[PATCH wayland-protocols v3] Add alpha-compositing protocol

[Derek Foreman]

This looks nice to me, and I have need of something like this.

A couple of comments below.

On 11/14/18 7:00 PM, Scott Anderson wrote:
> This protocol specifies a set of interfaces used to control the alpha
> compositing and blending of surface contents, based on the Chromium
> Wayland protocol of the same name [1].
> 
> Signed-off-by: Scott Anderson <scott.anderson at collabora.com>
> 
> [1] https://chromium.googlesource.com/chromium/src/+/master/third_party/wayland-protocols/unstable/alpha-compositing/alpha-compositing-unstable-v1.xml
> ---
> 
> Changes in v3:
>   - Rename "blending" event to "blending_equation"
>   - Add enum specifier to "blending_equation" event and "set_blending"
>     request.
>   - Fix typos
> 
> Changes in v2:
>   - Add additional "fromsource" blending equation used by [3]
>   - Allow the server to advertise which blending equations it supports
>   - Added a protocol error for using an unsupported equation
>   - Added a protocol error for using an invalid alpha value
>   - Added clarification about wl_surface.set_opaque_region
>   - Added clarification about per-pixel alpha values
> 
>  Makefile.am                                   |   1 +
>  unstable/alpha-compositing/README             |   5 +
>  .../alpha-compositing-unstable-v1.xml         | 175 ++++++++++++++++++
>  3 files changed, 181 insertions(+)
>  create mode 100644 unstable/alpha-compositing/README
>  create mode 100644 unstable/alpha-compositing/alpha-compositing-unstable-v1.xml
> 
> diff --git a/Makefile.am b/Makefile.am
> index 6394e26..ac6c9f8 100644
> --- a/Makefile.am
> +++ b/Makefile.am
> @@ -21,6 +21,7 @@ unstable_protocols =								\
>  	unstable/xdg-output/xdg-output-unstable-v1.xml				\
>  	unstable/input-timestamps/input-timestamps-unstable-v1.xml	\
>  	unstable/xdg-decoration/xdg-decoration-unstable-v1.xml	\
> +	unstable/alpha-compositing/alpha-compositing-unstable-v1.xml		\
>  	$(NULL)
>  
>  stable_protocols =								\
> diff --git a/unstable/alpha-compositing/README b/unstable/alpha-compositing/README
> new file mode 100644
> index 0000000..5826967
> --- /dev/null
> +++ b/unstable/alpha-compositing/README
> @@ -0,0 +1,5 @@
> +Alpha compositing protocol
> +
> +Maintainers:
> +David Reveman <reveman at chromium.org>
> +Alexandros Frantzis <alexandros.frantzis at collabora.com>
> diff --git a/unstable/alpha-compositing/alpha-compositing-unstable-v1.xml b/unstable/alpha-compositing/alpha-compositing-unstable-v1.xml
> new file mode 100644
> index 0000000..d3c41d5
> --- /dev/null
> +++ b/unstable/alpha-compositing/alpha-compositing-unstable-v1.xml
> @@ -0,0 +1,175 @@
> +<?xml version="1.0" encoding="UTF-8"?>
> +<protocol name="alpha_compositing_unstable_v1">
> +
> +  <copyright>
> +    Copyright 2016      The Chromium Authors.
> +    Copyright 2017-2018 Collabora Ltd
> +    Copyright 2018      NXP
> +
> +    Permission is hereby granted, free of charge, to any person obtaining a
> +    copy of this software and associated documentation files (the "Software"),
> +    to deal in the Software without restriction, including without limitation
> +    the rights to use, copy, modify, merge, publish, distribute, sublicense,
> +    and/or sell copies of the Software, and to permit persons to whom the
> +    Software is furnished to do so, subject to the following conditions:
> +
> +    The above copyright notice and this permission notice (including the next
> +    paragraph) shall be included in all copies or substantial portions of the
> +    Software.
> +
> +    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
> +    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
> +    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
> +    THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
> +    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
> +    FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
> +    DEALINGS IN THE SOFTWARE.
> +  </copyright>
> +
> +  <description summary="Protocol for more advanced compositing and blending">
> +    This protocol specifies a set of interfaces used to control the alpha
> +    compositing and blending of surface contents.
> +
> +    Warning! The protocol described in this file is experimental and backward
> +    incompatible changes may be made. Backward compatible changes may be added
> +    together with the corresponding interface version bump. Backward
> +    incompatible changes are done by bumping the version number in the protocol
> +    and interface names and resetting the interface version. Once the protocol
> +    is to be declared stable, the 'z' prefix and the version number in the
> +    protocol and interface names are removed and the interface version number is
> +    reset.
> +  </description>
> +
> +  <interface name="zwp_alpha_compositing_v1" version="1">
> +    <description summary="alpha compositing">
> +      The global interface exposing compositing and blending capabilities is
> +      used to instantiate an interface extension for a wl_surface object.
> +      This extended interface will then allow the client to specify the
> +      blending equation and alpha value used for compositing the wl_surface.
> +    </description>
> +
> +    <enum name="error">
> +      <entry name="blending_exists" value="0"
> +             summary="the surface already has a blending object associated"/>
> +    </enum>
> +
> +    <request name="destroy" type="destructor">
> +      <description summary="unbind from the blending interface">
> +        Informs the server that the client will not be using this
> +        protocol object anymore. This does not affect any other objects,
> +        blending objects included.
> +      </description>
> +    </request>
> +
> +    <request name="get_blending">
> +      <description summary="extend surface interface for blending">
> +        Instantiate an interface extension for the given wl_surface to
> +        provide surface blending. If the given wl_surface already has
> +        a blending object associated, the BLENDING_EXISTS protocol error
> +        is raised.
> +      </description>
> +
> +      <arg name="id" type="new_id" interface="zwp_blending_v1"
> +           summary="the new blending interface id"/>
> +      <arg name="surface" type="object" interface="wl_surface"
> +           summary="the surface"/>
> +    </request>
> +
> +    <event name="blending_equation">
> +      <description summary="supported blending equations">
> +        This event advertises the blending equations that the server
> +        supports. All the supported blending equations are advertised once
> +        when the client binds to this interface. A roundtrip after binding
> +        guarantees that the client has received all supported blending
> +        equations.
> +
> +        For the definition of the blending equations, see the
> +        zwp_blending_v1.blending_equation enum.
> +
> +        The server must always advertise the 'none' blending equation.

