Displaying PDF with Python, Qt5 and Poppler (Shallow Thoughts)

Akkana's Musings on Open Source Computing and Technology, Science, and Nature.

Fri, 27 Apr 2018

Displaying PDF with Python, Qt5 and Poppler

I had a need for a Qt widget that could display PDF. That turned out to be surprisingly hard to do. The Qt Wiki has a page on Handling PDF, which suggests only two alternatives: QtPDF, which is C++ only so I would need to write a wrapper to use it with Python (and then anyone else who used my code would have to compile and install it); or Poppler. Poppler is a common library on Linux, available as a package and used for programs like evince, so that seemed like the best route.

But Python bindings for Poppler are a bit harder to come by. I found a little one-page example using Poppler and Gtk3 via gi.repository ... but in this case I needed it to work with a Qt5 program, and my attempts to translate that example to work with Qt were futile. Poppler's page.render(ctx) takes a Cairo context, and Cairo is apparently a Gtk-centered phenomenon: I couldn't find any way to get a Cairo context from a Qt5 widget, and although I found some web examples suggesting renderToImage(), the Poppler available in gi.repository doesn't have that function.

But it turns out there's another Poppler: popplerqt5, available in the Debian package python3-poppler-qt5. That Poppler does have renderToImage, and you can take that image and paint it in a paint() callback or turn it into a pixmap you can use with a QLabel. Here's the basic sequence:

    document = Poppler.Document.load(filename)
    document.setRenderHint(Poppler.Document.TextAntialiasing)
    page = document.page(pageno)
    img = self.page.renderToImage(dpi, dpi)

    # Use the rendered image as the pixmap for a label:
    pixmap = QPixmap.fromImage(img)
    label.setPixmap(pixmap)

The line to set text antialiasing is not optional. Well, theoretically it's optional; go ahead, try it without that and see for yourself. It's basically unreadable.

Of course, there are plenty of other details to take care of. For instance, you can get the size of the rendered image:

    size = page.pageSize()
... after which you can use size.width() and size.height(). They're in points. There are 72 points per inch, so calculate accordingly in the dpi values you pass to renderToImage if you're targeting a specific DPI or need it to fit in a specific window size.

Window Resize and Efficient Rendering

Speaking of fitting to a window size, I wanted to resize the content whenever the window was resized, which meant redefining resizeEvent(self, event) on the widget. Initially my PDFWidget inherited from Qwidget with a custom paintEvent(), like this:

        # Create self.img once, early on:
        self.img = self.page.renderToImage(self.dpi, self.dpi)

    def paintEvent(self, event):
        qp = QPainter()
        qp.begin(self)
        qp.drawImage(QPoint(0, 0), self.img)
        qp.end()
(Poppler also has a function page.renderToPainter(), but I never did figure out how to get it to do anything useful.)

That worked, but when I added resizeEvent I got an infinite loop: paintEvent() called resizeEvent() which triggered another paintEvent(), ad infinitum. I couldn't find a way around that (GTK has similar problems -- seems like nearly everything you do generates another expose event -- but there you can temporarily disable expose events while you're drawing). So I rewrote my PDFWidget class to inherit from QLabel instead of QWidget, converted the QImage to a QPixmap and passed it to self.setPixmap(). That let me get rid of the paintEvent() function entirely and let QLabel handle the painting, which is probably more efficient anyway.

Showing all pages in a scrolled widget

renderToImage gives you one image corresponding to one page of the PDF document. More often, you'll want to see the whole document laid out, with all the pages. So you need a way to stack a bunch of widgets vertically, one for each page. You can do that with a QVBoxLayout on a widget inside a QScrollArea.

I haven't done much Qt5 programming, so I wasn't familiar with how these QVBoxes work. Most toolkits I've worked with have a VBox container widget to which you add child widgets, but in Qt5, you create a widget (no particular type -- a QWidget is enough), then create a layout object that modifies the widget, and add the sub-widgets to the layout object. There isn't much documentation for any of this, and very few examples of doing it in Python, so it took some fiddling to get it working.

Initial Window Size

One last thing: Qt5 doesn't seem to have a concept of desired initial window size. Most of the examples I found, especially the ones that use a .ui file, use setGeometry(); but that requires an (X, Y) position as well as (width, height), and there's no way to tell it to ignore the position. That means that instead of letting your window manager place the window according to your preferences, the window will insist on showing up at whatever arbitrary place you set in the code. Worse, most of the Qt5 examples I found online set the geometry to (0, 0): when I tried that, the window came up with the widget in the upper left corner of the screen and the window's titlebar hidden above the top of the screen, so there's no way to move the window to a better location unless you happen to know your window manager's hidden key binding for that. (Hint: on many Linux window managers, hold Alt down and drag anywhere in the window to move it. If that doesn't work, try holding down the "Windows" key instead of Alt.)

This may explain why I've been seeing an increasing number of these ill-behaved programs that come up with their titlebars offscreen. But if you want your programs to be better behaved, it works to self.resize(width, height) a widget when you first create it.

The current incarnation of my PDF viewer, set up as a module so you can import it and use it in other programs, is at qpdfview.py on GitHub.

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[ 19:01 Apr 27, 2018    More programming | permalink to this entry | comments ]
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