Segmentation fault when painting a png in updatePaintNode() using QSGTexture

mxyn · 30 Jun 2020, 07:26

I'm stumped. I don't know why this code is throwing a segmentation fault. I can't tell where the seg fault is, but I know that it prints up to debug "t8". So it looks like it is exiting updatePaintNode() without a problem.
I'm ultimately trying to paint multiple png graphics on the scene in different coordinates every frame. So my current baby step is PAINT ONE png item on the scene graph.

I tried this code in QPainter and it paints fine. So I know I'm accessing it with the right filename.

updatePaintNode(....)
{
    QPixmap px1("pics/RadarCarSmall4.png");
    QImage img("pics/RadarCarSmall5.png");
    painter->drawPixmap(10,10,40,40, px1);   // just trying different ways to access and paint
    painter->drawImage(QPointF(50,50), img);  //just trying different way to access and paint

Then I tried this code in updatePaintNode() and it throws a segmentation fault.

 QSGNode *root = static_cast<QSGNode *>(oldNode);
 if(!root) {
     root = new QSGNode;
 }
qDebug("1");
    root->removeAllChildNodes();
    QSGGeometryNode *n = new QSGGeometryNode();
    QSGGeometry * geo2 = new QSGGeometry(QSGGeometry::defaultAttributes_Point2D(), 4);  // create geometry
    geo2->setDrawingMode(QSGGeometry::DrawTriangleFan);
    geo2->vertexDataAsPoint2D()[0].set(100,100);
    geo2->vertexDataAsPoint2D()[1].set(100,150);
    geo2->vertexDataAsPoint2D()[2].set(150,150);
    geo2->vertexDataAsPoint2D()[3].set(150,100);
qDebug("4");
    QImage img("pics/RadarCarSmall5.png");                   // create texture
    QSGTexture * t = window()->createTextureFromImage(img);
    QSGOpaqueTextureMaterial *testM = new QSGOpaqueTextureMaterial;    
    testM->setTexture(t);                             // set texture

QSGFlatColorMaterial *z2 = new QSGFlatcolorMaterial;
z2->setColor(Qt::blue);
    n->setMaterial(testM);   // if I replace testM with z2, the segmentation fault goes away.
    n->setGeometry(geo2);
   root->appendChildNode(n);
qDebug("t8");
return root
}

It's very possible I'm not use window() correctly. All I did was #include <QQuickWindow>... I'm not sure if that's the right way to use it?

mxyn · wrote on 1 Jul 2020, 06:39

I found the problem. I should not be using QSGGeometryNode. I should be using QSGSimpleTextureNode.

   QSGSimpleTextureNode *test = new QSGSimpleTextureNode;
    QImage img;
    bool state = img.load(":/scenegraph/customgeometry/radarcar.png");
    QSGTexture * t =  window()->createTextureFromImage(img);
    test->setTexture(t);
    test->setRect(10, 10, 30, 30);

jsulm · M mxyn 30 Jun 2020, 07:16

@mxyn said in Segmentation fault when painting a png in updatePaintNode() using QSGTexture:

it throws a segmentation fault

Where exactly?

mxyn · 30 Jun 2020, 07:47

I updated my previous code so you can see where debug "t8" get's printed. It seems to seg fault after exiting the function.

jsulm · M mxyn 30 Jun 2020, 07:46

@mxyn Please show stack trace after the crash

jsulm · 30 Jun 2020, 08:08

@mxyn said in Segmentation fault when painting a png in updatePaintNode() using QSGTexture:

QImage img("pics/RadarCarSmall5.png"); // create texture
QSGTexture * t = window()->createTextureFromImage(img);

I'm wondering why you're doing this everytime you paint? This can be done once.

mxyn · wrote on 30 Jun 2020, 07:58

I would love to! ... How do I pull up the backtrace? I know how to run gdb.... and then backtrace. Is this what you are asking for?

