QPainter Fails to Paint

[gga96]

Good Morning All,

My app deals with burrowing down to the R G B pixel data in successive video frames. The frame analysis takes place in a dialog where method ShowFrame renders a zoomed QPixmap to screen.

My problem is I cannot get the QPainter to zoom the pixmap.

void IconDialog::ShowFrame(QString aFrameNm, int Zoom)
{
    QGraphicsPixmapItem *item;
    FrameNm = aFrameNm;
    scene->clear();
    Frame.load(aFrameNm);
    scene->setSceneRect(Frame.rect());
    QPixmap pixmap = QPixmap::fromImage(Frame);
    if (Zoom > 1) {
        QRectF sr = pixmap.rect();      //source rectangle
        int w = qRound(sr.width()/Zoom);
        int h = qRound(sr.height()/Zoom);
        QRectF tr;                      //target rectangle
        tr.setX(sr.width()-w);
        tr.setY(sr.height()-h);
        tr.setWidth(w);
        tr.setHeight(h);
        QPainter p;
        if (p.begin(this)) {
            p.drawPixmap(tr, pixmap, sr);
            p.end();
            }
        else {
            QMessageBox::information(0, "Error IconDialog", "Statement p.begin(this) failed, I don't know why.");
            }
        }
    item = new QGraphicsPixmapItem(pixmap);
    scene->addItem(item);
    ui->FrameGV->show();
}

void IconDialog::ShowFrame(QString aFrameNm, int Zoom)
{
    QGraphicsPixmapItem *item;
    FrameNm = aFrameNm;
    scene->clear();
    Frame.load(aFrameNm);
    scene->setSceneRect(Frame.rect());
    QPixmap pixmap = QPixmap::fromImage(Frame);
    if (Zoom > 1) {
        QRectF sr = pixmap.rect();      //source rectangle
        int w = qRound(sr.width()/Zoom);
        int h = qRound(sr.height()/Zoom);
        QRectF tr;                      //target rectangle
        tr.setX(sr.width()-w);
        tr.setY(sr.height()-h);
        tr.setWidth(w);
        tr.setHeight(h);
        QPainter p;
        if (p.begin(this)) {
            p.drawPixmap(tr, pixmap, sr);
            p.end();
            }
        else {
            QMessageBox::information(0, "Error IconDialog", "Statement p.begin(this) failed, I don't know why.");
            }
        }
    item = new QGraphicsPixmapItem(pixmap);
    scene->addItem(item);
    ui->FrameGV->show();
}

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What ever I try results in activating the QMessageBox, I cannot find a "last error" or a reason for failure.
I have followed the usual procedures to resolve the issue but without success.

I believe my code is at fault, I have missed something, but what?

Hope you can help.

Thank you.

Regards
Glen

[gga96]

I have found the errors I had made in the posted code (nothing like a good nights sleep).

Correction 1: Edit line 19 to read "if (p.begin(&pixmap)) {" that got painter working which exposed another error with Source and Target rectangles doing the wrong thing.

Correction2: Transpose the use of "sr" and "tr".

Sorry to have troubled you.

Regards
Glen

PS How do I mark this thread SOLVED?