App crashes when I try to get the position attribute of a QMesh (Qt 5.15.2)

[franco.amato]

I try to get the position attribute of a mesh using the following code

....code...
   Qt3DRender::QMesh *mesh = new Qt3DRender::QMesh();
    mesh->setMeshName("mesh");
    // Import mesh data from a ply file (monster.ply located at same level of the executable)
    mesh->setSource(QUrl::fromLocalFile("monster.ply"));
 
    // The call to mesh->geometry() crashes my app
    if (mesh->primitiveType() == Qt3DRender::QGeometryRenderer::Triangles)
    {
        for(int i = 0; i < mesh->geometry()->attributes().size(); ++i)
        {
            // To have access to data
            mesh->geometry()->attributes().at(i)->buffer()->setSyncData(true);
        }
 
        // Now I need to get the position of the vertices of the triangles
 
        Qt3DRender::QAttribute *positionAttribute = mesh->geometry()->attributes().at(0);
        ...more code...
    }

....code...
   Qt3DRender::QMesh *mesh = new Qt3DRender::QMesh();
    mesh->setMeshName("mesh");
    // Import mesh data from a ply file (monster.ply located at same level of the executable)
    mesh->setSource(QUrl::fromLocalFile("monster.ply"));

    // The call to mesh->geometry() crashes my app
    if (mesh->primitiveType() == Qt3DRender::QGeometryRenderer::Triangles)
    {
        for(int i = 0; i < mesh->geometry()->attributes().size(); ++i)
        {
            // To have access to data
            mesh->geometry()->attributes().at(i)->buffer()->setSyncData(true);
        }

        // Now I need to get the position of the vertices of the triangles

        Qt3DRender::QAttribute *positionAttribute = mesh->geometry()->attributes().at(0);
        ...more code...
    }

To copy to clipboard, switch view to plain text mode 

This is the content of the qmake script

QT += 3dcore 3drender 3dinput 3dextras
QT += widgets
 
SOURCES += main.cpp

QT += 3dcore 3drender 3dinput 3dextras
QT += widgets

SOURCES += main.cpp

To copy to clipboard, switch view to plain text mode 

I am using Qt5 LTS version.
Am I forgetting some plugins or am I doing something wrong?
I don't understand why I am getting such runtime error.

Franco

[d_stranz]

So where does the debugger say it crashes?

You have several places where you access member functions of "mesh" which return pointers. You don't check that those pointers are valid (non-null) before using them to dig further into the data structure.

mesh->geometry()->attributes().size()
mesh->geometry()->attributes().at(i)->buffer()->setSyncData(true)
mesh->geometry()->attributes().at(0)

mesh->geometry()->attributes().size()
mesh->geometry()->attributes().at(i)->buffer()->setSyncData(true)
mesh->geometry()->attributes().at(0)

To copy to clipboard, switch view to plain text mode 

How do you know that the call to geometry(), attributes(), at(), or buffer() returns a valid pointer? Do some error checking, put in some asserts(), don't just assume everything works.

[franco.amato]

I cannot call any method that belong to Qt3DRender::QGeometry
The error occurs on the call QMesh::geometry();

This code gives "Invalid pointer:

auto *geom = mesh->geometry();
if (geom == nullptr)
{
    qDebug() << "Invalid pointer";
    return 1;
}

auto *geom = mesh->geometry();
if (geom == nullptr)
{
    qDebug() << "Invalid pointer";
    return 1;
}

To copy to clipboard, switch view to plain text mode 

I can open the same mesh with a non qt app without errors so it maybe it's a bug of my current Qt version?

[d_stranz]

And what do you get as a Status if you call mesh->status() after assigning the source URL?

And once you see that it is returning Error, then think about why QUrl (and your runtime environment) doesn't find your monster file.

[franco.amato]

And what do you get as a Status if you call mesh->status() after assigning the source URL?
And once you see that it is returning Error, then think about why QUrl (and your runtime environment) doesn't find your monster file.