t8

Thread 9 "QThread" received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7fa8c4d1c0 (LWP 15010)]
0x0000007fb7179418 in QSGBatchRenderer::Renderer::renderUnmergedBatch(QSGBatchRenderer::Batch const*) () from /usr/lib/aarch64-linux-gnu/libQt5Quick.so.5
(gdb) backtrace
#0 0x0000007fb7179418 in QSGBatchRenderer::Renderer::renderUnmergedBatch(QSGBatchRenderer::Batch const*) () at /usr/lib/aarch64-linux-gnu/libQt5Quick.so.5
#1 0x0000007fb7179b60 in QSGBatchRenderer::Renderer::renderBatches() ()
at /usr/lib/aarch64-linux-gnu/libQt5Quick.so.5
#2 0x0000007fb717ed30 in QSGBatchRenderer::Renderer::render() ()
at /usr/lib/aarch64-linux-gnu/libQt5Quick.so.5
#3 0x0000007fb7170eec in QSGRenderer::renderScene(QSGBindable const&) ()
at /usr/lib/aarch64-linux-gnu/libQt5Quick.so.5
#4 0x0000007fb717134c in QSGRenderer::renderScene(unsigned int) ()
at /usr/lib/aarch64-linux-gnu/libQt5Quick.so.5
#5 0x0000007fb71a7cac in QSGDefaultRenderContext::renderNextFrame(QSGRenderer*, unsigned int) ()
at /usr/lib/aarch64-linux-gnu/libQt5Quick.so.5
#6 0x0000007fb7202d7c in QQuickWindowPrivate::renderSceneGraph(QSize const&) ()
at /usr/lib/aarch64-linux-gnu/libQt5Quick.so.5
#7 0x0000007fb72b71e0 in QQuickRenderControl::render() ()
at /usr/lib/aarch64-linux-gnu/libQt5Quick.so.5
#8 0x0000007fa8c550e0 in Worker::doRender(unsigned char*, unsigned char*) (this=0x7fa8c4c370, inFrame=0x8 <error: Cannot access memory at address 0x8>, outFrame=0xffffec00 <error: Cannot access memory at address 0xffffec00>) at gstqoverlay.cpp:344
#9 0x0000007fa8c568c0 in QtPrivate::FunctorCall<QtPrivate::IndexesList<0, 1>, QtPrivate::List<unsigned char*, unsigned char*>, bool, bool (Worker::)(unsigned char, unsigned char*)>::call(bool (Worker::)(unsigned char, unsigned char*), Worker*, void**) (arg=0x7f7effbc70, o=<optimized out>, f=<optimized out>) at /usr/include/aarch64-linux-gnu/qt5/QtCore/qobjectdefs_impl.h:136

mxyn · J jsulm 30 Jun 2020, 07:54

@jsulm Short answer is that I'm just trying to get one item on the screen for now. So I figured it's okay to plop this code in while I'm trying to get a handle on how QSGTexture is drawn.

Long answer, I'm drawing radar blips that will be moving across the screen. Each blip will be a different color, and the number of blips may go up or down. It is all represented in a model list that contains the colors and the coords of the blips. The model list will also be growing or shrinking with the number of blips I need to draw. So I plan to have each blip mapped to a node, and create this node from scratch each time. This is probably not the most efficient way to do it. I just started learning updatePaintNode() stuff 2 days ago. I would love to hear if you have a better idea on how to take advantage of the batch processing power of updatePaintNode().

In fact I had made another post hereasking people to give me their opinion on how to make another updatePaintNode() implementation better.

fcarney · 30 Jun 2020, 17:17

@mxyn said in Segmentation fault when painting a png in updatePaintNode() using QSGTexture:

QSGTexture * t = window()->createTextureFromImage(img);
QSGOpaqueTextureMaterial *testM = new QSGOpaqueTextureMaterial;

Check for null on anything that is returning a pointer and see if something is failing to be created.

mxyn · wrote on 30 Jun 2020, 17:17

@fcarney I tested for NULL, and none of these debugs printed out. meaning the vars aren't null

QImage img("pics/RadarCarSmall5.png");                   // create texture
QSGTexture * t = window()->createTextureFromImage(img);

if(img.isNull())
  qdebug("img is null");

if( t == NULL)
  qDebug( "t is null");

QSGOpaqueTextureMaterial *testM = new QSGOpaqueTextureMaterial;
testM.setTexture(t);
if( testM == NULL )
  qDebug("testM is NULL);

But I do suspect SOMETHING must be wrong with my texture... Not sure what though...

mxyn · wrote on 1 Jul 2020, 04:35

I decided to create a SIMPLE project in QTCreator to try to zero in on what's broken in the code. This code draws a triangle and then a rectangle. I tried setting the rectangle material to red. It worked! It tried setting the rectangle material to a texture, it failed! The guilty lines are:
//test->setMaterial(m2); // makes a red rectangle
test->setMaterial(tm); // nothing gets drawn

I'm hoping someone could try this code on their QTCreator and see if they have the same problem?

qquickcustomitem.cpp

#include "qquickcustomitem.h"
#include <QSGGeometry>
#include <QSGGeometryNode>
#include <QSGFlatColorMaterial>
#include <QDebug>
#include <QImage>
#include <QQuickWindow>
#include <QSGOpaqueTextureMaterial>
#include <QSGSimpleRectNode>

