Fix video backward seeking / stepping back sometimes getting stuck (in the presence of b-frames) (#8053)

### What

* Fixes #7956

Our PTS -> sample search algorithm wasn't quite right. Fixed this here
and added a unit test.
Unfortunately, I had to introduce a small auxiliary data structure to
keep this snappy / not O(n) for n samples, but it's very simple and
rather contained.

Tested on native & web. Web still suffers from
* #7961

but on Chrome this works fine now!



https://github.com/user-attachments/assets/7a20467d-4d66-456f-be99-debba4ff7a31


### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)
* [x] I have tested the web demo (if applicable):
* Using examples from latest `main` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/8053?manifest_url=https://app.rerun.io/version/main/examples_manifest.json)
* Using full set of examples from `nightly` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/8053?manifest_url=https://app.rerun.io/version/nightly/examples_manifest.json)
* [x] The PR title and labels are set such as to maximize their
usefulness for the next release's CHANGELOG
* [x] If applicable, add a new check to the [release
checklist](https://github.com/rerun-io/rerun/blob/main/tests/python/release_checklist)!
* [x] If have noted any breaking changes to the log API in
`CHANGELOG.md` and the migration guide

- [PR Build Summary](https://build.rerun.io/pr/8053)
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Author: Wumpf

Cargo.lock

@@ -6477,6 +6477,7 @@ dependencies = [
 name = "re_video"
 version = "0.20.0-alpha.1+dev"
 dependencies = [
+ "bit-vec",
  "cfg_aliases 0.2.1",
  "criterion",
  "crossbeam",

crates/store/re_video/Cargo.toml

@@ -47,6 +47,7 @@ re_build_info.workspace = true
 re_log.workspace = true
 re_tracing.workspace = true
 
+bit-vec.workspace = true
 crossbeam.workspace = true
 econtext.workspace = true
 itertools.workspace = true

crates/store/re_video/src/decode/ffmpeg_h264/ffmpeg.rs

@@ -516,12 +516,6 @@ fn read_ffmpeg_output(
                 // How do we know how large this buffer needs to be?
                 // Whenever the highest known DTS is behind the PTS, we need to wait until the DTS catches up.
                 // Otherwise, we'd assign the wrong PTS to the frame that just came in.
-                //
-                // Example how presentation timestamps and decode timestamps
-                // can play out in the presence of B-frames to illustrate this:
-                //    PTS: 1 4 2 3
-                //    DTS: 1 2 3 4
-                // Stream: I P B B
                 let frame_info = loop {
                     let oldest_pts_in_buffer =
                         pending_frame_infos.first_key_value().map(|(pts, _)| *pts);

crates/store/re_video/src/demux/mod.rs

@@ -9,6 +9,7 @@ pub mod mp4;
 
 use std::{collections::BTreeMap, ops::Range};
 
+use bit_vec::BitVec;
 use itertools::Itertools as _;
 
 use super::{Time, Timescale};
@@ -89,6 +90,11 @@ pub struct SamplesStatistics {
     ///
     /// If true, the video typically has no B-frames as those require frame reordering.
     pub dts_always_equal_pts: bool,
+
+    /// If `dts_always_equal_pts` is false, then this gives for each sample whether its PTS is the highest seen so far.
+    /// If `dts_always_equal_pts` is true, then this is left as `None`.
+    /// This is used for optimizing PTS search.
+    pub has_sample_highest_pts_so_far: Option<BitVec>,
 }
 
 impl SamplesStatistics {
@@ -104,9 +110,25 @@ impl SamplesStatistics {
             .iter()
             .all(|s| s.decode_timestamp == s.presentation_timestamp);
 
