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Qt-interest Archive, August 2007
OpenGL Perspective QTransform Issue

Message 1 in thread

    I am rendering a QGraphicsPixmapItem in a QGraphicsScene in a QGraphicsView.
    The QGraphicsView uses a QGLWidget viewport.

    I am trying to rotate an image around its Y-axis but from 72
degrees until 102 degrees the image disappears.
    So the image's perspective rotation around the Y-axis is as follows:
      * 0 -> 71 degrees: visible
      * 72 -> 102 degrees: INVISIBLE
      * 103 -> 180 degrees: visible

    Also noteworthy: At certain degrees I see just one pixel (at 79
degrees and at 101 degrees)

    Screenshot of the application to give you an idea of the kind of
perspective transformation I am trying to do:
        http://wesley.debianbox.be/images/SlideFlowDevelopment.png
(left part of the screen)

    The following code is an example project that illustrates my problem.
    It is also available as a tar.gz package here:
        http://wesley.debianbox.be/files/RotationProblem.tar.gz
        Pastebin version is here: http://qtnode.net/index.php/pastebin/4453

//////// main.cpp ////////

#include <QApplication>
#include "test.h"

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    test *t = new test();
    t->show();
    return app.exec();
}

//////// test.h ////////

#include <QWidget>
class QGraphicsScene;
class QGraphicsPixmapItem;
class QGraphicsView;

class test : public QWidget
{
    Q_OBJECT
public:
    test(QWidget *parent = 0);
private:
    qreal xx, yy;
    QGraphicsView *view;
    QGraphicsScene *sscene;
    QGraphicsPixmapItem *one;
private slots:
    void tl2(int frame);
};

//////// test.cpp ////////

#include <QtGui>
#include <QtOpenGL>
#include "test.h"

const char *IMAGE = "test.png"; // image to use

test::test(QWidget *parent) : QWidget(parent)
{
    xx = yy = 0.0;

    setWindowModality(Qt::NonModal);
    setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);
    resize(500, 500);

    sscene = new QGraphicsScene(this);
    view = new QGraphicsView(this);
    QGLWidget *glw = new QGLWidget();
    view->resize(450, 450);
    view->setViewport(glw);
    view->setScene(sscene);
    view->setBackgroundBrush(Qt::black);

    QVBoxLayout *vbox = new QVBoxLayout();

    QPalette palette;
    QBrush brush(QColor(0, 0, 0, 255));
    brush.setStyle(Qt::SolidPattern);
    palette.setBrush(QPalette::Active, QPalette::Window, brush);
    palette.setBrush(QPalette::Inactive, QPalette::Window, brush);
    palette.setBrush(QPalette::Disabled, QPalette::Window, brush);
    setPalette(palette);

    vbox->addWidget(view);
    setLayout(vbox);

    one = sscene->addPixmap(QPixmap(IMAGE).scaled(view->width()-10,
      view->height(), Qt::KeepAspectRatio));
    one->setPos((view->width()-one->sceneBoundingRect().width())/2,
      (view->height()-one->sceneBoundingRect().height())/2);
    view->setSceneRect(sscene->itemsBoundingRect());

    QTimeLine *time = new QTimeLine(7000);
    time->setFrameRange(0, 180);
    time->setUpdateInterval(20);
    connect(time, SIGNAL(frameChanged(int)), this, SLOT(tl2(int)));
    time->start();
}

void test::tl2(int frame)
{
    QTransform t, t1, t2;
    if (xx == 0.0) xx = one->sceneBoundingRect().width()/2;
    if (yy == 0.0) yy = one->sceneBoundingRect().height()/2;
    t1.translate(-xx, -yy);
    t1.rotate(frame, Qt::YAxis);
    t2.translate(xx, yy);
    t = t1 * t2;
    one->setTransform(t);
}

//////// test.pro ////////

TEMPLATE = app
TARGET =
DEPENDPATH += .
INCLUDEPATH += .

# Input
HEADERS += test.h
SOURCES += main.cpp test.cpp

QT += opengl


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Message 2 in thread

I haven't used QGraphicsView with OpenGL yet, so i might be totally off,
but this comes to my mind: did you check your near clip plane? if an
edge of the rotating quad gets too close to the camera it might get
clipped.

Cheers,
Peter 

gogogowesley wrote:
>     I am rendering a QGraphicsPixmapItem in a QGraphicsScene in a
> QGraphicsView.
>     The QGraphicsView uses a QGLWidget viewport.
> 
>     I am trying to rotate an image around its Y-axis but from 72
> degrees until 102 degrees the image disappears.
>     So the image's perspective rotation around the Y-axis is as
follows:
>       * 0 -> 71 degrees: visible
>       * 72 -> 102 degrees: INVISIBLE
>       * 103 -> 180 degrees: visible
> 

--
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Message 3 in thread

gogogowesley wrote:
> 
> 2007/8/9, Peter Prade <prade@xxxxxxxxxxx>:
> > I haven't used QGraphicsView with OpenGL yet, so i might be totally
off,
> > but this comes to my mind: did you check your near clip plane? if an
> > edge of the rotating quad gets too close to the camera it might get
> > clipped.
> >
> 
> Does anyone know how I should fix that? This is not pure OpenGL.

you probably wanted to write to the list, not to me in private.

i have no clue, but if i had to debug this, i'd try to give the quad a
different "depth" position.

Cheers,
Peter

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