QT4 – Font rendering problem when using QBitmap

[Hariprasasth]

Hello,

I am developing a application for my embedded device using QT4 with Freetype font library.
I have to create an image to display it on Monochrome display. Therefore, I created instances of QBitmap and QPainter.
When I draw the text, the text rendering (look) is different. Some part in the characters are quite dark.
Below is my test code:

class QBitmapTest
{
public:
QBitmapTest();
bool Text(wchar_t* f_pszFontName,int f_nXPos, int f_nYPos, int f_nWidth, int f_nHeight,int f_nFontHeight);
void saveAsImage();
private:
QBitmap* m_hBitmap;
QPen m_hPen;
QBrush m_hBrush;
QPainter* m_hdcMem;
};
BitmapTest::QBitmapTest()
{
m_hBitmap = new QBitmap(500,500);
m_hdcMem = new QPainter();
m_hBitmap->fill(Qt::color1);
if(m_hBitmap->isNull()!= true){
m_hdcMem->begin(m_hBitmap);
}
printf("Painter status: Dc %d ", m_hdcMem->isActive());
m_hPen.setWidth(1);
m_hPen.setColor(Qt::color0);
m_hPen.setStyle(Qt::SolidLine);
m_hBrush.setColor(Qt::color0);
m_hBrush.setStyle(Qt::SolidPattern);
//m_hdcMem->setBackgroundMode(Qt::OpaqueMode);
m_hdcMem->setPen(m_hPen);
m_hdcMem->setBrush(m_hBrush);
}

bool QBitmapTest::Text(wchar_t* f_pwFontName,int f_nXPos, int f_nYPos, int f_nWidth, int f_nHeight,int f_nFontHeight)
{
QFont font;
// font.setStyleStrategy(QFont::NoFontMerging);
// font.setStyleStrategy(QFont::PreferQuality);
// font.setStretch(QFont::SemiExpanded);
// font.setHintingPreference(QFont::PreferNoHinting);

if(f_pwFontName){
    QString fontName = QString::fromWCharArray(f_pwFontName);
    font.setFamily(fontName);
    font.setWeight(QFont::Normal);
    font.setPixelSize(f_nFontHeight);

    //  font.setPointSize(f_nFontHeight);
    //  font.setFixedPitch(true);
    //  font.setRawMode(true);
    //  m_hDC->setFont(font);
    qDebug() << "Font match" << font.exactMatch();
    QFontInfo chk(font);
    qDebug() << "Font match fi" << chk.exactMatch();
    qDebug() << "Font famly fi" << chk.family();
    qDebug() << "Font fixed pitch fi" << chk.fixedPitch();
    qDebug() << "Font point size fi" << chk.pointSize();

    m_hdcMem->setFont(font);
}

QString str("ABCDEF GHIJK LMNOP QRST WXYZ");
QRect rect;
rect.setLeft(f_nXPos);
rect.setTop(f_nYPos);
rect.setWidth(f_nWidth);
rect.setHeight(f_nHeight);
qDebug() << "Drawing";
m_hdcMem->drawText(rect,str);
return true;}

void QBitmapTest::saveAsImage()
{
QImage image = m_hBitmap->toImage();
image.save("test.bmp");
}

But when I change QBitmap to QPixmap, I don’t see such rendering issue. If I use QPixmap, I have to convert the pixels data (16 bpp) to monochrome image to display it.

Font name: Roboto Mono
Line1 – Font size: 10
Line2 – Font size: 12
Line3 – Font size: 13
Line4 – Font size: 15
Line5 – Font size: 17

Image with QBitmap:
See Line4 (E, F, L, M, NP, Q, R) 5 (B,D,E,J,K,L,P,R,T) and 6(M,N): Some part of characters are quite dark.

Image with QPixmap:

How can I resolve the rendering problem with QBitmap?

Thanks,
Hari

[mvuori]

Of course your QPixmap looks good - it is a grayscale image which has many bits per pixel allowing nice antialiasing of the letters. That is not possible with a 1 bit bitmap. Convert the QPixmap to a bitmap and you'll see similar result.

The thing to do is to find a font that works well (or at least better) on 1 bit screens.