How to reduce padding on QTableView cells?

sairun

NOTE: Qt version: 6.10.1, Linux

I'm trying to adjust the amount of space between the contents of the cells and their borders in a QTableView to achieve a display similar to a DNA alignment editor. Each cell will only display a character ('A', 'T', 'C', 'G', or '-') in different colors, probably with different backgrounds. The goal is to have a padding of 1 or 2 pixels per cell. In the example below I used

view->setStyleSheet("QTableView::item{border:0px;padding:1px;}");

but without any result. It doesn't matter if I change QTableView::item to QTableView alone, or omit border:0px;. The final result is always a table with a considerable amount of space surrounding each individual character (more pronounced at the left and right sides). However, the stylesheet method works for other attributes. For example, uncommenting the line

//view->setStyleSheet("QTableView::item{background-color:yellow;}");

renders the background of the cells yellow.

#include <QAbstractTableModel>
#include <QApplication>
#include <QMainWindow>
#include <QTableView>
#include <QHeaderView>
#include <QList>

class TableModel : public QAbstractTableModel {
    Q_OBJECT
private:
    int rows {10};
    int columns {10};
    char seqs[100];
    QList<QString> names;
public:
    explicit TableModel(QObject *parent = nullptr) {
        names.push_back("Species 01");
        names.push_back("Species 02");
        names.push_back("Species 03");
        names.push_back("Species 04");
        names.push_back("Species 05");
        names.push_back("Species 06");
        names.push_back("Species 07");
        names.push_back("Species 08");
        names.push_back("Species 09");
        names.push_back("Species 10");
        std::string d ="AGTATAATTATTCGGGCTGAGTTAGGTCAGCCAGGTAGATTCATTGGAGACGATCAGATTTATAATGTAGTTGTTACGGCGCATGCTTTTGTAATAATTT";
        std::copy(std::begin(d), std::begin(d) + sizeof(seqs), std::begin(seqs));
    }
    int rowCount(const QModelIndex &parent = QModelIndex()) const override {
        return rows;
    }
    int columnCount(const QModelIndex &parent = QModelIndex()) const override {
        return columns;
    }
    QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const override {
        switch( role ) {
            case Qt::DisplayRole:
                if( index.column() == 0 ) {
                    return QString("%1").arg( names[ index.row() ] );
                }
                else {
                    char base = seqs[index.row()*columns+index.column()];
                    return QString("%1").arg( base );
                }
                break;
            case Qt::TextAlignmentRole:
                return int(Qt::AlignCenter | Qt::AlignVCenter);
                break;
            default:
                return QVariant();
        }
    }
};

class MainWindow : public QMainWindow
{
    Q_OBJECT
private:
    QTableView  * view;
    TableModel  * model;
public:
    MainWindow(QWidget *parent = nullptr)
    {
        setMinimumSize( 400, 300 );
        model = new TableModel( this );
        view  = new QTableView( this );
        view->setModel( model );
        // view->horizontalHeader()->setSectionResizeMode(QHeaderView::Stretch);
        // view->setSizeAdjustPolicy( QAbstractScrollArea::AdjustToContents );
        // QHeaderView * header = view->horizontalHeader();
        // QHeaderView * rows = view->verticalHeader();
        // header->hide();
        // rows->hide();
        // view->verticalHeader()->setDefaultSectionSize(10);   
        // view->horizontalHeader()->setDefaultSectionSize(10); 
        // view->horizontalHeader()->setSectionResizeMode(QHeaderView::Fixed);
        // view->horizontalHeader()->resizeSections(QHeaderView::ResizeToContents);
        view->resizeColumnsToContents();
        view->setStyleSheet("QTableView::item{border:0px;padding:1px;}");
        //view->setStyleSheet("QTableView::item{background-color:yellow;}");
        setCentralWidget( view );
    }
};

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

#include "main.moc"

SGaist

Hi,

Did you consider implementing a QStyledItemDelegate ?
That way you will have more control on the rendering of your content.

sairun

@SGaist said in How to reduce padding on QTableView cells?:

Hi,

Did you consider implementing a QStyledItemDelegate ?
That way you will have more control on the rendering of your content.

