protocol: crop & scale RFC v3

Add cropping and scaling to wl_surface.

Add a global factory interface wl_scaler, which creates
wl_scaler_surface objects tied to a given wl_surface. The
wl_scaler_surface object can be used to set a cropping and scaling
transformation to change how a wl_buffer maps to wl_surface contents.

Changes in v2:

Take into account buffer_transform and buffer_scale, and try to explain
more clearly how the coordinate transformations work and what their
order is. Add, that crop and scale state is double-buffered. Explain
missing dst_x, dst_y.  Clarify that undefined content still is some
content, but NULL buffer implies no content nor size.

Changes in v3:

Disallow zero values for dst_width and dst_height.

Open issues:

Should this be a separate interface like here, or just a wl_surface
request?

If we keep this as a separate interface, rename wl_surface_scaler to
wl_viewport.

protocol/Makefile.am

@@ -7,7 +7,8 @@ protocol_sources =				\
 	workspaces.xml				\
 	text-cursor-position.xml		\
 	wayland-test.xml			\
-	xdg-shell.xml
+	xdg-shell.xml				\
+	scaler.xml
 
 if HAVE_XMLLINT
 .PHONY: validate

protocol/scaler.xml

@@ -0,0 +1,156 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<protocol name="scaler">
+
+  <copyright>
+    Copyright © 2013 Collabora, Ltd.
+
+    Permission to use, copy, modify, distribute, and sell this
+    software and its documentation for any purpose is hereby granted
+    without fee, provided that the above copyright notice appear in
+    all copies and that both that copyright notice and this permission
+    notice appear in supporting documentation, and that the name of
+    the copyright holders not be used in advertising or publicity
+    pertaining to distribution of the software without specific,
+    written prior permission.  The copyright holders make no
+    representations about the suitability of this software for any
+    purpose.  It is provided "as is" without express or implied
+    warranty.
+
+    THE COPYRIGHT HOLDERS DISCLAIM ALL WARRANTIES WITH REGARD TO THIS
+    SOFTWARE, INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND
+    FITNESS, IN NO EVENT SHALL THE COPYRIGHT HOLDERS BE LIABLE FOR ANY
+    SPECIAL, INDIRECT OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+    WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN
+    AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION,
+    ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF
+    THIS SOFTWARE.
+  </copyright>
+
+  <interface name="wl_scaler" version="1">
+    <description summary="surface cropping and scaling">
+      The global interface exposing surface cropping and scaling
+      capabilities is used to instantiate an interface extension for a
+      wl_surface object. This extended interface will then allow
+      cropping and scaling the surface contents, effectively
+      disconnecting the direct relationship between the buffer and the
+      surface size.
+    </description>
+
+    <request name="destroy" type="destructor">
+      <description summary="unbind from the cropping and scaling interface">
+	Informs the server that the client will not be using this
+	protocol object anymore. This does not affect any other objects,
+	wl_surface_scaler objects included.
+      </description>
+    </request>
+
+    <enum name="error">
+      <entry name="scaler_exists" value="0"
+             summary="the surface already has a scaler object associated"/>
+    </enum>
+
+    <request name="get_surface_scaler">
+      <description summary="extend surface interface for crop and scale">
+	Instantiate an interface extension for the given wl_surface to
+	crop and scale its content. If the given wl_surface already has
+	a wl_surface_scaler object associated, the scaler_exists
+	protocol error is raised.
+      </description>
+
+      <arg name="id" type="new_id" interface="wl_surface_scaler"
+           summary="the new scaler interface id"/>
+      <arg name="surface" type="object" interface="wl_surface"
+           summary="the surface"/>
+    </request>
+  </interface>
+
+  <interface name="wl_surface_scaler" version="1">
+    <description summary="crop and scale interface to a wl_surface">
+      An additional interface to a wl_surface object, which allows the
+      client to specify the cropping and scaling of the surface
+      contents.
+
+      This interface allows to define the source rectangle (src_x,
+      src_y, src_width, src_height) from where to take the wl_buffer
+      contents, and scale that to destination size (dst_width,
+      dst_height). This state is double-buffered, and is applied on the
+      next wl_surface.commit.
+
+      Before the first set request, the wl_surface still behaves as if
+      there was no crop and scale state. That is, no scaling is applied,
+      and the surface size is as defined in wl_surface.attach.
+
+      The crop and scale state causes the surface size to become
+      dst_width, dst_height. This overrides whatever the attached
+      wl_buffer size is, unless the wl_buffer is NULL. If the wl_buffer is
+      NULL, the surface has no content and therefore no size.
+
+      The coordinate transformations from buffer pixel coordinates up to
+      the surface-local coordinates happen in the following order:
+        1. buffer_transform (wl_surface.set_buffer_transform)
+        2. buffer_scale (wl_surface.set_buffer_scale)
+        3. crop and scale (wl_surface_scaler.set)
+      This means, that the source rectangle coordinates of crop and scale
+      are given in the coordinates after the buffer transform and scale,
+      i.e. in the coordinates that would be the surface-local coordinates
+      if the crop and scale was not applied.
+
+      If the source rectangle is partially or completely outside of the
+      wl_buffer, then the surface contents are undefined (not void), and
+      the surface size is still dst_width, dst_height.
+
+      The x, y arguments of wl_surface.attach are applied as normal to
+      the surface. They indicate how many pixels to remove from the
+      surface size from the left and the top. In other words, they are
+      still in the surface-local coordinate system, just like dst_width
+      and dst_height are.
+
+      If the wl_surface associated with the wl_surface_scaler is
+      destroyed, the wl_surface_scaler object becomes inert.
+
+      If the wl_surface_scaler object is destroyed, the crop and scale
+      state is removed from the wl_surface. The change will be applied
+      on the next wl_surface.commit.
+    </description>
+
+    <request name="destroy" type="destructor">
+      <description summary="remove scaling and cropping from the surface">
+	The associated wl_surface's crop and scale state is removed.
+	The change is applied on the next wl_surface.commit.
+      </description>
+    </request>
+
+    <enum name="error">
+      <entry name="bad_value" value="0"
+             summary="negative values in width or height"/>
+    </enum>
+
+    <request name="set">
+      <description summary="set the crop and scale state">
+	Set the crop and scale state of the associated wl_surface. See
+	wl_surface_scaler for the description, and relation to the
+	wl_buffer size.
+
+	The bad_value protocol error is raised if src_width or
+	src_height is negative, or if dst_width or dst_height is not
+	positive.
+
+	The crop and scale state is double-buffered state, and will be
+	applied on the next wl_surface.commit.
+
+	Arguments dst_x and dst_y do not exist here, use the x and y
+	arguments to wl_surface.attach. The x, y, dst_width, and dst_height
+	define the surface-local coordinate system irrespective of the
+	attached wl_buffer size.
+      </description>
+
+      <arg name="src_x" type="fixed" summary="source rectangle x"/>
+      <arg name="src_y" type="fixed" summary="source rectangle y"/>
+      <arg name="src_width" type="fixed" summary="source rectangle width"/>
+      <arg name="src_height" type="fixed" summary="source rectangle height"/>
+      <arg name="dst_width" type="int" summary="surface width"/>
+      <arg name="dst_height" type="int" summary="surface height"/>
+    </request>
+
+  </interface>
+</protocol>