[Solved] Problem with svg pictures and transparent background

JohanSolo

Dear experts,

I'm trying to code something which paints a widget using svg images. I have read that this is rather slow, and that the pictures have better to be pre-rendered first. I found this thread and thought that it would be straightforward to achieve what I wanted...
My problem is that my pictures are diamond-shaped, and the QPixmap I use is of course rectangular. I want therefore the 4 corners of the QPixmap to be transparent, I have thus the following piece of code, mostly inspired from q8phantom's snippet:

Converter -> load( fileName ); // Converter is a QSvgRenderer object pointer
// checks that the loaded file is valid, extract the source size, etc.
QPixmap image( destSize );
QPainter painter( &image );
Converter -> render( &painter );
image.fill( Qt::transparent );

In order to test that, I created a stupid little widget which displays 5 pictures converted from svg on a white background. The result shown here below, is clearly buggy to me, as there is this strange black background around the diamonds, background which even contains some text.
Example picture
I also checked that the obtained QPixmap is not transparent at all: if I put two diamonds with neighbouring edges, part of the one drawn first is hidden by the background from the second one.
What am I missing?

Some information: I'm using Qt 4.7.4 with gcc 4.6.1 on a 64 bit Mandriva 2011.

Thanks in advance,
Johan

Edit: I have the same strange behaviour using Qt 4.8.0 with gcc 4.6.2 on a 64 bit Fedora 16. I think the problem comes from the svg rendering: at some point I had a bug in my code and the svg was never rendered, which resulted in an "empty" QPixmap, i.e. totally transparent. As soon as I had the svg rendering, the background was buggy.

JohanSolo

Hello guys!

I can't believe nobody has a hint... Did I ask the question that shouldn't be asked? ;-)
More seriously, I also investigated on my own in the meantime, but still no solution. If you miss any kind of information, please let me know.

I put here a minimalist example :

buggywidget.hpp:

#ifndef BUGGYWIDGET_HPP
#define BUGGYWIDGET_HPP

#include <QtGui/QWidget>

class QPaintEvent;

class Widget : public QWidget
{
    Q_OBJECT

public:
    Widget(QWidget *parent = 0);
    ~Widget();
    void paintEvent(QPaintEvent *);
};

#endif // BUGGYWIDGET_HPP

buggywidget.cpp:

#include "buggywidget.hpp"

#include <QtGui/QPainter>
#include <QtSvg/QSvgRenderer>
#include <QtGui/QPixmap>

Widget::Widget(QWidget *parent)
    : QWidget(parent)
{
    setPalette( QPalette( Qt::white ) );
    setGeometry( 0, 0, 800, 600 );
    update();
}

Widget::~Widget()
{

}

void Widget::paintEvent(QPaintEvent *)
{
    QSvgRenderer * Converter = new QSvgRenderer( this );
    Converter -> load( QString( ":/VectorRedTile" ) );
    QPixmap image( 800, 240 );
    QPainter painter( &image );
    Converter -> render( &painter );
    image.fill( Qt::transparent );

    QPainter p( this );
    p.drawPixmap( 0, 180, image );
}

main.cpp:

#include <QtGui/QApplication>
#include "buggywidget.hpp"

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    Q_INIT_RESOURCE( TestResources );
    Widget w;
    w.show();

    return a.exec&#40;&#41;;
}

and BuggyTest.pro:

#-------------------------------------------------
#
# Project created by QtCreator 2012-03-13T07:27:52
#
#-------------------------------------------------

QT       += core gui svg

TARGET = BuggyTest.exe
TEMPLATE = app
MOC_DIR = "./bin/moc/"
OBJECTS_DIR = "./bin/objects/"
DESTDIR = "./bin/"
RESOURCES += TestResources.qrc


SOURCES += main.cpp\
        buggywidget.cpp

HEADERS  += buggywidget.hpp

With any svg file I use, the diamond is painted, now with a pure black background (I guess this background is just random). My problem is 100% reproducible with the given example.
I see however something I didn't see before (maybe I just didn't notice it before): there's a warning message saying "QPixmap::fill: Cannot fill while pixmap is being painted on". Is this related to my problem?

Thanks,
Johan

Edit: problem solved! I just had to write

Pixmap image( 800, 240 );
image.fill( Qt::transparent );
QPainter painter( &image );
Converter -> render( &painter );@
instead of 
@QPixmap image( 800, 240 );
QPainter painter( &image );
Converter -> render( &painter );
image.fill( Qt::transparent );

Sorry for the noise!

mlong

In your paintEvent() method, you should do the transparent fill before you do the painting.

[Edit: I should finish reading your post before I write mine. I see you figured it out! :) ]

JohanSolo

Thanks anyway!