+        let mut biggest_pts_so_far = Time::MIN;
+        let has_sample_highest_pts_so_far = (!dts_always_equal_pts).then(|| {
+            samples
+                .iter()
+                .map(move |sample| {
+                    if sample.presentation_timestamp > biggest_pts_so_far {
+                        biggest_pts_so_far = sample.presentation_timestamp;
+                        true
+                    } else {
+                        false
+                    }
+                })
+                .collect()
+        });
+
         Self {
             minimum_presentation_timestamp,
             dts_always_equal_pts,
+            has_sample_highest_pts_so_far,
         }
     }
 }
@@ -296,34 +318,83 @@ impl VideoData {
         })
     }
 
-    /// For a given decode (!) timestamp, returns the index of the latest sample whose
+    /// For a given decode (!) timestamp, returns the index of the first sample whose
     /// decode timestamp is lesser than or equal to the given timestamp.
-    pub fn latest_sample_index_at_decode_timestamp(&self, decode_time: Time) -> Option<usize> {
-        latest_at_idx(
-            &self.samples,
-            |sample| sample.decode_timestamp,
-            &decode_time,
-        )
+    fn latest_sample_index_at_decode_timestamp(
+        samples: &[Sample],
+        decode_time: Time,
+    ) -> Option<usize> {
+        latest_at_idx(samples, |sample| sample.decode_timestamp, &decode_time)
     }
 
-    /// For a given presentation timestamp, return the index of the latest sample
-    /// whose presentation timestamp is lesser than or equal to the given timestamp.
-    pub fn latest_sample_index_at_presentation_timestamp(
-        &self,
+    /// See [`Self::latest_sample_index_at_presentation_timestamp`], split out for testing purposes.
+    fn latest_sample_index_at_presentation_timestamp_internal(
+        samples: &[Sample],
+        sample_statistics: &SamplesStatistics,
         presentation_timestamp: Time,
     ) -> Option<usize> {
         // Find the latest sample where `decode_timestamp <= presentation_timestamp`.
         // Because `decode <= presentation`, we never have to look further backwards in the
         // video than this.
         let decode_sample_idx =
-            self.latest_sample_index_at_decode_timestamp(presentation_timestamp)?;
+            Self::latest_sample_index_at_decode_timestamp(samples, presentation_timestamp)?;
+
+        // It's very common that dts==pts in which case we're done!
+        let Some(has_sample_highest_pts_so_far) =
+            sample_statistics.has_sample_highest_pts_so_far.as_ref()
+        else {
+            debug_assert!(!sample_statistics.dts_always_equal_pts);
+            return Some(decode_sample_idx);
+        };
+        debug_assert!(has_sample_highest_pts_so_far.len() == samples.len());
 
         // Search backwards, starting at `decode_sample_idx`, looking for
         // the first sample where `sample.presentation_timestamp <= presentation_timestamp`.
-        // this is the sample which when decoded will be presented at the timestamp the user requested.
-        self.samples[..=decode_sample_idx]
-            .iter()
-            .rposition(|sample| sample.presentation_timestamp <= presentation_timestamp)
+        // I.e. the sample with the biggest PTS that is smaller or equal to the requested PTS.
+        //
+        // The tricky part is that we can't just take the first sample with a presentation timestamp that matches
+        // since smaller presentation timestamps may still show up further back!
+        let mut best_index = usize::MAX;
+        let mut best_pts = Time::MIN;
+        for sample_idx in (0..=decode_sample_idx).rev() {
+            let sample = &samples[sample_idx];
+
+            if sample.presentation_timestamp == presentation_timestamp {
+                // Clean hit. Take this one, no questions asked :)
+                // (assuming that each PTS is unique!)
+                return Some(sample_idx);
+            }
+
+            if sample.presentation_timestamp < presentation_timestamp
+                && sample.presentation_timestamp > best_pts
+            {
+                best_pts = sample.presentation_timestamp;
+                best_index = sample_idx;
+            }
+
+            if best_pts != Time::MIN && has_sample_highest_pts_so_far[sample_idx] {
+                // We won't see any bigger PTS values anymore, meaning we're as close as we can get to the requested PTS!
+                return Some(best_index);
+            }
+        }
+
+        None
+    }
+
+    /// For a given presentation timestamp, return the index of the first sample
+    /// whose presentation timestamp is lesser than or equal to the given timestamp.
+    ///
+    /// Remember that samples after (i.e. with higher index) may have a *lower* presentation time
+    /// if the stream has sample reordering!
+    pub fn latest_sample_index_at_presentation_timestamp(
+        &self,
+        presentation_timestamp: Time,
+    ) -> Option<usize> {
+        Self::latest_sample_index_at_presentation_timestamp_internal(
+            &self.samples,
+            &self.samples_statistics,
+            presentation_timestamp,
+        )
     }
 