Thanks @SGaist

I did consider implementing my own QStyledItemDelegate. In fact, I've tried it once but failed! I was hoping that there was a simpler way of styling Qt tables through style sheets.

sairun

As suggested by @Sgaist, I've implemented a "simple" demonstration of my QTableView below using a QItemDelegate to render the cell contents. I selected this delegate type instead of QStyledItemDelegate to have access to functions like drawDisplay() and drawFocus() instead of using painter->save() and painter->restore(). I moved the font settings from QTableModel::data() to QItemDelegate::paint() just to ensure the delegate was working.

The code produces a QTableView with the table cells more or less adjusted to their contents. The first column with the sequence names is correct, but the subsequent columns for the bases are a bit wide. Apparently, there's a minimum width for columns, but I don't know where it's defined! How do I change that? Is it through the QStyleOptionViewItem class?

#include <QAbstractTableModel>
#include <QApplication>
#include <QMainWindow>
#include <QTableView>
#include <QHeaderView>
#include <QItemDelegate>
#include <QList>
#include <QPainter>
#include <QFont>

class BaseDelegate : public QItemDelegate
{
    Q_OBJECT
public:
    explicit BaseDelegate(QObject *parent = nullptr) {}
    void paint(QPainter *painter, const QStyleOptionViewItem &option, const QModelIndex &index) const {
        //
        if ( index.column() > 0 ) {
            QString text = index.data(Qt::DisplayRole).toString();
            QStyleOptionViewItem myOption = option;
            QFont font("Courier New");
            font.setPointSize( 14 );
            myOption.font = font;
            myOption.displayAlignment = Qt::AlignCenter | Qt::AlignVCenter;
            drawDisplay( painter, myOption, myOption.rect, text );
            drawFocus( painter, myOption, myOption.rect );
        } else {
            QItemDelegate::paint(painter, option, index);
        }
    }
};

class TableModel : public QAbstractTableModel {
    Q_OBJECT
private:
    int rows {10};
    int columns {10};
    char seqs[100];
    QList<QString> names;
public:
    explicit TableModel(QObject *parent = nullptr) {
        names.push_back("Species 01");
        names.push_back("Species 02");
        names.push_back("Species 03");
        names.push_back("Species 04");
        names.push_back("Species 05");
        names.push_back("Species 06");
        names.push_back("Species 07");
        names.push_back("Species 08");
        names.push_back("Species 09");
        names.push_back("Species 10");
        std::string d ="AGTATAATTATTCGGGCTGAGTTAGGTCAGCCAGGTAGATTCATTGGAGACGATCAGATTTATAATGTAGTTGTTACGGCGCATGCTTTTGTAATAATTT";
        std::copy(std::begin(d), std::begin(d) + sizeof(seqs), std::begin(seqs));
    }
    int rowCount(const QModelIndex &parent = QModelIndex()) const override {
        return rows;
    }
    int columnCount(const QModelIndex &parent = QModelIndex()) const override {
        return columns;
    }
    QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const override {
        switch( role ) {
            case Qt::DisplayRole:
                if( index.column() == 0 ) {
                    return QString("%1").arg( names[ index.row() ] );
                }
                else {
                    char base = seqs[index.row()*columns+index.column()];
                    return QString("%1").arg( base );
                }
                break;
            default:
                return QVariant();
        }
    }
};

class MainWindow : public QMainWindow
{
    Q_OBJECT
private:
    QTableView  * view;
    TableModel  * model;
public:
    MainWindow(QWidget *parent = nullptr)
    {
        setMinimumSize( 400, 300 );
        model = new TableModel( this );
        view  = new QTableView( this );
        view->setModel( model );
        view->resizeColumnsToContents();
        view->setItemDelegate( new BaseDelegate );
        setCentralWidget( view );
    }
};

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

#include "main.moc"

SGaist

Your model can return a Qt::SizeHintRole.