[RFC wayland] protocol: Add wl_surface.buffer_damage

[Derek Foreman]

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.

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.

 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>
+
+    <!-- Version 4 additions -->
+    <request name="buffer_damage" since="4">
+      <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.
+
+	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.
+
+	Mixing wl_surface.buffer_damage and wl_surface.damage requests
+	on the same surface will result in undefined behaviour.
+      </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">
-- 
2.6.1