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POUsing QtConcurrent to load a Pixmap and paint it
 

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  1. POUsing QtConcurrent to load a Pixmap and paint it
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    <p>I'm trying to create a Tile rendering program. Heres some basic code.</p> <p>Header</p> <pre><code>class Tile: public QGraphicsItem { public: Tile(void); ~Tile(void); QGraphicsPixmapItem *tileItem; void update(QPainter *painter, const QStyleOptionGraphicsItem *option,QWidget *widget); protected: QRectF boundingRect() const; void paint(QPainter *painter, const QStyleOptionGraphicsItem *option,QWidget *widget); }; </code></pre> <p>CPP:</p> <pre><code>.Constructor etc . . void Tile::paint(QPainter *painter, const QStyleOptionGraphicsItem *option,QWidget *widget) { if(tileItem==NULL) { qDebug()&lt;&lt;"Loading Pixmap"; QPixmap p("c:\\qt\\tile\\tile0-0.png"); tileItem=new QGraphicsPixmapItem; tileItem-&gt;setPixmap(p); } tileItem-&gt;paint(painter,option,widget); } </code></pre> <p>I'm trying to make a application that will paste tiles of a big image onto a QGraphicsScene. But loading all the tiles at once is time consuming and takes up a lot of memory. So I'm subclassing QGraphicsItem and overriding paint. The paint method in the QGraphicsItem class is only called when a it comes into view inside the QGraphicsView. So by loading up my tile inside paint, I can basically create an application that loads tiles only when they come into view. This much is working so far.</p> <p>To make the user experience better I'm use QtConcurrent to try an load the tiles up in a seperate thread. SO here's the changes I've made.</p> <p>CPP</p> <pre><code>connect(&amp;watcher,SIGNAL(finished()),this,SLOT(updateSceneSlot())); void Tile::paint(QPainter *painter, const QStyleOptionGraphicsItem *option,QWidget *widget) { if(tileItem==NULL) { TilePainter=painter; TileOption=option; TileWidget=widget; qDebug()&lt;&lt;"Paint Thread id "&lt;&lt; QThread::currentThread(); future=QtConcurrent::run(LoadTilePixmap,this); watcher.setFuture(future); } else tileItem-&gt;paint(painter, option, widget); } </code></pre> <p>LoadTilePixmap function:</p> <pre><code>void LoadTilePixmap(Tile *temp,QPainter *painter, const QStyleOptionGraphicsItem *option,QWidget *widget) { qDebug()&lt;&lt;"Loading Pixmap"; QPixmap p("c:\\qt\\tile\\tile0-0.png"); temp-&gt;tileItem=new QGraphicsPixmapItem; temp-&gt;tileItem-&gt;setPixmap(p); qDebug()&lt;&lt;"Loaded Pixmap"; } void Tile::updateSceneSlot() { qDebug()&lt;&lt;"updateSceneSlot Thread id "&lt;&lt; QThread::currentThread(); tileItem-&gt;paint(TilePainter, TileOption, TileWidget); } </code></pre> <p>This code should work but it keeps crashing at runtime as soon as paint gets called. After adding breakpoints I narrowed the problem down to <code>temp-&gt;tileItem-&gt;paint(painter,option,widget);</code> which causes the crash.</p> <p>The output that I get is</p> <pre><code>Loading Pixmap Almost Loaded Pixmap First-chance exception at 0x6526174a (QtGuid4.dll) in Visualizer.exe: 0xC0000005: Access violation reading location 0xc88bffe1. Unhandled exception at 0x6526174a (QtGuid4.dll) in Visualizer.exe: 0xC0000005: Access violation reading location 0xc88bffe1. </code></pre> <p>Could anyone help me and let me know why the lastline/paint method crashes. How can I fix it?</p> <p>EDITED CODE TO UPDATE CHANGES</p>
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    1. COI think you are running into general concurrency issues. You don't have any synchronization for the variables you are trying to access in multiple threads. You have to consider that after launching the future, the call returns immediately. Paint could be called many times while the other thread is working. How do you know that 1) you haven't already scheduled the work and 2) the work is actually done? Paint() just tests to see if the pointer is null, but how does it really know that the other thread is done? The GUI thread is probably getting switched in during the pixmap creation.
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    2. COThis is the first time I'm trying multithreading so I really don't know if I'm doing it right. According to Qt concurrent documentation, Qt's supposed to take care of synchronization of variables. Secondly I even tried a signal slot mechanism where a queued slot is being used. So basically after the thread finishes execution it emits a signal which is put into a queue. The slot (which calls paint) is then called turn by turn. I think that should have resolved concurrency issues, don't know though. I've solved it now by calling update instead of paint. Update schedules a paint call. It works.
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      1. USsleeping.ninja
 

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