;

if(l > 25.0)
{
l = 25.0;
lx = 25.0 * hourOffset + horizOffset;
}

if(lx > x2)
{
l -= 1;
lx -= hourOffset;
}

if(f > l)
return;

qDebug() << " First:" << f << "Last:" << l;
qDebug() << " " << fx << lx;

if(true)
{
painter->setPen(Qt::blue);
painter->setFont(QFont("ARIAL", 10, QFont::Bold));

const QString fmt = QStringLiteral("%1:00");
double huorX = horizOffset;
for(int i = 0; i < 25; i++)
{
qDebug() << "Shift Hour:" << i << "X:" << huorX << horizOffset << hourOffset;
painter->drawText(QPointF(huorX, 15), fmt.arg(i));
huorX += hourOffset;
}
}

size_t n = size_t(l - f + 1);
QLineF *arr = new QLineF[n];
double lineX = fx;
for(std::size_t i = 0; i < n; i++)
{
arr[i] = QLineF(lineX, t, lineX, b);
lineX += hourOffset;
}

painter->setPen(linePen);
painter->drawLines(arr, int(n));
delete [] arr;
}

Thanks in advance