     /// For a given decode (!) timestamp, return the index of the group of pictures (GOP) index containing the given timestamp.
@@ -553,4 +624,102 @@ mod tests {
         assert_eq!(latest_at_idx(&v, |v| *v, &11), Some(9));
         assert_eq!(latest_at_idx(&v, |v| *v, &1000), Some(9));
     }
+
+    #[test]
+    fn test_latest_sample_index_at_presentation_timestamp() {
+        // This is a snippet of real world data!
+        let pts = [
+            512, 1536, 1024, 768, 1280, 2560, 2048, 1792, 2304, 3584, 3072, 2816, 3328, 4608, 4096,
+            3840, 4352, 5376, 4864, 5120, 6400, 5888, 5632, 6144, 7424, 6912, 6656, 7168, 8448,
+            7936, 7680, 8192, 9472, 8960, 8704, 9216, 10496, 9984, 9728, 10240, 11520, 11008,
+            10752, 11264, 12544, 12032, 11776, 12288, 13568, 13056,
+        ];
+        let dts = [
+            0, 256, 512, 768, 1024, 1280, 1536, 1792, 2048, 2304, 2560, 2816, 3072, 3328, 3584,
+            3840, 4096, 4352, 4608, 4864, 5120, 5376, 5632, 5888, 6144, 6400, 6656, 6912, 7168,
+            7424, 7680, 7936, 8192, 8448, 8704, 8960, 9216, 9472, 9728, 9984, 10240, 10496, 10752,
+            11008, 11264, 11520, 11776, 12032, 12288, 12544,
+        ];
+
+        // Checking our basic assumptions about this data:
+        assert_eq!(pts.len(), dts.len());
+        assert!(pts.iter().zip(dts.iter()).all(|(pts, dts)| dts <= pts));
+
+        // Create fake samples from this.
+        let samples = pts
+            .into_iter()
+            .zip(dts)
+            .map(|(pts, dts)| Sample {
+                is_sync: false,
+                decode_timestamp: Time(dts),
+                presentation_timestamp: Time(pts),
+                duration: Time(1),
+                byte_offset: 0,
+                byte_length: 0,
+            })
+            .collect::<Vec<_>>();
+
+        let sample_statistics = SamplesStatistics::new(&samples);
+        assert_eq!(sample_statistics.minimum_presentation_timestamp, Time(512));
+        assert!(!sample_statistics.dts_always_equal_pts);
+
+        // Test queries on the samples.
+        let query_pts = |pts| {
+            VideoData::latest_sample_index_at_presentation_timestamp_internal(
+                &samples,
+                &sample_statistics,
+                pts,
+            )
+        };
+
+        // Check that query for all exact positions works as expected using brute force search as the reference.
+        for (idx, sample) in samples.iter().enumerate() {
+            assert_eq!(Some(idx), query_pts(sample.presentation_timestamp));
+        }
+
+        // Check that for slightly offsetted positions the query is still correct.
+        // This works because for this dataset we know the minimum presentation timesetampe distance is always 256.
+        for (idx, sample) in samples.iter().enumerate() {
+            assert_eq!(
+                Some(idx),
+                query_pts(sample.presentation_timestamp + Time(1))
+            );
+            assert_eq!(
+                Some(idx),
+                query_pts(sample.presentation_timestamp + Time(255))
+            );
+        }
+
+        // A few hardcoded cases - both for illustrative purposes and to make sure the generic tests above are correct.
+
+        // Querying before the first sample.
+        assert_eq!(None, query_pts(Time(0)));
+        assert_eq!(None, query_pts(Time(123)));
+
+        // Querying for the first sample
+        assert_eq!(Some(0), query_pts(Time(512)));
+        assert_eq!(Some(0), query_pts(Time(513)));
+        assert_eq!(Some(0), query_pts(Time(600)));
+        assert_eq!(Some(0), query_pts(Time(767)));
+
+        // The next sample is a jump in index!
+        assert_eq!(Some(3), query_pts(Time(768)));
+        assert_eq!(Some(3), query_pts(Time(769)));
+        assert_eq!(Some(3), query_pts(Time(800)));
+        assert_eq!(Some(3), query_pts(Time(1023)));
+
+        // And the one after that should jump back again.
+        assert_eq!(Some(2), query_pts(Time(1024)));
+        assert_eq!(Some(2), query_pts(Time(1025)));
+        assert_eq!(Some(2), query_pts(Time(1100)));
+        assert_eq!(Some(2), query_pts(Time(1279)));
+
+        // And another one!
+        assert_eq!(Some(4), query_pts(Time(1280)));
+        assert_eq!(Some(4), query_pts(Time(1281)));
+
+        // Test way outside of the range.
+        // (this is not the last element in the list since that one doesn't have the highest PTS)
+        assert_eq!(Some(48), query_pts(Time(123123123123123123)));
+    }
 }

