QGraphicsItem disappear when zoom is too large.

[mcosta]

What is the value of length??

if length is 2, you're painting 4 points

[Zikoel]

length is a global variable and the value is 20.

[mcosta]

From QT Docs

void QGraphicsItem:aint ( QPainter * painter, const QStyleOptionGraphicsItem * option, QWidget * widget = 0 ) [pure virtual]
...
Make sure to constrain all painting inside the boundaries of boundingRect() to avoid rendering artifacts (as QGraphicsView does not clip the painter for you).
...

How do you set the boundingRect() ?
Show the code of CMCanvasPointer::boundingRect().

[SixDegrees]

What zoom level does the object disappear at? You may be over- or under-flowing the painter's underlying units.