Access violation in QImage from QRenderCapture

Argostin

Hi there,
I'm trying to use QRenderCapture to get QImage that I'll use later but when I access the QImage from the QRenderCaptureReply I get read access violation.
This happens only when I try to use the binary information of the actual image (Qimage::pixel(int x, int y), QImage::bits(), QImage::save(const QString &fileName)), when I try to access the metadata of the QImage it works properly (QImage::size(), QImage::width(), QImage::isNull()).
I tried this example https://code.woboq.org/qt5/qt3d/tests/manual/rendercapture-cpp/ and got it to work until I try to access the image data and once again it gives me read access violation.
Any ideas on how to fix the problem or at least where to look for a solution?

Argostin

As a last resort, I installed a new version of Qt (I was with 6.1.2 and now I'm with 6.2.0) and now it works properly. No more crashing and read access violations.
I guess it is some kind of a bug.

SGaist

Hi,

Can you provide a minimal compilable example that shows this ?

Which version of Qt are you using ?
On which platform ?

Argostin

I am using Qt 6.1.2 on Windows 10.

Argostin

Here is my code:

main.cpp:

#include <windows.h>
#include <QtWidgets/qapplication.h>
#include "My3DWindow.h"

int WINAPI WinMain(
_In_ HINSTANCE hInstance,
_In_opt_ HINSTANCE hPrevInstance,
_In_ LPSTR lpCmdLine,
_In_ int nShowCmd
) {
int temp = 0;
QApplication app(temp, &lpCmdLine);
My3dWindow myWindow;
myWindow.start();

return app.exec();
}

My3dWindow.h:

#pragma once

#include <Qt3DCore/QEntity>
#include <Qt3DExtras/qt3dwindow.h>
#include <Qt3DRender/QCamera>
#include <Qt3DRender/QRenderCapture>

class My3dWindow : public Qt3DExtras::Qt3DWindow {
Q_OBJECT
public:
void start();
private:
void createSceneGraph(); //1
void createCamera(); //2
void createFrameGraph(); //3
Qt3DCore::QEntity* rootEntity;
Qt3DRender::QCamera* cameraObject;
Qt3DRender::QRenderCapture* renderCapture;
Qt3DRender::QRenderCaptureReply* reply;

private slots:
void onImageRendered();

private:
void requestRenderCapture();
};

My3dWindow.cpp:

#include "My3dWindow.h"

#include <Qt3DExtras/QCuboidMesh>
#include <Qt3DExtras/QDiffuseSpecularMaterial>
#include <Qt3DRender/QRenderSurfaceSelector>
#include <Qt3DRender/QViewport>
#include <Qt3DRender/QCameraSelector>
#include <Qt3DRender/QClearBuffers>
#include <Qt3DExtras/QOrbitCameraController>
#include <Qt3DRender/QCameraLens>

void My3dWindow::start() {
    reply = nullptr;
    createSceneGraph();
    createCamera();
    createFrameGraph();
    this->show();
    requestRenderCapture();
}

void My3dWindow::createSceneGraph() {
rootEntity = new Qt3DCore::QEntity();
Qt3DCore::QEntity* cubeEntity = new Qt3DCore::QEntity(rootEntity);

Qt3DExtras::QCuboidMesh* cuboidMesh = new Qt3DExtras::QCuboidMesh();
cubeEntity->addComponent(cuboidMesh);

Qt3DExtras::QDiffuseSpecularMaterial* material = new
Qt3DExtras::QDiffuseSpecularMaterial();
QColor materialColor(200, 100, 50);
material->setDiffuse(materialColor);
material->setAmbient(materialColor);
cubeEntity->addComponent(material);

this->setRootEntity(rootEntity);
}

void My3dWindow::createCamera() {
    static constexpr QVector3D cameraUpVector(0.0f, 1.0f, 0.0f);
    static constexpr QVector3D cameraPosition(40.0f, 40.0f, 40.0f);
    static constexpr QVector3D cameraViewCenter(0.0f, 0.0f, 0.0f);
    static constexpr float cameraLinearSpeed = 50.0f;
    static constexpr float cameraLookSpeed = 180.0f;

    cameraObject = this->camera();
    cameraObject->setProjectionType(Qt3DRender::QCameraLens::PerspectiveProjection);
    cameraObject->setUpVector(cameraUpVector);
    cameraObject->setPosition(cameraPosition);
    cameraObject->setViewCenter(cameraViewCenter);

