Category: GStreamer


During the past few months I’ve been occasionally working on integrating GStreamer better with wayland. GStreamer already has a ‘waylandsink’ element in gst-plugins-bad, but the implementation is very limited. One of the things I’ve been working on was to add GstVideoOverlay support in it, and recently, I managed to make this work nicely embedded in a GTK+ window.

GStreamer Wayland GTK Demo

gtk+ video player demo running on weston

I’m happy to say that it works pretty well, even though GTK does not support wayland sub-surfaces, which was being thought of as a problem initially. It turns out there is no problem with that, and both the GTK and GstVideoOverlay APIs are sufficient to make this work. However, there needs to be a small addition in GstVideoOverlay to allow smooth resizing. Currently, I have a GstWaylandVideo API that includes those additions.

This essentially means that as soon as this work is merged (hopefully soon), there is nothing stopping applications like totem from being ported to wayland :D

I believe embedding waylandsink in Qt should also work without any problems, I just haven’t tested it.

If you are interested, check the code. The code of the above running demo can also be found here, and the ticket for merging this branch is being tracked here.

I should say many thanks here to my employer, Collabora, for sponsoring this work.

CommonsFest

Hello all! Long time no blog. The reason I decided to blog this time is that I want to spread the word about CommonsFest, a different festival that will take place in less than two weeks (9-11 May) in my home city, Heraklion, in Crete.

What is it?

CommonsFest-Logo-BlueCommonsFest is a festival that aims to promote freedom of knowledge (or free knowledge) and peer-to-peer collaboration for the creation and management of the Commons. A philosophy that has spread through free software communities and extends to many aspects of our daily lives, such as the arts, governance, construction of machinery, tools and other goods. In other words, it aims to raise the awareness of people about the philosophy of open source in its generalisation, involving open source code, open hardware, creative commons and similar initiatives like open source ecology, open governance, etc.

Why?

This festival was conceived and organized for the first time in Heraklion last year by a group of volunteers, myself being one of them, who felt that it is worth promoting this philosophy to people that are unaware of its existence, especially in this time of economic crisis where it is clear that people need to cooperate more and share in order to go forward. This year we are repeating it, aiming to be better and achieve even more. Our long term vision is to spread this philosophy to as many people as possible around the world and eventually improve our lives by changing the way people think. We believe in this idea that the world would be a better place if we all shared our knowledge and worked together, equally, for the well-being of all of us.

I love the idea, how can I be part of it?

There are many ways you can help. First of all, this year, the festival is crowd-funded to cover our expenses (mainly printing and transportation for the speakers). If you like the idea and would like to support us directly, you could give a small donation. Another way you can help is to just share this with other people that may (or may not) be interested – even if it’s just to raise their awareness about the subject ;). Finally, you could help to achieve our goals by organizing something similar in your area. All our material (logos, texts, slideshows, etc) is freely licensed under creative commons licences, so you can also use them if you want.

Where can I learn more?

Have a look at our website, our fund-raising campaign, our facebook, google+ and twitter pages. You can also contact the organizing team by email.

CF-CrowdFunding-Campaign-Banner

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.

QtGStreamer 0.10.1

This weekend I released QtGStreamer 0.10.1, the first stable version of QtGStreamer. This release marks the beginning of the stable 0.10 series of QtGStreamer that will continue for the lifetime of GStreamer 0.10. For those of you that don’t yet know what QtGStreamer is, it is a set of libraries that provide Qt-style C++ bindings for GStreamer, plus extra helper classes and elements for better integration of GStreamer in Qt applications.

I must say thanks a lot to Mauricio, the co-developer of QtGStreamer, who helped me a lot with the design and code, to the GStreamer community, who accepted this project under the GStreamer umbrella with great enthusiasm, to Nokia for sponsoring it, to Collabora for assigning me and Mauricio to work on it and to all those developers who are already using it in their projects and have helped us by providing feedback.

The future

Development of course does not stop here. It just started. We will try to improve the bindings as much as we can by exporting more and more of GStreamer’s functionality, by adding more and more convenience methods/classes and/or gstreamer elements that ease the use of GStreamer in Qt applications and by collecting opinions and ideas from all of you out there that will use this API. This last bit is quite important imho, so, if you have any suggestions to make about things that you don’t like or things that you would like to see implemented, please file a bug to let us know.

Use in KDE

I am quite happy to see that this library already has early adopters in KDE. Apart of course from my telepathy-kde-call-ui (ex kcall), which is the “father” of QtGStreamer, QtGStreamer is also used in kamoso, a cheese-like camera app, whose authors, Alex Fiestas and Aleix Pol, have been very patient waiting for me to release QtGStreamer before they release kamoso and have also been very supportive during all this time (thanks!).

Personal thoughts

I must say this project was fun to develop. During development, I learned a lot about C++ that I didn’t know before and I also learned how GObject works, which I must say is quite interesting, although ugly for my taste. Learning more about C++ was my main source of interest from the beginning of the project, and for some period of time I couldn’t even imagine that this project would ever reach here, but I kept coding it for myself. Obviously, I am more than happy now that this finally evolved into something that is also useful for others and has wide acceptance :)

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