desktop-shell: How to enable really alpha blending of weston background?

Pekka Paalanen ppaalanen at
Tue Feb 23 09:29:34 UTC 2016

On Tue, 23 Feb 2016 10:11:39 +0900
YoungJun Jo <dtoartist at> wrote:

> Hello,
> Thanks for your reply.
> What i want is alpha blending for my chips.
> As i mentioned, My chips have four independent H/W overlays.
> For example,
> H/W Overlay 1 => Weston, Applications, etc..
> H/W Overlay 2 => Video play.
> H/W Overlay 3 = > Empty.
> H/W Overlay 4 = > Empty.
> I also know that weston don't know H/W Overlays above mentioned.


ok. The current state of Weston is slightly sad, as the support for
hardware overlays is disabled by default, because enabling it causes
performance problems. All that will be fixed once the patch series to
support atomic/nuclear KMS in the DRM backend by Daniel Stone will
land. I think Daniel is polishing it for submission whenever he has
time for it.

I would expect the situation to improve for Weston 1.11, but that's no

Btw. if you are using an fbdev backed, you have already lost: it will
never support hardware overlays. Use the DRM-backend instead. It is
possible to activate overlay support already with the debug key binding
Mod+Shift+space, v.

> Scenario:
> 1. If i click play button on my video application, it will be call alpha
> blending API.
> 2. When weston receives alpha blending api, weston background will be
> transparent.
> 3. And i could see video playing from H/W Layer 2.(Still H/W Layer1 shows
> GUI of video control buttons)
> So i want really alpha blending of weston background.

I suppose you could do it like that, assuming that:
- you will only ever support your one special video player
- your video player is not cooperating with the display server, but it
  is bypassing the display server instead and holding the display
  server as a hostage (otherwise the display server, e.g. weston, can
  mess up your video view)
- your video player is likely specific to your hardware and cannot work
- you will have to use a modified Weston, as upstream will never accept
  what you would need for this
- you will need to create a protocol extension for the "alpha blending
  API" or whatever to tell weston to get out of the way
- you won't have any application window mapped while playing e.g.
  fullscreen video
- you will waste power by always having to have a fullscreen completely
  transparent overlay on top the video overlay, rather than having the
  video on the top-most overlay

and probably many more limitations that make the approach very painful
to use in anything but a simple single-purpose device with a throw-away
software stack.

Essentially, you are using a display server (weston) to get a window
system, and then your aim is to remove the window system and bypass the
display server, telling weston to not mess with your video player.

Btw. I forgot a few things earlier. Not only you need to have the
backend tell the renderer use an alpha-capable framebuffer format, you
also need to hack clients/desktop-shell.c to not force the background
to be opaque. I'm not sure, but there might also be further things to
adjust in a backend to make sure the alpha bits actually get into the

The proper solution using Wayland and Weston as they are intended would
involve the following:
- ensure your display hardware has proper DRM drivers supporting atomic
- wait for the atomic support to land in Weston, which...
- puts Weston in control of all hardware planes, so that...
- Weston will automatically use all hardware planes for everything it
- use any general purpose video player you want which supports
  linux_dmabuf Wayland protocol extension...
- and that will get the video on the hw overlay automatically when it
  is possible, and through hardware accelerated composition when the
  overlay cannot be used.

When the video player uses EGL or linux_dmabuf to push the video frames
to Weston, then Weston has the possibility to show them on a hardware

As Weston will be in charge of everything on the display, the
fullscreen video playback won't suffer from the same waste of power as
your approach.

The proper solution is likely to result in a near-optimal use of
hardware resources in all cases automatically, both where overlays can
and cannot be used. The proper solution is also generic in that it is
not tied to a particular hardware platform.

In summary, do what you must, but I would strongly recommend the proper
solution if at all possible for you.


> 2016-02-22 21:36 GMT+09:00 YoungJun Jo <dtoartist at>:
> > Environment
> > OS : Linux kernel 3.10
> > Version : Weston 1.8.0
> > H/W spec : embedded soc based on ARM
> >
> > Hello, All
> >
> > I have a question about weston background.
> > I want enable alpha blending to background, so i have typed
> > 'background-color=0x00ffffff' on weston.ini.
> > But that's not really alpha blending. Because alpha value of pixel is a
> > 'ff'.
> > My chips have four H/W layers, top of the layer is UI layer and second
> > layer is video layer.
> >
> > I reviewed source code of 'desktop-shell.c' but i can't find any hints.
> >
> > Need some additional ideas to solve this problem. Any feedback is
> > appreciated.
> >
> > Thanks.
> >  

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