[Bug 743248] New: Add new control binding GstAppControlBinding

[GStreamer (bugzilla.gnome.org)]

https://bugzilla.gnome.org/show_bug.cgi?id=743248
  GStreamer | gstreamer (core) | unspecified

           Summary: Add new control binding GstAppControlBinding
    Classification: Platform
           Product: GStreamer
           Version: unspecified
        OS/Version: All
            Status: UNCONFIRMED
          Severity: normal
          Priority: Normal
         Component: gstreamer (core)
        AssignedTo: gstreamer-bugs at lists.freedesktop.org
        ReportedBy: hfink at toolsonair.com
         QAContact: gstreamer-bugs at lists.freedesktop.org
     GNOME version: ---


Created an attachment (id=295004)
 View: https://bugzilla.gnome.org/attachment.cgi?id=295004
 Review: https://bugzilla.gnome.org/review?bug=743248&attachment=295004

patch

GstAppControlBinding is a new GstControlBinding subclass that emits a signal
('update-value') in order to update the value of its bound property. This
allows applications to connect to this signal and to set controllable
properties along the time-line, without being tied to GstControlSource /
GstDirectControlBinding. A patch of a first implementation, including basic
unit tests and documentation is provided.

Example use case:

A live mixing application uses compositor/glvideomixer to animate multiple
video layers along an application-defined time-line. This is done by setting
the properties xpos/ypos/width/height of compositor's sink pads accordingly.
Using GstControlSource / GstDirectControlBinding doesn't work here because: 

-) 0..1 sources would be mapped to the full property range. That doesn't really
make sense for properties using pixel coordinates (e.g.
xpos/ypos/width/height).

-) Since this a *live* mixing application, changes of a video layer's
properties (i.e. sink pad's xpos/ypos/width/height properties) need to have
immediate effect (e.g. animating a transition based on real-time user input).
One could try to just set xpos/ypos/width/height using a single g_object_set
call, but since this is not guaranteed to be atomic, this could result in one
frame to already have updated xpos/ypos values, but not yet updated
width/height values, resulting in undesired animation artefacts. Using
GstAppControlBinding, the application simply binds to xpos/ypos/width/height,
which results in the signal being emitted before each rendered frame for all
bound properties. The application could then make sure that the right
xpos/ypos/width/height tuple is used for the requested render time.

Further remarks:

(1) Since GstAppControlBinding does have to map GstControlSource's 0..1 value
to something useful (like GstDirectControlBinding does), we could support
further PARAM_SPEC types (like strings, chars, whatever). However, until a good
use case for this comes up, GstAppControlBinding only supports the same types
as GstDirectControlBinding.

(2) The performance of the installed signal ('update-value') might be improved
by passing more advanced flags to g_signal_new (accumulators, etc). Out of my
lacking experience with this, I have only installed a basic signal for now.

(3) If GstAppControlBinding is asked for an array of values, it is currently
emitting multiple signals, for each time-sample of the requested array, in
order to fill the array. This could be optimized by using another signal that
returns an array of GValues instead of just a single GValue.

(4) The GValue instance returned by the 'update-value' signal is currently
expected to hold the same type as the bound property. Alternatively, type
conversion could be attempted, if supported.

(5) Similar to GstDirectControlBinding, GstAppControlBinding does not set the
target property, if the value hasn't changed.

(6) I tried to extend the documentation to document GstAppControlBinding, I
hope I haven't missed anything :)

(7) Credits: I have added Stefan Sauer (ensonic) to the license headers,
because most of the code was based on his GstDirectControlBinding
implementation, and I have also added Sebastian Dröge (slomo), because
GstAppControlBinding was his idea.

I am looking forward to your feedback!

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