why QPainter cause "image composition".

cainiao123

I use the method "QPrinter.drawText( )" to genarate a image, and set it in a lable.
If i genarate several times, it will cause "image composition".
I don't user the QPrinter.setCompositionMode(xxx), how it could be? below is the codes , please help me !

void MainWindow::on_pushButton_clicked()
{
static int i=0;

QImage *mainimg = new QImage();
*mainimg = QImage(240+1,240+1,QImage::Format_ARGB32); //size 240*240
QPainter painter(mainimg);

painter.setPen(QColor(Qt::black));
painter.drawRect(0,0,240,240);
painter.drawLine(0,120,240,120);

QFont f12(u8"宋体",12);
painter.setFont(f12);
painter.drawText(10,20,"测试重复生成图片");

char text[100];
sprintf(text,"%d%d%d%d%d",i,i,i,i,i);
painter.drawText(10,50,text);
qDebug()<<i++<<":"<<text;
painter.end();

QPixmap map = QPixmap::fromImage(*mainimg);
ui->label_test->setPixmap(map);

delete mainimg;

}

Christian Ehrlicher

@cainiao123 said in why QPainter cause "image composition".:

QImage *mainimg = new QImage();
mainimg = QImage(240+1,240+1,QImage::Format_ARGB32); //size 240240

Your create a memleaks here for no reason - create the QImage on the stack.

Also you don't initialize your QImage with a background color so everywhere where you don't paint anything random stuff will be shown (for instance your old data). Clear your QImage with QPainter::fillRect() or similar.

cainiao123

This is the result of the program running.

Christian Ehrlicher

@cainiao123 said in why QPainter cause "image composition".:

QImage *mainimg = new QImage();
mainimg = QImage(240+1,240+1,QImage::Format_ARGB32); //size 240240

Your create a memleaks here for no reason - create the QImage on the stack.

Also you don't initialize your QImage with a background color so everywhere where you don't paint anything random stuff will be shown (for instance your old data). Clear your QImage with QPainter::fillRect() or similar.

cainiao123

@Christian-Ehrlicher thank you very much
I set the background color to the QPainter and make a lot of test,
It seems to be no problem now~
thank you again.