[RFC v2] surface crop & scale protocol extension
ppaalanen at gmail.com
Fri Nov 8 03:33:22 PST 2013
this is the v2 of the crop and scale extension, as RFC.
The v1 was in:
Based on v1, Jonny Lamb has been working on a Weston implementation,
starting from the protocol side, and working towards the renderer
implementations. That work is still very much in progress.
The primary use case for the crop & scale extension is for hardware
accelerated, zero-copy video display. Hardware video decoders produce
complete frames or ultimately wl_buffers that are attached to
a wl_surface. The crop & scale extension allows the client to
request the compositor to crop and scale the frame so it fits the
client's purposes. Optimally, a compositor implements this by
programming a hardware overlay unit to do the requested cropping and
scaling. The major benefit is that the CPU never has to even see the
Probably a common case is to have a video in a sub-surface, so that
window decorations and overlaid GUI elements can be in other
(sub-)surfaces, and avoid touching the pixels in the video frame
buffers. Video as a browser element will need cropping when the element
is partially off-view. Scaling should be useful in general, e.g.
showing a video scaled up (or down) but still in a window.
However, I also see that crop & scale can be used to present videos
with non-square pixels in fullscreen (e.g. without sub-surfaces). Crop
& scale is needed in this case, because the fullscreening methods in
wl_shell do not allow changing the aspect ratio, or cropping parts of
the video out to fill the whole output area when video and output
aspect ratios differ. The fullscreening methods support only adding
black borders, but not scale-to-fill-and-crop.
Changes since v1:
The changes I have made are very small, and can be seen in patch form
The changes are:
- improve wording, add missing details
- take buffer_scale into account
- rewrite the coordinate transformations more clearly
In the end, I did not get much else out from the discussions of v1.
I think some people do not like the structure of the protocol
additions, namely adding yet another global factory interface and a
sub-interface to wl_surface. If the concensus really is to move this
into a single wl_surface request, that is fine.
But, to me this would not make sense as a request in wl_subsurface. The
crop & scale state is supposed to be driven by the application
component that produces the wl_buffers and attaches them to a
wl_surface. The wl_subsurface interface OTOH is to be used by the
parent component, for e.g. setting the sub-surface position. The
situation is similar to a compositor vs. clients: clients define the
surface size and content, compositor the position. Also, the crop &
scale state is not well suited to be "forced on" by a parent software
component, as it changes how e.g. input event coordinates relate to the
wl_buffer contents. Finally, there is the fullscreen video use case I
The interface names are still so-and-so, I haven't come up with
anything better. wl_scaler is pretty ok, but I feel something better
would be in place for wl_surface_scaler. How would wl_viewport sound
A minor benefit of keeping crop & scale as a separate interface is that
it could be made optional, a per-backend or renderer thing.
For your reviewing pleasure, here is the v2:
<?xml version="1.0" encoding="UTF-8"?>
Copyright Â© 2013 Collabora, Ltd.
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software and its documentation for any purpose is hereby granted
without fee, provided that the above copyright notice appear in
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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
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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
<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
<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.
<entry name="scaler_exists" value="0"
summary="the surface already has a scaler object associated"/>
<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.
<arg name="id" type="new_id" interface="wl_surface_scaler"
summary="the new scaler interface id"/>
<arg name="surface" type="object" interface="wl_surface"
<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
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
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.
<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.
<entry name="bad_value" value="0"
summary="negative values in width or height"/>
<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
If any of src_width, src_height, dst_width, and dst_height is
negative, the bad_value protocol error is raised.
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.
<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"/>
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