QListViewDelegate paint issue



  • I am working on an image viewer app. I am using a QListView called thumbview and a delegate to customize the iconview. When I click on a thumb the item is highlighted with a white border. When I click on another thumb its border turns white, but the previous selection sometimes leaves white border remnants, as illustrated by the red arrow.

    Alt text

    If I move the mouse outside the listview area the thumbs redraw correctly, cleaning up the remnants.

    void ThumbViewDelegate::paint(QPainter *painter, const QStyleOptionViewItem &option,
                               const QModelIndex &index) const
     {
        painter->save();
    
        QFont font = qvariant_cast<QFont>(index.data(Qt::FontRole));
        font.setPixelSize(10);
        QFontMetrics fm(font);
    
        QIcon icon = qvariant_cast<QIcon>(index.data(Qt::DecorationRole));
        QSize iconsize = icon.actualSize(option.decorationSize);
    
        QString filenameText = qvariant_cast<QString>(index.data(Qt::EditRole));
    
        QRect item = option.rect;
        QRect iconRect(item.left()+4, item.top()+4, iconsize.width(), iconsize.height());
        QPainterPath iconPath;
        iconPath.addRoundRect(iconRect, 12, 12);
    
        QRect textRect = item;
        textRect.setTop(item.top()+item.height()-fm.height()-10);
        QPainterPath textPath;
        textPath.addRoundedRect(textRect, 8, 8);
    
        painter->setRenderHint(QPainter::Antialiasing, true);
        painter->setRenderHint(QPainter::TextAntialiasing, true);
    
        painter->setFont(font);
    //    painter->fillPath(textPath, Qt::gray);
    //    painter->drawRoundedRect(textRect, 6, 6);
        painter->drawText(textRect, Qt::AlignCenter, filenameText);
    
        painter->setPen(Qt::darkGray);
        painter->setClipping(true);
        painter->setClipPath(iconPath);
        painter->drawPixmap(iconRect, icon.pixmap(iconsize.width(),iconsize.height()));
        painter->setClipping(false);
    
        painter->setPen(Qt::darkGray);
        painter->drawPath(iconPath);
    
        painter->setPen(Qt::darkGray);
        if (option.state & QStyle::State_Selected) {
            QPen pen(Qt::white);
            pen.setWidth(2);
            painter->setPen(pen);
        }
        painter->drawRoundedRect(item, 8, 8);
    
        painter->restore();
    }
    

    Any help much appreciated.

    Rory


  • Qt Champions 2016

    @Rory_1
    Hello,
    I think this might be a problem:

    painter->setPen(Qt::darkGray);
    if (option.state & QStyle::State_Selected) {
        QPen pen(Qt::white);
        pen.setWidth(2);
        painter->setPen(pen);
    }
    

    You seem to draw in 2px width with the white pen, but then use only the default 1px gray pen. What happens if you disable the antialiasing option (for debugging purposes), is a gray line visible next to the white one?

    Kind regards.



  • I tried that with/without the aliasing. The pen width and aliasing does not make any difference. I want to know what is happening when I move the mouse away as it is triggering a refresh of the listview that clears up the problem. I tried creating a signal/slot back to my listview to update the model but that did not help.


  • Qt Champions 2016

    @Rory_1

    I want to know what is happening when I move the mouse away as it is triggering a refresh of the listview that clears up the problem.

    Hello,
    You probably are getting a full repaint request, and that will erase the background, and when your code runs after that it will display properly. I still think that there is some strange problem with the painting over the white line, when the mouse leaves your item. I believe that is the case, because it is partially erased. I suggest using a red line for the inactive items, setting the width of the pen to 2px for the inactive items and disabling antialiasing all at the same time. When you get the partially erased border, take a look for a red line next to the white one. If you see such a thing, that simply means that you're not fully erasing the active item border.

    Kind regards.



  • I eliminated everything except the border. The top-left item is the previously selected item and the top-right is the currently selected item.

    Alt text
    and then after I mouse-over the scrollbar to the right:
    Alt text

    Here is the code:

    void ThumbViewDelegate::paint(QPainter *painter, const QStyleOptionViewItem &option,
                               const QModelIndex &index) const
     {
        painter->save();
    
        QRect itemRect = option.rect;
        if (option.state & QStyle::State_Selected) {
            QPen selectedPen(Qt::white);
            selectedPen.setWidth(2);
            painter->setPen(selectedPen);
        } else {
            QPen activePen(Qt::red);
            activePen.setWidth(2);
            painter->setPen(activePen);
        }
        painter->drawRoundedRect(itemRect, 8, 8);
    
        painter->restore();
    }

  • Qt Champions 2016

    Hello,
    Now this is clearer to diagnose. Try erasing the background before painting, like this:

    painter->fillRect(option.rect, option.backgroundBrush);
    // ... Rest of your painting code
    

    or

    painter->eraseRect(option.rect);
    

    As a side note, I also advise you to use the test function to check for a flag, like this:

    if (option.state.testFlag(QStyle::State_Selected))  {
        // ... Flag is set
    }
    

    Kind regards.



  • I tried all of the above: erasing rect and testFlag. No change what-so-ever to behavior. The weird thing is this behavior does not always occur. Sometimes the previous and/or currently selected items paint correctly.

    I am going to track which events are firing when I mouse-over leaving the listview. Maybe I can get a work-around using a signal/slot.


  • Qt Champions 2016

    @Rory_1
    Take note that your highlighted border is not properly drawn as well. It seems half the drawing occurs at the old item, and the other half at the new item. This is pretty strange. Do you mind sharing a bit more of your code, for example how you set up your delegate and any event overrides you might have in your list view?

