From db53fb79527decd9d5c80a67efc60d9c12488e06 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jonas=20R=C3=B6ger?= <jonas.roeger@tu-dortmund.de>
Date: Thu, 19 Mar 2026 00:27:07 +0100
Subject: [PATCH] feat(avpipe): add SmearFrames

---
 app/mosh-pipe.cpp          |  2 +-
 include/mosh-me/avpipe.hpp | 23 +++++++++++++++
 src/avpipe.cpp             | 60 +++++++++++++++++++++++++++++++++++++-
 3 files changed, 83 insertions(+), 2 deletions(-)

diff --git a/app/mosh-pipe.cpp b/app/mosh-pipe.cpp
index 67f9d43..bba5766 100644
--- a/app/mosh-pipe.cpp
+++ b/app/mosh-pipe.cpp
@@ -15,7 +15,7 @@ int main(int argc, char *argv[]) {
 
   try {
     auto pipe = mosh_me::VideoMuxer::link(std::filesystem::path(argv[2]))
-                << mosh_me::DropIFrames::link()
+                << mosh_me::SmearFrames::link(3, 5)
                 << mosh_me::MoshableVideoEncoder::link()
                 << mosh_me::VideoDecoder::link()
                 << mosh_me::VideoPacketSource::link(argv[1]);
diff --git a/include/mosh-me/avpipe.hpp b/include/mosh-me/avpipe.hpp
index 3211159..944a810 100644
--- a/include/mosh-me/avpipe.hpp
+++ b/include/mosh-me/avpipe.hpp
@@ -60,7 +60,30 @@ class DropIFrames
           std::pair<std::reference_wrapper<av::VideoEncoderContext>, FrameMeta>,
           std::pair<std::reference_wrapper<av::VideoEncoderContext>,
                     FrameMeta>> {
+private:
+  uint32_t remaining_ = 0;
+
 public:
+  DropIFrames(uint32_t keep_n = 0);
+  mosh_me::PipeData<av::Packet> pull() noexcept override;
+  std::pair<std::reference_wrapper<av::VideoEncoderContext>, FrameMeta>
+  propagateMeta() noexcept override;
+};
+
+class SmearFrames
+    : public AsPipeLink<
+          SmearFrames, av::Packet, av::Packet,
+          std::pair<std::reference_wrapper<av::VideoEncoderContext>, FrameMeta>,
+          std::pair<std::reference_wrapper<av::VideoEncoderContext>,
+                    FrameMeta>> {
+private:
+  std::vector<av::Packet> back_buffer_;
+  std::vector<av::Packet>::iterator read_head_;
+  uint32_t n_repeat_ = 0;
+  uint32_t current_ix_ = 0;
+
+public:
+  SmearFrames(uint32_t n_frames = 1, uint32_t n_repeat = 2);
   mosh_me::PipeData<av::Packet> pull() noexcept override;
   std::pair<std::reference_wrapper<av::VideoEncoderContext>, FrameMeta>
   propagateMeta() noexcept override;
diff --git a/src/avpipe.cpp b/src/avpipe.cpp
index be5df93..9e1e79f 100644
--- a/src/avpipe.cpp
+++ b/src/avpipe.cpp
@@ -117,6 +117,8 @@ mosh_me::MoshableVideoEncoder::propagateMeta() noexcept {
   return {ctx_, frame_meta};
 }
 
+mosh_me::DropIFrames::DropIFrames(uint32_t keep_n) : remaining_(keep_n) {}
+
 mosh_me::PipeData<av::Packet> mosh_me::DropIFrames::pull() noexcept {
   for (;;) {
     auto input = fetchInput();
@@ -124,8 +126,10 @@ mosh_me::PipeData<av::Packet> mosh_me::DropIFrames::pull() noexcept {
       return std::unexpected(input.error());
     }
 
-    if (input->isKeyPacket()) {
+    if (input->isKeyPacket() && remaining_ == 0) {
       continue;
+    } else if (input->isKeyPacket()) {
+      remaining_--;
     }
 
     return input;
@@ -144,6 +148,60 @@ mosh_me::VideoMuxer::VideoMuxer(const std::filesystem::path &path) {
   ctx_.openOutput(path);
 }
 
+mosh_me::SmearFrames::SmearFrames(uint32_t n_frames, uint32_t n_repeat)
+    : n_repeat_(n_repeat) {
+  back_buffer_.reserve(n_frames);
+  read_head_ = back_buffer_.begin();
+}
+
+mosh_me::PipeData<av::Packet> mosh_me::SmearFrames::pull() noexcept {
+  for (;;) {
+    auto input = fetchInput();
+    if (!input) {
+      // TODO: dump back_buffer
+      return std::unexpected(input.error());
+    }
+
+    // Do not repeat iframes
+    if (input->isKeyPacket()) {
+      current_ix_ = 0;
+      back_buffer_.clear();
+      read_head_ = back_buffer_.begin();
+      return input;
+    }
+
+    if (back_buffer_.size() == back_buffer_.capacity()) {
+      // Full, dump the back buffer
+      if (read_head_ == back_buffer_.end() && current_ix_ == n_repeat_ - 1) {
+        current_ix_ = 0;
+        back_buffer_.clear();
+        read_head_ = back_buffer_.begin();
+      } else if (read_head_ == back_buffer_.end()) {
+        current_ix_++;
+        read_head_ = back_buffer_.begin();
+        auto frame = *read_head_++;
+        frame.setDts(input->dts());
+        frame.setPts(input->pts());
+        return frame;
+      } else {
+        auto frame = *read_head_++;
+        frame.setDts(input->dts());
+        frame.setPts(input->pts());
+        return frame;
+      }
+    } else {
+      // Fill back_buffer
+      back_buffer_.push_back(*input);
+    }
+  }
+}
+
+std::pair<std::reference_wrapper<av::VideoEncoderContext>, mosh_me::FrameMeta>
+
+mosh_me::SmearFrames::propagateMeta() noexcept {
+  return fetchMeta();
+}
+
 mosh_me::PipeData<void> mosh_me::VideoMuxer::pull() noexcept {
   std::error_code ec;