QPainter::drawText and floating point precision

[kssm]

Hi!

I want to draw text with floating point precision,
but so far I have not succeeded.

QPainter::drawText takes a QPointF as first
argument, but it seems like it could
just as well just have taken a QPoint as argument instead.

I've tried to set the SmoothPixmapTransform, Antialiasing,
and HighQualityAntialiasing rendering hints, but none of them
seems to make a difference.

I'm using Qt 4.8.

Is floating point precision supported in Qt 4.8? (Am I
just doing something wrong?)

And in case not, will it help to use Qt 5 instead?

Thanks for any help and clarification about what Qt does.
If Qt doesn't support floating point precision when drawing
text, I'll just have to do it manually instead, but I rather not.

-K

---

Added after 4 minutes:

I should also add that I'm painting to a QImage.

I've also tried to explicitly tell Qt to use the
raster engine, but it makes no difference.

I'm developing on Linux, but the program
runs on OSX and Windows as well.

[Lykurg]

Have you tried to use QPainter::TextAntialiasing?

[kssm]

Have you tried to use QPainter::TextAntialiasing?

Yes, but since TextAntialiasing is ON by default, I didn't wrote that I had tried that, because then someone
would comment that TextAntialising is ON by default.

---

Added after 1 6 minutes:

I've dived into the Qt source, and as far as I see, QPainter::drawPath is the
drawing primitive used to draw text on QImages on Raster, and that method should
provide floating point precision.

Unless anyone comes up with a bright idea, I'll just compile Qt myself, and insert
some debug printings here and there to see what's happening.

[Lykurg]

Yes, but since TextAntialiasing is ON by default, I didn't wrote that I had tried that, because then someone
would comment that TextAntialising is ON by default.

Ups, I was not aware that it is turned ON by default.

[kssm]

I found a solution. Glyphs painting are cached, but it seems like the keys used for caching
are in integer format, (or that the cached glyphs are stored in integer format).

If I comment out line 3195 (or thereabout) in qpaintengine_raster.cpp, which looks like this:
"if (!drawCachedGlyphs(glyphs.size(), glyphs.constData(), positions.constData(), fontEngine))"

drawText works properly.

Is there some other way to turn off caching, besides commenting out that line?

Or perhaps this should be considered a bug? I guess some programs would run slower
if the glyph caching used floats for key formats, although float keys would be correct.

At least I haven't seen this limitation documented anywhere, and that should be considered
a bug.

[kssm]

Using painter.translate() doesn't help either.

I guess we are supposed to draw text using drawGlyphRun in these situations?
I hope that works, if not I might have to use a patched version of Qt for the program.

Using painter.translate() doesn't help either.

I guess we are supposed to draw text using drawGlyphRun in these situations?
I hope that works, if not I might have to use a patched version of Qt for the program.

Using painter.translate() doesn't help either.

I guess we are supposed to draw text using drawGlyphRun in these situations?
I hope that works, if not I might have to use a patched version of Qt for the program.

Using painter.translate() doesn't help either.

I guess we are supposed to draw text using drawGlyphRun in these situations?
I hope that works, if not I might have to use a patched version of Qt for the program.

---

Added after 19 minutes:

Nope, drawGlyphRun doesn't work either:

static void myDrawText(QPainter *painter, QPointF p, QString text){
  QTextLayout textLayout(text, painter->font(), painter->device());
 
  //int leading = fontMetrics.leading();
  qreal height = 0;
  textLayout.beginLayout();
  while (1) {
    QTextLine line = textLayout.createLine();
    if (!line.isValid())
      break;
 
    line.setLineWidth(50.0f);//lineWidth);
    height += 5.4; //leading;
    line.setPosition(QPointF(0, height));
    height += line.height();
  }
  textLayout.endLayout();
 
 
  //textLayout.draw(painter, p);
 
  QList<QGlyphRun> glyphs = textLayout.glyphRuns();
 
  QList<QGlyphRun>::iterator i;
  for (i = glyphs.begin(); i != glyphs.end(); ++i) {
    printf("Drawing something\n");
    QGlyphRun glyph = *i;
    //glyph.setRawFont(QRawFont::fromFont(painter->font()));
    painter->drawGlyphRun(p, (const QGlyphRun &)glyph);
  } 
}

static void myDrawText(QPainter *painter, QPointF p, QString text){
  QTextLayout textLayout(text, painter->font(), painter->device());
  
  //int leading = fontMetrics.leading();
  qreal height = 0;
  textLayout.beginLayout();
  while (1) {
    QTextLine line = textLayout.createLine();
    if (!line.isValid())
      break;
    
    line.setLineWidth(50.0f);//lineWidth);
    height += 5.4; //leading;
    line.setPosition(QPointF(0, height));
    height += line.height();
  }
  textLayout.endLayout();
  

  //textLayout.draw(painter, p);
 
  QList<QGlyphRun> glyphs = textLayout.glyphRuns();

  QList<QGlyphRun>::iterator i;
  for (i = glyphs.begin(); i != glyphs.end(); ++i) {
    printf("Drawing something\n");
    QGlyphRun glyph = *i;
    //glyph.setRawFont(QRawFont::fromFont(painter->font()));
    painter->drawGlyphRun(p, (const QGlyphRun &)glyph);
  } 
}

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Guess I'll just rip out some code from the Qt source and insert it into my program.
(they're both GPL)

---

Added after 43 minutes:

Got it. This works:

void myDrawText(QPainter *painter, QPointF p, QString text){
 
  QRawFont rawFont = QRawFont::fromFont(painter->font());
  QVector<quint32> indexes = rawFont.glyphIndexesForString(text);
 
  painter->save();
  printf("num indexes: %d\n",indexes.count());
  for(unsigned int i=0; i<indexes.count(); i++){
    QPainterPath path = rawFont.pathForGlyph(indexes[i]);
    painter->translate(QPointF(p.x(), p.y() + i*0));
    painter->fillPath(path,painter->pen().brush());
  }
  painter->restore();
}

void myDrawText(QPainter *painter, QPointF p, QString text){

  QRawFont rawFont = QRawFont::fromFont(painter->font());
  QVector<quint32> indexes = rawFont.glyphIndexesForString(text);

  painter->save();
  printf("num indexes: %d\n",indexes.count());
  for(unsigned int i=0; i<indexes.count(); i++){
    QPainterPath path = rawFont.pathForGlyph(indexes[i]);
    painter->translate(QPointF(p.x(), p.y() + i*0));
    painter->fillPath(path,painter->pen().brush());
  }
  painter->restore();
}

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