private Rectangle GetVisibleRectangle(Control c)
{
// rectangle du controle en coordonnées écran
Rectangle rect = c.RectangleToScreen(c.ClientRectangle);
c = c.Parent;
while (c != null)
{
rect = Rectangle.Intersect(rect, c.RectangleToScreen(c.ClientRectangle));
c = c.Parent;
}
// rectangle en coordonnées relatives au client
rect = pnlCanvas.RectangleToClient(rect);
return(rect);
}