I changed the code to:

QUrl data = QUrl::fromLocalFile("statue.ply");
 
Qt3DRender::QMesh *bodyMesh = new Qt3DRender::QMesh();
bodyMesh->setMeshName("bodyMesh");
bodyMesh->setSource(data);

QUrl data = QUrl::fromLocalFile("statue.ply");

Qt3DRender::QMesh *bodyMesh = new Qt3DRender::QMesh();
bodyMesh->setMeshName("bodyMesh");
bodyMesh->setSource(data);

To copy to clipboard, switch view to plain text mode 

And now it works (i don't know why)

About the status:

The following code:

bodyMesh->setSource(data);
 
    auto status = bodyMesh->status();
    if (status !=Qt3DRender::QMesh::Ready)
    {
        qDebug() << "An error occurred when loading the mesh.\n"
                 << "Error code: " << status;
    }

bodyMesh->setSource(data);

    auto status = bodyMesh->status();
    if (status !=Qt3DRender::QMesh::Ready)
    {
        qDebug() << "An error occurred when loading the mesh.\n"
                 << "Error code: " << status;
    }

To copy to clipboard, switch view to plain text mode 

Shows the output:

An error occurred when loading the mesh.
Error code: Qt3DRender::QMesh::None

I don't why it's showing that error if I can see the mesh in the window

[d_stranz]

I don't why it's showing that error if I can see the mesh in the window

Weird. Don't know. Possibly the mesh hasn't finished (or even started) loading yet since there is no delay between the setSource() and status() calls.

And now it works (i don't know why)

Maybe the mesh needs to have a name property set on it first? Does it work if you replace "statue" with "monster" (with the monster file in the same location as the statue file)? If not, it could be that Qt's implementation of the PLY format reader doesn't support all of the elements that your external reader does.

[franco.amato]

Weird. Don't know. Possibly the mesh hasn't finished (or even started) loading yet since there is no delay between the setSource() and status() calls.

Maybe the mesh needs to have a name property set on it first? Does it work if you replace "statue" with "monster" (with the monster file in the same location as the statue file)? If not, it could be that Qt's implementation of the PLY format reader doesn't support all of the elements that your external reader does.

I have another model called "monster.ply". Changing the name in the setSource() method caused the other model to be loaded and I always get the same error

[d_stranz]

I have another model called "monster.ply". Changing the name in the setSource() method caused the other model to be loaded and I always get the same error

But is it now working otherwise? You can access the geometry() without a crash? If so, then I wouldn't worry about the confusing error.

[franco.amato]

But is it now working otherwise? You can access the geometry() without a crash? If so, then I wouldn't worry about the confusing error.

I still get the crash adding the code I posted at the beginning.
I really have to have it working because I have to populate a graph with the mesh vertex.
I am willing to pay for a help

[d_stranz]

I am willing to pay for a help

How about just click "Thanks"?

OK, I got curious, so I decided to write a test program. I found an example PLY file online and used that, and then I hacked up the Qt3D Basic Shapes Example. Like you, I observed that after loading the mesh, the Status was "None" and the geometry() call returns a NULL pointer. However, I could see the mesh when I displayed the window. So I dug a little deeper and found that the Status does not become Ready and the geometry() is not valid until after the mesh is displayed.

Here is my complete example. I'm using Qt 5.15.3 but it should also work in Qt6:

Qt3DExample.h

#pragma once
 
#include <QtWidgets/QMainWindow>
 
#include <QEntity>
#include <QMesh>
 
class Qt3DExample : public QMainWindow
{
    Q_OBJECT
 
public:
    Qt3DExample(QWidget *parent = Q_NULLPTR);
 
protected:
    void  keyPressEvent( QKeyEvent * pEvent );
 
private:
    Qt3DRender::QMesh * mpMesh = nullptr;
    Qt3DCore::QEntity * mpRootEntity = nullptr;
    Qt3DCore::QEntity * mpMeshEntity = nullptr;
};

#pragma once

#include <QtWidgets/QMainWindow>

#include <QEntity>
#include <QMesh>

class Qt3DExample : public QMainWindow
{
    Q_OBJECT

public:
    Qt3DExample(QWidget *parent = Q_NULLPTR);

protected:
    void  keyPressEvent( QKeyEvent * pEvent );

private:
    Qt3DRender::QMesh * mpMesh = nullptr;
    Qt3DCore::QEntity * mpRootEntity = nullptr;
    Qt3DCore::QEntity * mpMeshEntity = nullptr;
};

To copy to clipboard, switch view to plain text mode 

Qt3DExample.cpp:

