During the past month I’ve been working on a new GStreamer element called qtvideosink. The purpose of this element is to allow painting video frames from GStreamer on any kind of Qt surface and on any platform supported by Qt. A “Qt surface” can be a QWidget, a QGraphicsItem in a QGraphicsView, a QDeclarativeItem in a QDeclarativeView, and even off-screen surfaces like QImage, QPixmap, QGLPixelBuffer, etc… The initial reason for working on this new element was to support GStreamer video in QML, which is something that many people have asked me about in the past. Until now there was only QtMultimedia supporting this, with some code in phonon being in progress as well. But of course, the main disadvantage with both QtMultimedia and phonon is that although they support this feature with GStreamer as the backend, they don’t allow you to mix pure GStreamer code with their QML video item, therefore they are useless in case you need to do something more advanced using the GStreamer API directly. Hence the need for something new.

My idea with qtvideosink was to implement something that would be a standalone GStreamer element, which would not require the developer to use a specific high level API in order to paint video on QML. In the past I have also written another similar element, qwidgetvideosink, which is basically the same idea, but for QWidgets. After looking at the problem a bit more carefully, I realized that in fact qwidgetvideosink and qtvideosink would share a lot of their internal logic and therefore I could probably do one element generic enough to do both painting on QWidgets and on QML and perhaps more surfaces. And so I did.

I started by taking the code of qtgst-qmlsink, a project that was started by a colleague here at Collabora last year, with basically the same intention, but which was never finished properly. This project was initially based on QtMultimedia’s GStreamer backend. As a first step, I did some major refactoring to clean it up from its QtMultimedia dependencies and to make it an independent GStreamer plugin (as it used to be a library). Then I merged it with qwidgetvideosink, so that they can share the common parts of the code and also wrote a unit test for it. Sadly, the unit test proved something that I was suspecting already: the original QtMultimedia code was quite buggy. But I must say I enjoyed fixing it. It was a good opportunity for me to learn a lot of things on video formats and on OpenGL.

How does it work

First of all, you can create the sink with the standard gst_element_factory_make method (or its equivalent in the various bindings). You will notice that this sink provides two signals, an action signal (a slot in Qt terminology) called “paint” and a normal signal called “update”. “update” is emitted every time the sink needs the surface to be repainted. This is meant to be connected directly to QWidget::update() or QGraphicsItem::update() or something similar. The “paint” slot takes a QPainter pointer and a rectangle (x, y, width, height as qreals) as its arguments and paints the video inside the given rectangle using the given painter. This is meant to be called from the widget’s paint event or the graphics item’s paint() function. So, all you need to do is to take care of those two signals and qtvideosink will do everything else.

Getting OpenGL into the game

You may be wondering how this sink does the actually painting. Using QPainter, using OpenGL or maybe something else? Well, there are actually two variants of this video sink. The first one, qtvideosink, just uses QPainter. It is able to handle only RGB data (only a subset of the formats that QImage supports) and does format conversion and scaling in software. The second one, however, qtglvideosink, uses OpenGL/OpenGLES with shaders. It is able to handle both RGB and YUV formats and does format conversion and scaling in hardware. It is used in exactly the same way as qtvideosink, but it requires a QGLContext pointer to be set on its “glcontext” property before its state is set to READY. This of course means that the underlying surface must support OpenGL (i.e. it must be one of QGLWidget, QGLPixelBuffer or QGLFrameBufferObject). To get this working on QGraphicsView/QML, you just need to set a QGLWidget as the viewport of QGraphicsView and use this widget’s QGLContext in the sink.

qtglvideosink uses either GLSL shaders or ARB fragment program shaders if GLSL is not supported. This means it should work on pretty much every GPU/driver combination that exists for linux on both desktop and emebedded systems. In case no shaders are supported, it will fail to change its state to READY and then you can just substitute it with qtvideosink, which is guaranteed to work on all platforms supported by Qt.

qtglvideosink also has an extra feature: it supports the GstColorBalance interface. Color adjustment is done in the shaders together with the format conversion. qtvideosink doesn’t support this, as it doesn’t make sense. Color adjustment would need to be implemented in software and this can be done better by plugging a videobalance element before the sink. No need to duplicate code.

So, which variant to use?

If you are interested in painting video on QGraphicsView/QML, then qtglvideosink is the best choice of all sinks. And if for any reason the system doesn’t support OpenGL shaders, qtvideosink is the next choice. Now if you intend to paint video on normal QWidgets, it is best to use one of the standard GStreamer sinks for your platform, unless you have a reason not to. QWidgets can be transformed to native system windows by calling their winId() method and therefore any sink that implements the GstXOverlay interface can be embedded in them. On X11 for example, xvimagesink is the best choice. However, if you need to do something more tricky and embedding another window doesn’t suit you very well, you could use qtglvideosink in a QGLWidget (preferrably) or qtvideosink / qwidgetvideosink on a standard QWidget.

Note that qwidgetvideosink is basically the same thing as qtvideosink, with the difference that it takes a QWidget pointer in its “widget” property and handles everything internally for painting on this widget. It has no signals. Other than that, it still does painting in software with QPainter, just like qtvideosink. This is just there to keep compatibility with code that may already be using it, as it already exists in QtGStreamer 0.10.1.

This is actually 0.10 stuff… What about GStreamer 0.11/1.0?

Well, if you are interested in 0.11, you will be happy to hear that there is already a partial 0.11 port around. Two weeks ago I was at the GStreamer 1.0 hackfest at Malaga, Spain, and one of the things I did there was porting qtvideosink to 0.11. I must say the port was quite easy to do. However, last week I added some more stuff in the 0.10 version that I haven’t ported yet to 0.11. I’ll get to that soon, it shouldn’t take long.

Try it out

The code lives in the qt-gstreamer repository. The actual video sinks are independent from the qt-gstreamer bindings, but qt-gstreamer itself has some helper classes for using them. Firstly there is QGst::Ui::VideoWidget, a QWidget subclass which will accept qtvideosink, qtglvideosink and qwidgetvideosink just like any other video sink and will transparently do all the required work to paint the video in it. Secondly, there is QGst::Ui::GraphicsVideoWidget and QGst::Ui::GraphicsVideoSurface. Those two are meant to be used together to paint video on a QGraphicsView or QML. You can find more about them at the documentation in graphicsvideosurface.h (this will soon be on the documentation website). Finally, there is a QtGStreamer QML plugin, which exports a “VideoItem” element if you “import QtGStreamer 0.10”. This is also documented in the GraphicsVideoSurface header. All of this will soon be released in the upcoming qt-gstreamer 0.10.2.