[PATCH weston] libweston: don't accumulate damage from transparent views

Fri May 4 12:09:55 UTC 2018

Hi Pekka,

Sorry for late response.

> -----Original Message-----
> From: Pekka Paalanen [mailto:ppaalanen at gmail.com]
> Sent: Donnerstag, 19. April 2018 12:36
> To: Ucan, Emre (ADITG/ESB)
> Cc: wayland-devel at lists.freedesktop.org
> Subject: Re: [PATCH weston] libweston: don't accumulate damage from
> transparent views
> 
> On Thu, 19 Apr 2018 09:26:59 +0000
> "Ucan, Emre (ADITG/ESB)" <eucan at de.adit-jv.com> wrote:
> 
> > Hi Pekka,
> >
> > If we remove the view from scenegraph, application will be blocked.
> > Because it is not getting any surface frame events. It is not OK to
> > block unexpected applications. Especially if the application is
> > sending output of a camera or digital TV, weston should always get
> > latest buffer from the application.
> 
> Hi Emre,
> 
> sounds like the application is broken if it gets literally blocked by
> that.
> 
> Applications very much have to be able to deal with frame callbacks
> not coming back in a timely fashion. This is what compositors use to
> throttle down well-behaving clients. It's not blocking the app, unless
> the app is badly written.
> 
> An invisible surface is a very good reason to try and throttle down the
> client.
> 
> > You might say that camera application can use wl_display_sync instead
> > wl_surface_frame.
> 
> Not necessarily. It would certainly make the app greedy instead of
> well-behaved in my opinion.
> 
> But, this would definitely make the latest buffer from the application
> always available to the compositor. You shouldn't need wl_display.sync
> even, the app can simply keep on committing new buffers whenever it
> wants.

You are right that this issue can be work around for an EGL application when we set
swapInterval to 0, and send a new buffer every time internal state of the application requires to
change its contents. But this does not fix the greediness problem. Because application state can change
more often than display refresh rate.

> 
> If the application is a video player that just cannot avoid decoding or
> receiving frames anyway, then the savings of not posting those frames
> may be insignificant. The major saving the throttling aims for is with
> apps that paint on demand, where painting is a heavy task that can
> easily be skipped.
> 
> > This would cause stutter and frame drop issues,
> > because camera stream and display are not in synch.
> 
> Are your camera and display both using the exact same refresh rate,
> just not synchronized by hardware?
> 
> I fail to see how the frame callbacks would synchronize anything any
> better than just always committing new buffers whenever the app has
> one. It's either the compositor or the client that eventually makes the
> decision to drop or repeat frames to account for differing refresh
> rates. But if it is the app doing it, it could interpolate video frames
> to match the display refresh cycle.
> 
> Or is the camera framerate an exact integer fraction of the display
> refresh rate?
Ideally yes. Let's think that camera is sending buffer with 60 Hz and display has also the same refresh rate.
But camera and display could be still out of sync, so that camera sends its buffer 8ms after Vsync. Therefore,
it will miss the repaint window of weston.

It is also possible that camera buffers sometimes hit, sometimes miss the repaint window. This would cause visible video stutter.
Because delay of displaying a buffer would swing between 0,5 Vsync to 1,5 Vsync ( Repaint window is 0.5 Vsync).
Therefore, it is not ok to send buffer every time when it is ready. We should send it directly after repaint, so that (hopefully) we won't miss the repaint window.
> 
> > I know that I have to modify weston_compositor_pick_view(). I will
> > send a patch if this one is accepted.
> 
> It still seems like a hack to me.
> 
> If the view is not visible (does not matter how it is not visible), it
> should not be getting frame callbacks, because there is no point for
> the client to update surface as it is not visible. Weston is missing
> some implementation bits to postpone frame callbacks for e.g. surfaces
> that are completely occluded. I suppose being completely transparent
> could be another case.
You are right that it is a kind of hack. But I don't know how it should work otherwise especially for camera/video use-cases.