#include "Qt3DExample.h"
 
#include <QMesh>
#include <Qt3DWindow>
#include <QCamera>
#include <QInputAspect>
#include <QPointLight>
#include <QTransform>
#include <QFirstPersonCameraController>
#include <QPhongMaterial>
 
Qt3DExample::Qt3DExample(QWidget *parent)
    : QMainWindow(parent)
{
    // Root entity
    mpRootEntity = new Qt3DCore::QEntity();
 
    Qt3DExtras::Qt3DWindow * pView = new Qt3DExtras::Qt3DWindow();
    QWidget * pCentralWidget = QWidget::createWindowContainer( pView );
    pCentralWidget->setMinimumSize( 200, 200 );
    pCentralWidget->setMaximumSize( 2000, 2000 );
    resize( 1000, 1000 );
 
    Qt3DInput::QInputAspect * input = new Qt3DInput::QInputAspect;
    pView->registerAspect( input );
 
    // Camera
    Qt3DRender::QCamera * cameraEntity = pView->camera();
 
    cameraEntity->lens()->setPerspectiveProjection( 45.0f, 16.0f / 9.0f, 0.1f, 1000.0f );
    cameraEntity->setPosition( QVector3D( 0, 0, 20.0f ) );
    cameraEntity->setUpVector( QVector3D( 0, 1, 0 ) );
    cameraEntity->setViewCenter( QVector3D( 0, 0, 0 ) );
 
    Qt3DCore::QEntity * lightEntity = new Qt3DCore::QEntity( mpRootEntity );
    Qt3DRender::QPointLight * light = new Qt3DRender::QPointLight( lightEntity );
    light->setColor( "white" );
    light->setIntensity( 1 );
    lightEntity->addComponent( light );
    Qt3DCore::QTransform * lightTransform = new Qt3DCore::QTransform( lightEntity );
    lightTransform->setTranslation( cameraEntity->position() );
    lightEntity->addComponent( lightTransform );
 
    // For camera controls
    Qt3DExtras::QFirstPersonCameraController * camController = new Qt3DExtras::QFirstPersonCameraController( mpRootEntity );
    camController->setCamera( cameraEntity );
 
     // Set root object of the scene
    pView->setRootEntity( mpRootEntity );
 
    mpMesh = new Qt3DRender::QMesh();
    mpMesh->setMeshName( "mesh" );
 
    // Import mesh data from a ply file (Copperkey.ply located at same level of the executable)
    QUrl	url = QUrl::fromLocalFile( "Copperkey.ply" );
    mpMesh->setSource( url );
    auto status = mpMesh->status();				// Returns None
    auto pGeometry = mpMesh->geometry();		// Returns NULL
 
    // Mesh Transform
    Qt3DCore::QTransform * meshTransform = new Qt3DCore::QTransform();
    meshTransform->setScale( 0.1f );
    meshTransform->setRotation( QQuaternion::fromAxisAndAngle( QVector3D( 0.0f, 1.0f, 0.0f ), 25.0f ) );
    meshTransform->setTranslation( QVector3D( 5.0f, 4.0f, 0.0f ) );
 
    // Mesh material
    Qt3DExtras::QPhongMaterial * meshMaterial = new Qt3DExtras::QPhongMaterial();
    meshMaterial->setDiffuse( QColor( QRgb( 0xbeb32b ) ) );
 
    // Mesh entity
    mpMeshEntity = new Qt3DCore::QEntity( mpRootEntity );
    mpMeshEntity->addComponent( mpMesh );
    mpMeshEntity->addComponent( meshMaterial );
    mpMeshEntity->addComponent( meshTransform );
 
    setCentralWidget( pCentralWidget );
}
 
void Qt3DExample::keyPressEvent( QKeyEvent * pEvent )
{
   auto status = mpMesh->status();                 // Returns Ready
   auto pGeometry = mpMesh->geometry();     // Returns a valid pointer
 
   // Set a breakpoint on this line:
   int	foo = 42;
}

#include "Qt3DExample.h"

#include <QMesh>
#include <Qt3DWindow>
#include <QCamera>
#include <QInputAspect>
#include <QPointLight>
#include <QTransform>
#include <QFirstPersonCameraController>
#include <QPhongMaterial>