Would it make sense to require the server to advertise 'none' last, so
clients can treat it as an end-of-list?

> +      </description>
> +      <arg name="blending_equation" type="uint"
> +	   enum="zwp_blending_v1.blending_equation"
> +	   summary="the blending equation"/>
> +    </event>
> +  </interface>
> +
> +  <interface name="zwp_blending_v1" version="1">
> +    <description summary="blending interface to a wl_surface">
> +      An additional interface to a wl_surface object, which allows the
> +      client to specify the blending equation used for compositing and
> +      an alpha value applied to the whole surface.
> +
> +      When the blending object is created its blending equation is
> +      'none' and its alpha is 1.0, i.e., it's inactive by default. Clients
> +      can activate it by setting the blending equation and alpha value.
> +
> +      Use of this interface has no effect on the surface's opaque region
> +      as set by wl_surface.set_opaque_region. Clients must make sure to set
> +      their opaque region correctly to prevent repaint artifacts.
> +
> +      If the wl_surface associated with the blending object is destroyed,
> +      the blending object becomes inert.
> +
> +      If the blending object is destroyed, the blending state is removed
> +      from the wl_surface. The change will be applied on the next
> +      wl_surface.commit.
> +    </description>
> +
> +    <enum name="blending_equation">
> +      <description summary="different blending equations for compositing">
> +        Blending equations that can be used when compositing a surface.
> +      </description>
> +      <entry name="none" value="0" summary="blending object is inactive"/>
> +      <entry name="opaque" value="1" summary="(one, zero)"/>
> +      <entry name="premultiplied" value="2" summary="(one, one_minus_src_alpha)"/>
> +      <entry name="straight" value="3" summary="(src_alpha, one_minus_src_alpha)"/>
> +      <entry name="fromsource" value="4" summary="(src_alpha, src_alpha)"/>
> +    </enum>
> +
> +    <enum name="error">
> +      <entry name="invalid_equation" value="0"
> +             summary="the blending equation is not supported"/>
> +      <entry name="invalid_alpha" value="1"
> +             summary="the requested alpha value is outside the interval [0, 1]"/>
> +    </enum>
> +
> +    <request name="destroy" type="destructor">
> +      <description summary="remove blending from the surface">
> +        The associated wl_surface's blending state is removed.
> +        The change is applied on the next wl_surface.commit.
> +      </description>
> +    </request>
> +
> +    <request name="set_blending">
> +      <description summary="set the blending equation">
> +        Set the blending equation for compositing the wl_surface.
> +
> +        If the requested format was not advertised by the
> +        zwp_alpha_compositing_v1.blending event, the INVALID_EQUATION protocol
> +        error is raised.
> +
> +        The blending equation state is double-buffered state,
> +        and will be applied on the next wl_surface.commit.
> +      </description>
> +      <arg name="equation" type="uint" enum="blending_equation"
> +	   summary="the new blending equation"/>
> +    </request>
> +
> +    <request name="set_alpha">
> +      <description summary="set the alpha value">
> +        Set the alpha value applied to the whole surface for compositing. This
> +        alpha value is applied as an additional step after the processing of
> +        per-pixel alpha values for the wl_surface.
> +
> +        If the requested alpha is outside the interval [0, 1], the
> +        INVALID_ALPHA protocol error is raised.
> +
> +        The alpha value state is double-buffered state,
> +        and will be applied on the next wl_surface.commit.
> +      </description>
> +      <arg name="value" type="fixed" summary="the new alpha value"/>

I worry about using wl_fixed here, as it only has 8 bits of decimal
precision, and >8bpc seems to be increasingly common these days...

Thanks,
Derek

> +    </request>
> +  </interface>
> +
> +</protocol>
>