> 
> I know there is a problem that when the surface becomes visible again,
> it would take a frame cycle to have the client send an updated buffer.
> This could be worked around on either side: the compositor could be
> sending frame callbacks at a slow rate even if the surface is not
> visible, or the client could be updating the surface content at a slow
> rate even if it doesn't get frame callbacks.

But your proposal is also a hack. Maybe better solution would be that we introduce a repaint_window event to wl_output interface, 
so that camera/video applications can synchronize themselves with this event instead of surface frame events. 
Compositor can send a timestamp and duration of repaint window to the clients.

> 
> > > -----Original Message-----
> > > From: Pekka Paalanen [mailto:ppaalanen at gmail.com]
> > > Sent: Donnerstag, 19. April 2018 11:12
> > > To: Ucan, Emre (ADITG/ESB)
> > > Cc: wayland-devel at lists.freedesktop.org
> > > Subject: Re: [PATCH weston] libweston: don't accumulate damage from
> > > transparent views
> > >
> > > On Wed, 18 Apr 2018 15:44:26 +0200
> > > Emre Ucan <eucan at de.adit-jv.com> wrote:
> > >
> > > > If view is set to be entirely transparent,
> > > > there is no need to accumulate its damage.
> > > >
> > > > This is an important optimization for
> > > > using view transparency. Because otherwise
> > > > transparent views are rendered like an
> > > > opaque view, and their pixel values
> > > > are set to 0 in fragment shader.
> 
> I do see the value here, but I'm not sure the below is the right way to
> do it. You suppress damage, but you don't avoid actually painting the
> view if something else still causes damage.
> 
> One possibility would be to exclude the view from
> weston_compositor::view_list which is used for both rendering and input
> picking. This would happen in weston_compositor_build_view_list(). It
> could also exclude completely occluded views, but we can leave that for
> another time.
> 
> Excluding the view from the rendering list will avoid sending frame
> callbacks. It will also affect Presentation feedback in the expected
> way: the updates the compositor has decided to not show (e.g. by setting
> view alpha to zero) will not result in a "presented" event.
> 
> Would you like to examine this path instead?

You are right that it would be a better solution to not have invisible views in the compositor view list.
Daniel also implemented to discard occluded views here: " https://patchwork.freedesktop.org/patch/202693/".
I can reuse it. But my real intention for this patch was to mitigate the performance penalty of hiding a surface via using opacity instead of visibility.
Technically, it is of course correct that invisible surfaces do not get any frame events. But we need some other solution to synchronize video applications with wayland compositors...

Best Regards,

Emre Ucan

> 
> 
> Thanks,
> pq
> 
> > > >
> > > > Signed-off-by: Emre Ucan <eucan at de.adit-jv.com>
> > > > ---
> > > >  libweston/compositor.c | 5 ++++-
> > > >  1 file changed, 4 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/libweston/compositor.c b/libweston/compositor.c
> > > > index a9de4ac..4bcf120 100644
> > > > --- a/libweston/compositor.c
> > > > +++ b/libweston/compositor.c
> > > > @@ -2122,7 +2122,10 @@ compositor_accumulate_damage(struct
> > > weston_compositor *ec)
> > > >  		pixman_region32_init(&opaque);
> > > >
> > > >  		wl_list_for_each(ev, &ec->view_list, link) {
> > > > -			if (ev->plane != plane)
> > > > +			/* If view is set to be entirely
> > > > transparent,
> > > > +			 * there is no need to accumulate its
> > > > damage.
> > > > +			 */
> > > > +			if (ev->plane != plane || ev->alpha ==
> > > > 0.0f) continue;
> > > >
> > > >  			view_accumulate_damage(ev, &opaque);
> > >
> > > Hi,
> > >
> > > why this instead of removing the whole view from the scenegraph?
> > >
> > > You would also need to exclude the view in
> > > weston_compositor_pick_view(), and adding these rendering special
> > > cases around doesn't feel very good to me.
> > >
> > >
> > > Thanks,
> > > pq