[RFC wayland] protocol: Add wl_surface.buffer_damage

[Pekka Paalanen]

On Fri,  6 Nov 2015 12:55:19 -0600
Derek Foreman <derekf at osg.samsung.com> wrote:

> wl_surface.damage uses surface local co-ordinates.
> 
> Buffer scale and buffer transforms came along, and EGL surfaces
> have no understanding of them.
> 
> Theoretically, clients pass damage rectangles - in Y-inverted surface
> co-ordinates) to EGLSwapBuffersWithDamage, and the EGL implementation
> passed them on to wayland.  However, for this to work the EGL
> implementation must be able to flip those rectangles into the space
> the compositor is expecting, but it's unable to do so because it
> doesn't know the height of the transformed buffer.
> 
> So, currently, EGLSwapBuffersWithDamage is unusable and EGLSwapBuffers
> has to pass (0,0) - (INT32_MAX, INT32_MAX) damage to function.
> 
> wl_surface.buffer_damage allows damage to be registered on a surface
> in buffer co-ordinates, avoiding this problem.
> 
> Credit where it's due, these ideas are not entirely my own:
> Over a year ago the idea of changing damage co-ordinates to buffer
> co-ordinates was suggested (by Jason Ekstrand), and it was at least
> partially rejected and abandoned.  At the time it was also suggested
> (by Pekka Paalanen) that adding a new wl_surface.buffer_damage request
> was another option.
> 

Hi Derek,

please mention https://bugs.freedesktop.org/show_bug.cgi?id=78190 in
this patch.

> Signed-off-by: Derek Foreman <derekf at osg.samsung.com>
> ---
> 
> Necro-posting on:
> http://lists.freedesktop.org/archives/wayland-devel/2014-February/013440.html
> and
> http://lists.freedesktop.org/archives/wayland-devel/2014-February/013442.html
> 
> This came up on IRC again the other day, and it's still an unsolved problem...
> I'm posting this as RFC to see if anyone's interested in it - I'll do an
> implementation if we can get an agreement on the protocol text.

Thanks for picking this up!

>  protocol/wayland.xml | 44 ++++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 42 insertions(+), 2 deletions(-)
> 
> diff --git a/protocol/wayland.xml b/protocol/wayland.xml
> index 9c22d45..1cb2f66 100644
> --- a/protocol/wayland.xml
> +++ b/protocol/wayland.xml
> @@ -176,7 +176,7 @@
>      </event>
>    </interface>
>  
> -  <interface name="wl_compositor" version="3">
> +  <interface name="wl_compositor" version="4">
>      <description summary="the compositor singleton">
>        A compositor.  This object is a singleton global.  The
>        compositor is in charge of combining the contents of multiple
> @@ -986,7 +986,7 @@
>      </event>
>    </interface>
>  
> -  <interface name="wl_surface" version="3">
> +  <interface name="wl_surface" version="4">
>      <description summary="an onscreen surface">
>        A surface is a rectangular area that is displayed on the screen.
>        It has a location, size and pixel contents.
> @@ -1327,6 +1327,46 @@
>        </description>
>        <arg name="scale" type="int"/>
>      </request>

I know Jasper suggested deprecating wl_surface.damage, but I see no
reason for that. wl_surface.damage is well-defined - it is only misused.

We could add some wording to have both refer to each other as an
alternative way to post damage. Especially to wl_surface.damage should
mention buffer_damage as doing what you'd usually expect.

> +
> +    <!-- Version 4 additions -->
> +    <request name="buffer_damage" since="4">

The name "buffer_damage" is slightly unfortunate. See:
https://www.khronos.org/registry/egl/extensions/KHR/EGL_KHR_swap_buffers_with_damage.txt

What we are doing in Wayland is always "surface damage"[*], while that
EGL extension reserves "buffer damage" for a different purpose. I feel
it might be confusing.

Could we come up with a another name for this request?
- wl_surface.damage_pixels
- wl_surface.damage_by_buffer

eh... buffer_damage is fine if nothing else sticks. The specification
language below is clear anyway, IMO.

> +      <description summary="mark part of the surface damaged using buffer co-ordinates">
> +	This request is used to describe the regions where the pending
> +	buffer is different from the current surface contents, and where
> +	the surface therefore needs to be repainted. The pending buffer
> +	must be set by wl_surface.attach before sending damage. The
> +	compositor ignores the parts of the damage that fall outside of
> +	the surface.
> +
> +	Damage is double-buffered state, see wl_surface.commit.
> +
> +	The damage rectangle is specified in buffer coordinates.
> +
> +	The initial value for pending damage is empty: no damage.
> +	wl_surface.damage adds pending damage: the new pending damage
> +	is the union of old pending damage and the given rectangle.
> +
> +	wl_surface.commit assigns pending damage as the current damage,
> +	and clears pending damage. The server will clear the current
> +	damage as it repaints the surface.

Essentially a copy from wl_surface.damage without changing anything but
the coordinate system. Ok.

> +
> +	This request differs from wl_surface.damage in only one way - it
> +	takes damage in buffer co-ordinates instead of surface local
> +	co-ordinates.  This is desirable because EGL implementations
> +	are unaware of buffer scale and buffer transform and can only
> +	provide damage in buffer co-ordinates.  Damage in buffer
> +	co-ordinates is required for EGLSwapBuffersWithDamage to work
> +	efficiently.

Not sure if explaining the EGL side is needed here. Besides EGL, it
could be any drawing library, and with wl_viewport there are much more
use cases where buffer_damage is preferable.

> +	Mixing wl_surface.buffer_damage and wl_surface.damage requests
> +	on the same surface will result in undefined behaviour.

Why undefined? The compositor will always transform between surface and
buffer coordinate systems: surface to buffer for texture updates, and
buffer to surface for repaint damage.

I suppose you can avoid accumulating two different regions depending on
the coordinate space until wl_surface.commit by saying only one
coordinate space is guaranteed to work at a time. Is that reason
enough, or the only reason?

> +      </description>
> +
> +      <arg name="x" type="int"/>
> +      <arg name="y" type="int"/>
> +      <arg name="width" type="int"/>
> +      <arg name="height" type="int"/>
> +    </request>
>     </interface>
>  
>    <interface name="wl_seat" version="5">


Thanks,
pq


[*] There is an off-topic rabbit hole to be dug here, if we would allow
the compositor to cache shm-based textures... ;-)
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 811 bytes
Desc: OpenPGP digital signature
URL: <http://lists.freedesktop.org/archives/wayland-devel/attachments/20151109/4cf89be7/attachment.sig>