Qt3DExample::Qt3DExample(QWidget *parent)
    : QMainWindow(parent)
{
    // Root entity
    mpRootEntity = new Qt3DCore::QEntity();

    Qt3DExtras::Qt3DWindow * pView = new Qt3DExtras::Qt3DWindow();
    QWidget * pCentralWidget = QWidget::createWindowContainer( pView );
    pCentralWidget->setMinimumSize( 200, 200 );
    pCentralWidget->setMaximumSize( 2000, 2000 );
    resize( 1000, 1000 );

    Qt3DInput::QInputAspect * input = new Qt3DInput::QInputAspect;
    pView->registerAspect( input );

    // Camera
    Qt3DRender::QCamera * cameraEntity = pView->camera();

    cameraEntity->lens()->setPerspectiveProjection( 45.0f, 16.0f / 9.0f, 0.1f, 1000.0f );
    cameraEntity->setPosition( QVector3D( 0, 0, 20.0f ) );
    cameraEntity->setUpVector( QVector3D( 0, 1, 0 ) );
    cameraEntity->setViewCenter( QVector3D( 0, 0, 0 ) );

    Qt3DCore::QEntity * lightEntity = new Qt3DCore::QEntity( mpRootEntity );
    Qt3DRender::QPointLight * light = new Qt3DRender::QPointLight( lightEntity );
    light->setColor( "white" );
    light->setIntensity( 1 );
    lightEntity->addComponent( light );
    Qt3DCore::QTransform * lightTransform = new Qt3DCore::QTransform( lightEntity );
    lightTransform->setTranslation( cameraEntity->position() );
    lightEntity->addComponent( lightTransform );

    // For camera controls
    Qt3DExtras::QFirstPersonCameraController * camController = new Qt3DExtras::QFirstPersonCameraController( mpRootEntity );
    camController->setCamera( cameraEntity );

     // Set root object of the scene
    pView->setRootEntity( mpRootEntity );

    mpMesh = new Qt3DRender::QMesh();
    mpMesh->setMeshName( "mesh" );

    // Import mesh data from a ply file (Copperkey.ply located at same level of the executable)
    QUrl	url = QUrl::fromLocalFile( "Copperkey.ply" );
    mpMesh->setSource( url );
    auto status = mpMesh->status();				// Returns None
    auto pGeometry = mpMesh->geometry();		// Returns NULL

    // Mesh Transform
    Qt3DCore::QTransform * meshTransform = new Qt3DCore::QTransform();
    meshTransform->setScale( 0.1f );
    meshTransform->setRotation( QQuaternion::fromAxisAndAngle( QVector3D( 0.0f, 1.0f, 0.0f ), 25.0f ) );
    meshTransform->setTranslation( QVector3D( 5.0f, 4.0f, 0.0f ) );

    // Mesh material
    Qt3DExtras::QPhongMaterial * meshMaterial = new Qt3DExtras::QPhongMaterial();
    meshMaterial->setDiffuse( QColor( QRgb( 0xbeb32b ) ) );

    // Mesh entity
    mpMeshEntity = new Qt3DCore::QEntity( mpRootEntity );
    mpMeshEntity->addComponent( mpMesh );
    mpMeshEntity->addComponent( meshMaterial );
    mpMeshEntity->addComponent( meshTransform );

    setCentralWidget( pCentralWidget );
}

void Qt3DExample::keyPressEvent( QKeyEvent * pEvent )
{
   auto status = mpMesh->status();                 // Returns Ready
   auto pGeometry = mpMesh->geometry();     // Returns a valid pointer

   // Set a breakpoint on this line:
   int	foo = 42;
}

To copy to clipboard, switch view to plain text mode 

main.cpp:

#include "Qt3DExample.h"
#include <QtWidgets/QApplication>
 
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    Qt3DExample w;
    w.show();
    return a.exec();
}

#include "Qt3DExample.h"
#include <QtWidgets/QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    Qt3DExample w;
    w.show();
    return a.exec();
}

To copy to clipboard, switch view to plain text mode 

Set a breakpoint on the line shown in the keypress event. Run the program. After the mesh appears on screen, type any key to hit the breakpoint.

You might have to change the scale factor and translation in the meshTransform to suit your PLY object.