    Qt3DExtras::QOrbitCameraController* camController = new
Qt3DExtras::QOrbitCameraController(rootEntity);
    camController->setLinearSpeed(cameraLinearSpeed);
    camController->setLookSpeed(cameraLookSpeed);
    camController->setCamera(cameraObject);
    camController->setEnabled(true);
}

void My3dWindow::createFrameGraph() {
    Qt3DRender::QRenderSurfaceSelector* surfaceSelector = new
Qt3DRender::QRenderSurfaceSelector;
    surfaceSelector->setSurface(this);
    /*surfaceSelector*/ {
        Qt3DRender::QViewport* viewport = new
Qt3DRender::QViewport(surfaceSelector);
        viewport->setNormalizedRect(QRectF(0, 0, 1.0, 1.0));
        /*viewport*/ {
            Qt3DRender::QCameraSelector* cameraSelector = new
Qt3DRender::QCameraSelector(viewport);
            cameraSelector->setCamera(cameraObject);
            /*cameraSelector*/ {
                Qt3DRender::QClearBuffers* clearBuffers = new
Qt3DRender::QClearBuffers(cameraSelector);

clearBuffers->setBuffers(Qt3DRender::QClearBuffers::ColorDepthBuffer);
                clearBuffers->setClearColor(QColor(50, 50, 50));
                /*clearBuffers*/ {
                    renderCapture = new
Qt3DRender::QRenderCapture(clearBuffers);
                }
            }
        }
    }

    this->setActiveFrameGraph(surfaceSelector);
}

void My3dWindow::requestRenderCapture() {
    if (!reply) {
        reply = renderCapture->requestCapture();
        connect(reply, &Qt3DRender::QRenderCaptureReply::completed,
this, &My3dWindow::onImageRendered);
    }
}

void My3dWindow::onImageRendered() {
QImage* qimage = new QImage(reply->image());

qDebug() << "Image size in bytes:" << qimage->sizeInBytes();
qDebug() << "Width:" << qimage->width() << "Height:" << qimage->height();


QColor rgb = qimage->pixel(QPoint(0, 0));
    qDebug() << "Color: " << " " << rgb.red() << " " << rgb.green() <<
" " << rgb.blue();


reply->deleteLater();
reply = nullptr;

requestRenderCapture();
}

It stops as soon as it hits the "QColor rgb = qimage->pixel(QPoint(0, 0));" line, and it gives me the read access violation

ollarch

@Argostin said in Access violation in QImage from QRenderCapture:

QImage* qimage = new QImage(reply->image());

Here you are creating an image on every function call.

@Argostin said in Access violation in QImage from QRenderCapture:

reply->deleteLater();
reply = nullptr;

You create an image that does not copy the data, it stores the pointer to the data and finally you are deleting it.

Argostin

Hi @ollarch
I don't think this is part of the main problem, because the program stops on the first iteration so the code:

reply->deleteLater();
reply = nullptr;

isn't even executed.

But still, how do you suggest I should rework it?

ollarch

Hi,

If the image have to be used after deleting the reply you need to copy it.

Have you checked the call stack on debugger just when it crash?

Argostin

Here is the call stack:

Also, "RenderCapture" is the name of the project

JonB

@Argostin
Prior to your qimage->pixel() what happens if you go qimage->copy()? Does that violate too?

Since QPoint(0, 0) is a null QPoint, humour me by trying some coordinates other than that?

Argostin

@JonB
Yes, qimage->copy() does violate in the same way and I've tried several different points but none of them worked.
Still, thanks for the suggestions.

Argostin

As a last resort, I installed a new version of Qt (I was with 6.1.2 and now I'm with 6.2.0) and now it works properly. No more crashing and read access violations.
I guess it is some kind of a bug.

JonB

@Argostin LOL, well done!

ollarch

@Argostin Good job!