Custom QIcon using QIconEngine and transparency

Question

I'm trying to implement something like "composed icon" in Qt.

Target: I need to dynamicaly set color just for the portion of icon.

My idea: Compose this icon by two other. One icon that will be coloured as desired (perhaps by ColorizeEffect) and blend it under second icon that acts as overlay layer.

Issue: I tried QIconEngine and implementing its paint method. ColorizeEffect seems not to be working (even when I try the hack with temporary QLabel for that - when strength is set > 0.0, the QIcon formed by that is empty). But that's not the main problem. The thing is, no matter what I do, I get some default coloured background for this "composed" icon.

Here is fragment of my code:

class QComposedIconEngine: public QIconEngine
{
public:
   QComposedIconEngine();
   ~QComposedIconEngine();
   virtual void paint ( QPainter * painter, const QRect & rect, QIcon::Mode mode, QIcon::State state );
   virtual QIconEngine * clone(void) const;
public:
   QIcon m_qIconA;
   QIcon m_qIconB;
   QColor m_qColor;
};

And here is my implementation of paint(...):

void CLxQComposedIconEngine::paint ( QPainter * painter, const QRect & rect, QIcon::Mode mode, QIcon::State state )
{
   QBrush brush = painter->background();
   QColor color = brush.color();
   brush.setColor( Qt::transparent );
   painter->setBackground( brush );
   painter->eraseRect( rect );
   painter->setCompositionMode( QPainter::CompositionMode_SourceOver );
   m_qIconA.paint( painter, rect, Qt::AlignCenter, mode, state );
};

And here is how I create that "Composed" icon:

QComposedIconEngine * qIconEngine = new QComposedIconEngine();
QIcon i1;
QIcon i2;
i1.addPixmap(...);
i2.addPixmap(...);
qIconEngine->m_qIconA = i1;
qIconEngine->m_qIconB = i2;
QIcon i3( qIconEngine );

I expect i1 and i3 to look exactly the same. And besides the damn background it really is. But I need to make it transparent.

(even when I leave my paint(...) method empty, the damn background is there!)

Does anybody know how to make the background transparent? Thanks.

Kierkegaard

Doc of QIconEngine says "use QIconEngineV2". Have you tried that ? – Prude

Of course. No effect. Fortunatelly I solved this problem. It is in the way QIconEngine is called from outside. I'm going to compile the answer by myself... :-) – Kierkegaard

Answer 1

I solved this. The problem was, that the reimplemented paint(...) method is almost every time called via pixmap(...) method - which can be reimplemented too. If you leave default implementation, this method creates new QPixmap, creates QPainter on it and calls paint(...) witch this painter. But that is the problem - since the created QPixmap contains no alpha channel it cannot be semi-transparent. So you need to reimplement pixmap(...) method like this:

QPixmap CLxQComposedIconEngine::pixmap ( const QSize & size, QIcon::Mode mode, QIcon::State state )
{   
   QImage img( size, QImage::Format_ARGB32 );
   img.fill(qRgba(0,0,0,0));
   QPixmap pix = QPixmap::fromImage(img, Qt::NoFormatConversion );
   {
      QPainter painter ( &pix );
      QRect r( QPoint(0.0,0.0), size );
      this->paint(&painter, r, mode, state);      
   }
   return pix;       
};

The key are the first two rows... Without the img.fill() it would be not transparent.

Kierkegaard

QPixmap pix(size); pix.fill(Qt::transparent);is sufficient, you don't need to make a QImage. – Tb