crates/viewer/re_data_ui/src/video.rs

@@ -268,6 +268,24 @@ fn frame_info_ui(ui: &mut egui::Ui, frame_info: &FrameInfo, video_data: &re_vide
     .on_hover_text("Raw presentation timestamp prior to applying the timescale.\n\
                     This specifies the time at which the frame should be shown relative to the start of a video stream.");
 
+    // Judging the following to be a bit too obscure to be of relevance outside of debugging Rerun itself.
+    #[cfg(debug_assertions)]
+    {
+        if let Some(has_sample_highest_pts_so_far) = video_data
+            .samples_statistics
+            .has_sample_highest_pts_so_far
+            .as_ref()
+        {
+            if let Some(sample_idx) = video_data
+                .latest_sample_index_at_presentation_timestamp(frame_info.presentation_timestamp)
+            {
+                ui.list_item_flat_noninteractive(
+                    PropertyContent::new("Highest PTS so far").value_text(has_sample_highest_pts_so_far[sample_idx].to_string())
+                ).on_hover_text("Whether the presentation timestamp (PTS) at the this frame is the highest encountered so far. If false there are lower PTS values prior in the list.");
+            }
+        }
+    }
+
     // Information about the current group of pictures this frame is part of.
     // Lookup via decode timestamp is faster, but it may not always be available.
     if let Some(gop_index) =

crates/viewer/re_renderer/src/video/player.rs

@@ -258,7 +258,15 @@ impl VideoPlayer {
             // special case: handle seeking backwards within a single GOP
             // this is super inefficient, but it's the only way to handle it
             // while maintaining a buffer of only 2 GOPs
-            if requested_sample_idx < self.current_sample_idx {
+            //
+            // Note that due to sample reordering (in the presence of b-frames), if can happen
+            // that `self.current_sample_idx` is *behind* the `requested_sample_idx` even if we're
+            // seeking backwards!
+            // Therefore, it's important to compare presentation timestamps instead of sample indices.
+            // (comparing decode timestamps should be equivalent to comparing sample indices)
+            let current_pts = self.data.samples[self.current_sample_idx].presentation_timestamp;
+            let requested_pts = self.data.samples[requested_sample_idx].presentation_timestamp;
+            if requested_pts < current_pts {
                 self.reset()?;
                 self.enqueue_gop(requested_gop_idx, video_data)?;
                 self.enqueue_gop(requested_gop_idx + 1, video_data)?;