[RFC] wl_surface scale and crop protocol extension

[Pekka Paalanen]

Hi all,

below is my first draft for a wl_surface scaling and cropping
extension. I called it wl_scaler in the lack of a better name. It is
designed similarly to the other wl_surface extensions wl_shell_surface
and wl_subsurface.

There probably isn't any interesting details to debate, right? ;-)

I'd like to have a better name for it, and you might want the set
request split into two or three, or not, but otherwise I'm quite
satisfied with it. I tried to take into account all existing protocol
that interacts with this.

Comments? Is the language clear?


Thanks,
pq


<?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 1:1 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, allowing 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 the buffer size.

      On compositing, source rectangle coordinates are evaluated after
      wl_surface.set_buffer_transform is evaluated. This means that
      changing the buffer transform and correspondingly the client
      rendering does not require sending new source rectangle
      coordinates to keep the exact same image source rectangle. In
      other words, the source rectangle is given in the
      not-scaled-and-cropped surface coordinates, not buffer data
      coordinates.

      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.

      The surface-local coordinate system is tied to the space of
      dst_width and dst_height. That is, surface-local coordinates are
      relative to the scaled and cropped surface, not the wl_buffer
      providing the content.

      If the source rectangle is partially or completely outside of the
      wl_buffer, then the surface contents are undefined, 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.

	If any of src_width, src_height, dst_width, and dst_height is
	negative, the bad_value protocol error is raised.
      </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>