[RFC] wl_surface scale and crop protocol extension
ppaalanen at gmail.com
Tue Apr 30 05:33:42 PDT 2013
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
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?
<?xml version="1.0" encoding="UTF-8"?>
Copyright © 2013 Collabora, Ltd.
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<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
<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, 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
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
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.
<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.
<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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