transcriberbin: Add caption-source property

By using this new property, application can select exclusive caption
source. There are three source types
- Both: Inband and transcription captions are combined if exist.
  This is default behavior.
- Inband: Transcription buffers will be dropped
- Transcription: Caption meta of each video buffer will be dropped

In this version, transcriberbin doesn't provide any hint
for application to help caption source decision. That can be done
by application's strategy, passthrough status or probing inband
caption meta for example.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/684>

video/closedcaption/src/transcriberbin/imp.rs

@@ -15,6 +15,8 @@ use std::sync::Mutex;
 
 use once_cell::sync::Lazy;
 
+use super::CaptionSource;
+
 static CAT: Lazy<gst::DebugCategory> = Lazy::new(|| {
     gst::DebugCategory::new(
         "transcriberbin",
@@ -27,6 +29,7 @@ const DEFAULT_PASSTHROUGH: bool = false;
 const DEFAULT_LATENCY: gst::ClockTime = gst::ClockTime::from_seconds(4);
 const DEFAULT_ACCUMULATE: gst::ClockTime = gst::ClockTime::ZERO;
 const DEFAULT_MODE: Cea608Mode = Cea608Mode::RollUp2;
+const DEFAULT_CAPTION_SOURCE: CaptionSource = CaptionSource::Both;
 
 struct State {
     framerate: Option<gst::Fraction>,
@@ -43,6 +46,7 @@ struct State {
     textwrap: gst::Element,
     tttocea608: gst::Element,
     cccapsfilter: gst::Element,
+    transcription_valve: gst::Element,
 }
 
 struct Settings {
@@ -51,6 +55,7 @@ struct Settings {
     passthrough: bool,
     accumulate_time: gst::ClockTime,
     mode: Cea608Mode,
+    caption_source: CaptionSource,
 }
 
 impl Default for Settings {
@@ -63,6 +68,7 @@ impl Default for Settings {
             latency: DEFAULT_LATENCY,
             accumulate_time: DEFAULT_ACCUMULATE,
             mode: DEFAULT_MODE,
+            caption_source: DEFAULT_CAPTION_SOURCE,
         }
     }
 }
@@ -98,6 +104,7 @@ impl TranscriberBin {
             &state.tttocea608,
             &ccconverter,
             &state.cccapsfilter,
+            &state.transcription_valve,
         ])?;
 
         gst::Element::link_many(&[
@@ -109,6 +116,7 @@ impl TranscriberBin {
             &state.tttocea608,
             &ccconverter,
             &state.cccapsfilter,
+            &state.transcription_valve,
         ])?;
 
         let transcription_audio_sinkpad = gst::GhostPad::with_target(
@@ -117,7 +125,7 @@ impl TranscriberBin {
         )?;
         let transcription_audio_srcpad = gst::GhostPad::with_target(
             Some("src"),
-            &state.cccapsfilter.static_pad("src").unwrap(),
+            &state.transcription_valve.static_pad("src").unwrap(),
         )?;
 
         state
@@ -190,6 +198,31 @@ impl TranscriberBin {
         state.internal_bin.add_pad(&internal_video_sinkpad)?;
         state.internal_bin.add_pad(&internal_video_srcpad)?;
 
+        let element_weak = element.downgrade();
+        let comp_sinkpad = &state.cccombiner.static_pad("sink").unwrap();
+        // Drop caption meta from video buffer if user preference is transcription
+        comp_sinkpad.add_probe(gst::PadProbeType::BUFFER, move |_, probe_info| {
+            let element = match element_weak.upgrade() {
+                None => return gst::PadProbeReturn::Remove,
+                Some(element) => element,
+            };
+
+            let trans = TranscriberBin::from_instance(&element);
+            let settings = trans.settings.lock().unwrap();
+            if settings.caption_source != CaptionSource::Transcription {
+                return gst::PadProbeReturn::Pass;
+            }
+
+            if let Some(gst::PadProbeData::Buffer(buffer)) = &mut probe_info.data {
+                let buffer = buffer.make_mut();
+                while let Some(meta) = buffer.meta_mut::<gst_video::VideoCaptionMeta>() {
+                    meta.remove().unwrap();
+                }
+            }
+
+            gst::PadProbeReturn::Ok
+        });
+
         element.add(&state.internal_bin)?;
 
         state
@@ -434,6 +467,9 @@ impl TranscriberBin {
         let audio_queue_passthrough = gst::ElementFactory::make("queue", None)?;
         let video_queue = gst::ElementFactory::make("queue", None)?;
         let cccapsfilter = gst::ElementFactory::make("capsfilter", None)?;
+        let transcription_valve = gst::ElementFactory::make("valve", None)?;
+
+        transcription_valve.set_property_from_str("drop-mode", "transform-to-gap");
 
         Ok(State {
             framerate: None,
@@ -449,6 +485,7 @@ impl TranscriberBin {
             textwrap,
             tttocea608,
             cccapsfilter,
+            transcription_valve,
             tearing_down: false,
         })
     }
@@ -602,6 +639,16 @@ impl ObjectImpl for TranscriberBin {
                     gst::Element::static_type(),
                     glib::ParamFlags::READWRITE | gst::PARAM_FLAG_MUTABLE_READY,
                 ),
+                glib::ParamSpecEnum::new(
+                    "caption-source",
+                    "Caption source",
+                    "Caption source to use. \
+                    If \"Transcription\" or \"Inband\" is selected, the caption meta \
+                    of the other source will be dropped by transcriberbin",
+                    CaptionSource::static_type(),
+                    DEFAULT_CAPTION_SOURCE as i32,
+                    glib::ParamFlags::READWRITE | gst::PARAM_FLAG_MUTABLE_PLAYING,
+                ),
             ]
         });
 
@@ -673,6 +720,21 @@ impl ObjectImpl for TranscriberBin {
                     }
                 }
             }
+            "caption-source" => {
+                let mut settings = self.settings.lock().unwrap();
+                settings.caption_source = value.get().expect("type checked upstream");
+
+                let s = self.state.lock().unwrap();
+                if let Some(state) = s.as_ref() {
+                    if settings.caption_source == CaptionSource::Inband {
+                        gst::debug!(CAT, obj: obj, "Use inband caption, dropping transcription");
+                        state.transcription_valve.set_property("drop", true);
+                    } else {
+                        gst::debug!(CAT, obj: obj, "Stop dropping transcription");
+                        state.transcription_valve.set_property("drop", false);
+                    }
+                }
+            }
             _ => unimplemented!(),
         }
     }
@@ -708,6 +770,10 @@ impl ObjectImpl for TranscriberBin {
                     ret.to_value()
                 }
             }
+            "caption-source" => {
+                let settings = self.settings.lock().unwrap();
+                settings.caption_source.to_value()
+            }
             _ => unimplemented!(),
         }
     }

video/closedcaption/src/transcriberbin/mod.rs

@@ -11,6 +11,15 @@ use gst::prelude::*;
 
 mod imp;
 
+#[derive(Debug, Eq, PartialEq, Ord, PartialOrd, Hash, Clone, Copy, glib::Enum)]
+#[repr(u32)]
+#[enum_type(name = "GstTranscriberBinCaptionSource")]
+pub enum CaptionSource {
+    Both,
+    Transcription,
+    Inband,
+}
+
 glib::wrapper! {
     pub struct TranscriberBin(ObjectSubclass<imp::TranscriberBin>) @extends gst::Bin, gst::Element, gst::Object;
 }