    Kind regards.



  • Thanks very much for your help! I have to go out for the afternoon, but I will follow-up later.



  • I have created a small project that shows the problem. It contains:

    • testdelegate. pro
    • main.cpp
    • mw.h
    • mw.cpp
    • testdelegate.h
    • testdelegate.cpp

    testDelegate. pro:

    QT       += core gui
    
    greaterThan(QT_MAJOR_VERSION, 4): QT += widgets
    
    TARGET = testDelegate
    TEMPLATE = app
    
    
    SOURCES += main.cpp\
            mw.cpp \
        thumbviewdelegate.cpp
    
    HEADERS  += mw.h \
        thumbviewdelegate.h
    

    main.cpp:

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

    mw.h: (mainwindow)

    #ifndef MW_H
    #define MW_H
    
    #include <QMainWindow>
    #include <QtWidgets>
    #include "thumbviewdelegate.h"
    
    class MW : public QMainWindow
    {
        Q_OBJECT
    
    public:
        explicit MW(QWidget *parent = 0);
    
    private:
        QHBoxLayout *mainLayout;
        QListView *thumbView;
        QStandardItemModel *thumbViewModel;
        QStandardItem *thumbIitem;
        ThumbViewDelegate *thumbviewDelegate;
    };
    
    #endif // MW_H
    

    mw.cpp:

    #include "mw.h"
    
    MW::MW(QWidget *parent) :
        QMainWindow(parent)
    {
        resize(600, 600);
        thumbView = new QListView;
        mainLayout = new QHBoxLayout;
        mainLayout->setContentsMargins(0, 0, 0, 0);
        mainLayout->setSpacing(0);
        mainLayout->addWidget(thumbView);
        QWidget *centralWidget = new QWidget;
        centralWidget->setLayout(mainLayout);
        setCentralWidget(centralWidget);
    
        thumbView->setSelectionMode(QAbstractItemView::ExtendedSelection);
        thumbView->setViewMode(QListView::IconMode);
        thumbView->setResizeMode(QListView::Adjust);
        thumbView->setWordWrap(true);
        thumbView->setDragEnabled(true);
        thumbView->setEditTriggers(QAbstractItemView::NoEditTriggers);
        thumbView->setUniformItemSizes(false);
    
        thumbViewModel = new QStandardItemModel();
        thumbView->setModel(thumbViewModel);
        thumbViewModel->clear();
        thumbviewDelegate = new ThumbViewDelegate(this);
        thumbView->setItemDelegate(thumbviewDelegate);
        thumbView->setSpacing(10);
    
        for (int i = 0; i < 10; ++i) {
            thumbIitem = new QStandardItem;
            thumbViewModel->appendRow(thumbIitem);
        }
    }
    

    testdelegate.h:

    #ifndef THUMBVIEWDELEGATE_H
    #define THUMBVIEWDELEGATE_H
    
    #include <QtWidgets>
    
    class ThumbViewDelegate : public QStyledItemDelegate
    {
        Q_OBJECT
    
    public:
        ThumbViewDelegate(QObject *parent = 0);
    
        void paint(QPainter *painter, const QStyleOptionViewItem &option,
                   const QModelIndex &index) const;
    
        QSize sizeHint(const QStyleOptionViewItem &option,
                       const QModelIndex &index ) const;
    };
    
    #endif // THUMBVIEWDELEGATE_H
    

    testdelegate.cpp:

    #include "thumbviewdelegate.h"
    
    ThumbViewDelegate::ThumbViewDelegate(QObject *parent)
    {
    }
    
    QSize ThumbViewDelegate::sizeHint(const QStyleOptionViewItem &  option ,
                                  const QModelIndex & index) const
    {
        return(QSize(200,200));
    }
    
    void ThumbViewDelegate::paint(QPainter *painter, const QStyleOptionViewItem &option,
                               const QModelIndex &index) const
     {
        painter->save();
    
        if (option.state.testFlag(QStyle::State_Selected)) {
            QPen selectedPen(Qt::red);
            selectedPen.setWidth(5);
            painter->setPen(selectedPen);
        } else {
            QPen activePen(Qt::black);
            activePen.setWidth(5);
            painter->setPen(activePen);
        }
        painter->drawRect(option.rect);
    
        painter->restore();
    }

  • Qt Champions 2016

    Hello,
    Right, It took me a few minutes, but I've found the problem - you're painting outside of your allotted space. This is why the behavior you're getting is so erratic - you depend on when the list view will (or will not) erase it's contents. Brace yourself ;), here is the problematic fragment:

    painter->drawRect(option.rect);
    

    When you draw a stroked rectangle the area you provide is for the insides (the pen requires additional space).
    This means that you draw your border in actuality outside of the item rectangle. This is also the reason why the rounded corners actually get erased (they happen to be inside option.rect). The fix is quite simple, here is an example how you could solve your problem:

    painter->save();
    
    // Make the rect smaller to accommodate the border.
    static const qint32 penSize = 4;
    QPoint strokeOffset(penSize, penSize);
    QRect borderRect(option.rect.topLeft() + strokeOffset, option.rect.bottomRight() - strokeOffset);
    
    // ... Rest of your code
    
    painter->drawRect(borderRect);    // Draw the border properly
    

    I hope this helps.
    Kind regards.



  • Thank you so much! I don't think I would have figured that out. My first Christmas present :*)


  • Qt Champions 2016

    @Rory_1
    You're welcome. Good luck with your application!

    Kind regards.


  • Moderators

    just for addition: see this
    So i ncase you draw anti-aliased, the border-offset would just be the half of the border-width.


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