How to remove/clear previously drawn lines before redrawing new lines in Qt? - qt

Question

I am drawing few lines using paintEvent(QPaintEvent * event). Sometime later if I want to draw new lines then the previously drawn lines are not cleared/removed. How could I draw the new lines only, by removing/clearing old lines. Is there any property to clear the previously drawn lines.Please let me know.
void QGraphWidget::paintEvent(QPaintEvent * event)
{
const QRect & rect = event->rect();
QPainter painter(this);
painter.setRenderHint(QPainter::Antialiasing);
drawLines(painter, rect);//calling painter.drawLine() inside
}
Thanks...

Answer 1

In my opinion, the correct approach is to clear the area before drawing new lines. This can be achieved either by using the autoFillBackground property as proposed by Stephen Chu, or by calling manually the QPainter::eraseRect() before drawing your lines in the QGraphWidget::paintEvent method.
void QGraphWidget::paintEvent(QPaintEvent * event)
{
const QRect & rect = event->rect();
QPainter painter(this);
painter.eraseRect(rect);
painter.setRenderHint(QPainter::Antialiasing);
drawLines(painter, rect);
}
Another option is to draw "negatively" the lines from the previous call to QGraphWidget::paintEvent. If you store the coordinate of your old lines, you might first draw lines using the background brush and then draw your new lines using the foreground brush. See code sample that might fit into you drawLines method. Note that if you draw something else in your widget, drawing the lines negatively might erase some of the other graphics which is why the first approach, erasing all the GraphWidget area, is better.
// save the current brush
QBrush currentBrush = painter.brush();
// draw the old line with the background brush
painter.setBrush(background());
painter.drawLine(oldLine);
// draw the new line with the current brush
painter.setBrush(currentBrush);
painter.drawLine(newLine);

Answer 2

You want to fill your widget with window background color before redraw. Set autoFillBackground to true after you create the widget and Qt will handle this for you

Answer 3

I don't think there is a specific call to remove the line. If you're repainting the entire area each time paintEvent() function is called you shouldn't see previous lines. If you're not repainting the area, you'll have to draw over the line yourself. This code for instance is drawing a line in a different position each time the method is invoked:
QPainter painter(this);
painter.setBrush(QBrush(Qt::red));
painter.drawRect(rect());
painter.setPen(QPen(Qt::yellow));
static int i = 0;
painter.drawLine(QPointF(i, i), QPointF(i, height() - i));
i++;
but "previous lines" are cleared completely. If you want to keep those you'll have to repaint only a specific area or you'll have to repaint those.

Answer 4

This is one way to delete whole line from QT.
me->setFocus();
int pos;
QTextCursor tc= me->textCursor();
pos=tc.columnNumber();
tc.select(QTextCursor::LineUnderCursor);
QString str=tc.selectedText();
tc.removeSelectedText();
tc.movePosition(QTextCursor::NextBlock,QTextCursor::MoveAnchor);
tc.insertText(str);
tc.insertBlock();
tc.movePosition(QTextCursor::PreviousBlock,QTextCursor::MoveAnchor);
tc.movePosition(QTextCursor::StartOfLine,QTextCursor::MoveAnchor);
me->setTextCursor(tc);
return true;