nleobject: stop using media-duration-factor

The property had been deprecated and is unused.

This property is not needed. Any internal time effect that an nleoperation
wraps is itself responsible for converting seek/segment timestamps.
Previously, the ghostpads were performing a rate conversion after the
rate element had already done so, essentially doubling their effect on
seeks and segment times. This was always unnecessary, but went unnoticed
by the tempochange test because it was using an identity element rather
than an actual rate-changing element.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-editing-services/-/merge_requests/160>

1 files changed, 40 insertions, 0 deletions

diff --git a/plugins/nle/nleoperation.c b/plugins/nle/nleoperation.c
index 6f37eaa4..62e27a77 100644
--- a/plugins/nle/nleoperation.c
+++ b/plugins/nle/nleoperation.c
@@ -31,6 +31,46 @@
  * A NleOperation performs a transformation or mixing operation on the
  * data from one or more #NleSources, which is used to implement filters or
  * effects.
+ *
+ * ## Time Effects
+ *
+ * An #nleoperation that wraps a #GstElement that transforms seek and
+ * segment times is considered a time effect. Nle only tries to support
+ * time effect's whose overall seek transformation:
+ *
+ * + Maps the time `0` to `0`. So initial time-shifting effects are
+ *   excluded (the #nlesource:inpoint can sometimes be used instead).
+ * + Is monotonically increasing. So reversing effects, and effects that
+ *   jump backwards in the stream are excluded.
+ * + Can handle a reasonable #GstClockTime, relative to the project. So
+ *   this would exclude a time effect with an extremely large speed-up
+ *   that would cause the converted #GstClockTime seeks to overflow.
+ * + Is 'continuously reversible'. This essentially means that for every
+ *   seek position found on the sink pad of the element, we can, to 'good
+ *   enough' accuracy, calculate the corresponding seek position that was
+ *   received on the source pad. Moreover, this calculation should
+ *   correspond to how the element transforms its #GstSegment
+ *   @time field. This is needed so that a seek can result in an accurate
+ *   segment.
+ *
+ * Note that a constant-rate-change effect that is not extremely fast or
+ * slow would satisfy these conditions.
+ *
+ * For such a time effect, they should be configured in nle such that:
+ *
+ * + Their #nleoperation:inpoint is `0`. Otherwise this will introduce
+ *   seeking problems in its #nlecomposition.
+ * + They must share the same #nleoperation:start and
+ *   #nleoperation:duration as all nleobjects of lower priority in its
+ *   #nlecomposition. Otherwise this will introduce jumps in playback.
+ *
+ * Note that, at the moment, nle only converts the #GstSegment
+ * @time field which means the other fields, such as @start and @stop, can
+ * end up with non-meaningful values when time effects are involved.
+ * Moreover, it does not convert #GstBuffer times either, which can result
+ * in them having non-meaningful values. In particular, this means that
+ * #GstControlBinding-s will not work well with #nlecomposition-s when
+ * they include time effects.
  */
 
 static GstStaticPadTemplate nle_operation_src_template =