QQuickCustomItem::QQuickCustomItem(QQuickItem *parent) :
 QQuickItem(parent),
 m_color(Qt::red),
 m_needUpdate(true)
{
 setFlag(QQuickItem::ItemHasContents);
}

QColor QQuickCustomItem::color() const
{
 return m_color;
}
void QQuickCustomItem::setColor(const QColor &color)
{
 if(m_color != color) {
 m_color = color;
 m_needUpdate = true;
 update();
 colorChanged();
 }
}


QSGNode *QQuickCustomItem::updatePaintNode(QSGNode *oldNode, QQuickItem::UpdatePaintNodeData
*updatePaintNodeData)
{
    Q_UNUSED(updatePaintNodeData)
    QSGGeometryNode *root = static_cast<QSGGeometryNode *>(oldNode);

    if(!root) {
        root = new QSGGeometryNode;
        QSGGeometry *geometry = new QSGGeometry(QSGGeometry::defaultAttributes_Point2D(), 3);
        geometry->setDrawingMode(GL_TRIANGLE_FAN);
        geometry->vertexDataAsPoint2D()[0].set(width() / 2, 0);
        geometry->vertexDataAsPoint2D()[1].set(width(), height());
        geometry->vertexDataAsPoint2D()[2].set(0, height());

        root->setGeometry(geometry);
        root->setFlag(QSGNode::OwnsGeometry);
        root->setFlag(QSGNode::OwnsMaterial);
    }

    if(m_needUpdate) {
        QSGFlatColorMaterial *material = new QSGFlatColorMaterial;
        material->setColor(m_color);
        root->setMaterial(material);
        m_needUpdate = false;
    }

    QSGGeometryNode *test = new QSGGeometryNode;
    QSGGeometry *geometry2 = new QSGGeometry(QSGGeometry::defaultAttributes_Point2D(), 4);
    geometry2->setDrawingMode(QSGGeometry::DrawTriangleFan);
    geometry2->vertexDataAsPoint2D()[0].set(100,100);
    geometry2->vertexDataAsPoint2D()[1].set(100,150);
    geometry2->vertexDataAsPoint2D()[2].set(150, 150);
    geometry2->vertexDataAsPoint2D()[3].set(150, 100);

    QImage img;
    bool state = img.load(":/scenegraph/customgeometry/radarcar.png");
    QSGTexture * t =  window()->createTextureFromImage(img);
    QSGOpaqueTextureMaterial *tm = new QSGOpaqueTextureMaterial;
    tm->setTexture(t);

    QSGFlatColorMaterial *m2 = new QSGFlatColorMaterial;
    m2->setColor(Qt::red);

    test->setGeometry(geometry2);
    //test->setMaterial(m2);  // makes a red rectangle
    test->setMaterial(tm);    // nothing gets drawn
    root->appendChildNode(test);
    return root;
}

qquickcustomitem.h

#ifndef QQUICKCUSTOMITEM_H
#define QQUICKCUSTOMITEM_H

#include <QObject>
#include <QQuickItem>
#include <QColor>


class QQuickCustomItem : public QQuickItem
{
    Q_OBJECT
    Q_PROPERTY(QColor color READ color WRITE setColor NOTIFY colorChanged)
public:
    QQuickCustomItem(QQuickItem *parent = Q_NULLPTR);
protected:
    QSGNode *updatePaintNode(QSGNode *oldNode, UpdatePaintNodeData *updatePaintNodeData);
    QColor color() const;
    void setColor(const QColor &color);
private:
    QColor m_color;
    bool m_needUpdate;
signals:
    void colorChanged();
};


#endif // QQUICKCUSTOMITEM_H

mxyn · wrote on 1 Jul 2020, 06:39

I found the problem. I should not be using QSGGeometryNode. I should be using QSGSimpleTextureNode.

   QSGSimpleTextureNode *test = new QSGSimpleTextureNode;
    QImage img;
    bool state = img.load(":/scenegraph/customgeometry/radarcar.png");
    QSGTexture * t =  window()->createTextureFromImage(img);
    test->setTexture(t);
    test->setRect(10, 10, 30, 30);

BilbonSacquet · wrote on 16 Mar 2021, 01:22

QSGGeometry * geo2 = new QSGGeometry(QSGGeometry::defaultAttributes_Point2D(), 4); // create geometry

The crash is due to the 2D point which is not enough to hold the texture, the call with QSGGeometry::defaultAttributes_TexturedPoint2D() (QSGGeometry::TexturedPoint2D) will solve the issue.