diff --git a/blocky/CMakeLists.txt b/blocky/CMakeLists.txt
index fc24274..e73b709 100644
--- a/blocky/CMakeLists.txt
+++ b/blocky/CMakeLists.txt
@@ -7,9 +7,13 @@ target_sources(blocky
 
     bytes.hh
     common.hh
+    video_encoder.hh
+    video_decoder.hh
 )
 target_link_libraries(blocky
   PUBLIC
     args
     liblocky
+    minimp4
+    openh264
 )
diff --git a/blocky/main.cc b/blocky/main.cc
index 0bce957..56ed901 100644
--- a/blocky/main.cc
+++ b/blocky/main.cc
@@ -2,6 +2,7 @@
 #include <exception>
 #include <iomanip>
 #include <iostream>
+#include <optional>
 #include <tuple>
 
 #include "common.hh"
@@ -10,267 +11,278 @@
 
 #include <args.hxx>
 
+#include "video_decoder.hh"
+#include "video_encoder.hh"
+
 
 using namespace blky;
 
-int main(int argc, char** argv) {
-  args::ArgumentParser parser(
-      "liblocky command line tool",
-      "liblocky allow you to embed bits into video data secretly");
 
-  args::MapFlag<std::string, DataFlow> from {
-      parser, kDataFlowList, "input layer specifier", {"from"}, kDataFlowMap, args::Options::Required};
+args::ArgumentParser parser(
+    "liblocky command line tool",
+    "liblocky allow you to embed a bit array into video data secretly");
 
-  args::MapFlag<std::string, DataFlow> to {
-      parser, kDataFlowList, "output layer specifier", {"to"}, kDataFlowMap, args::Options::Required};
+args::MapFlag<std::string, DataFlow> from {
+    parser, kDataFlowList, "input layer specifier", {"from"}, kDataFlowMap, args::Options::Required};
 
-  args::Group src_group {
-    parser, "source specifier", args::Group::Validators::Xor, args::Options::Required};
-  args::Flag src_stdin {src_group, "src-stdin", "read from stdin", {"src-stdin", "stdin"}};
-  args::Flag src_stdin_hex {src_group, "src-stdin-hex", "read hex text from stdin", {"src-stdin-hex", "stdin-hex"}};
+args::MapFlag<std::string, DataFlow> to {
+    parser, kDataFlowList, "output layer specifier", {"to"}, kDataFlowMap, args::Options::Required};
 
-  args::Group dst_group  {
-    parser, "destination specifier", args::Group::Validators::Xor, args::Options::Required};
-  args::Flag dst_stdout {dst_group, "dst-stdout", "write to stdout", {"dst-stdout", "stdout"}};
-  args::Flag dst_stdout_hex {dst_group, "dst-stdout-hex", "write to stdout as hex text", {"dst-stdout-hex", "stdout-hex"}};
+args::Group src_group {
+  parser, "source specifier", args::Group::Validators::Xor, args::Options::Required};
+args::Flag src_stdin {src_group, "src-stdin", "read from stdin", {"src-stdin", "stdin"}};
+args::Flag src_stdin_hex {src_group, "src-stdin-hex", "read hex text from stdin", {"src-stdin-hex", "stdin-hex"}};
+args::ValueFlag<std::string> src_video {src_group, "path", "video input", {"src-video"}};
 
-  args::Group param_group {
-    parser, "general parameters", args::Group::Validators::DontCare
-  };
-  args::ValueFlag<std::tuple<uint32_t, uint32_t>> param_block_num {
-    param_group,
-    "int>0,int>0",
-    "number of features",
-    {"feature-num"},
-    {16, 16}
-  };
-  args::ValueFlag<uint32_t> param_block_first {
-    param_group,
-    "int>=0",
-    "an index of first block where feature will be embedded. used when encoding",
-    {"feature-first-index"},
-    0
-  };
-  args::ValueFlag<uint8_t> param_feat_bits {
-    param_group,
-    "int>0",
-    "number of bits that can be represented by a single feature",
-    {"feature-bits"},
-    1
-  };
-  args::ValueFlag<uint8_t> param_seed {
-    param_group,
-    "int>0",
-    "seed number for hopping randomization",
-    {"seed"},
-    123
-  };
+args::Group dst_group  {
+  parser, "destination specifier", args::Group::Validators::Xor, args::Options::Required};
+args::Flag dst_stdout {dst_group, "dst-stdout", "write to stdout", {"dst-stdout", "stdout"}};
+args::Flag dst_stdout_hex {dst_group, "dst-stdout-hex", "write to stdout as hex text", {"dst-stdout-hex", "stdout-hex"}};
+args::ValueFlag<std::string> dst_video {dst_group, "path", "video output", {"dst-video"}};
 
-  args::Group probgen_group {
-    parser, "params for feature probability generator", args::Group::Validators::DontCare
-  };
-  args::ValueFlag<double> probgen_false_positive {
-    probgen_group,
-    "0<=double<=1",
-    "false positive ratio in feature probability generation",
-    {"probgen-false-positive"},
-    0
-  };
-  args::ValueFlag<double> probgen_false_negative {
-    probgen_group,
-    "0<=double<=1",
-    "false negative ratio in feature probability generation",
-    {"probgen-false-negative"},
-    0
-  };
-  args::ValueFlag<uint64_t> probgen_seed {
-    probgen_group,
-    "int>0",
-    "random seed",
-    {"probgen-seed"},
-    1
-  };
-  args::Flag probgen_normalize {
-    probgen_group,
-    "probgen-normalize",
-    "normalize probabilities",
-    {"probgen-normalize"},
-  };
+args::Group param_group {
+  parser, "general parameters", args::Group::Validators::DontCare
+};
+args::ValueFlag<std::tuple<uint32_t, uint32_t>> param_block_num {
+  param_group,
+  "int>0,int>0",
+  "number of features",
+  {"feature-num"},
+  {16, 16}
+};
+args::ValueFlag<uint32_t> param_block_first {
+  param_group,
+  "int>=0",
+  "an index of first block where feature will be embedded. used when encoding",
+  {"feature-first-index"},
+  0
+};
+args::ValueFlag<uint8_t> param_feat_bits {
+  param_group,
+  "int>0",
+  "number of bits that can be represented by a single feature",
+  {"feature-bits"},
+  1
+};
+args::ValueFlag<uint8_t> param_seed {
+  param_group,
+  "int>0",
+  "seed number for hopping randomization",
+  {"seed"},
+  123
+};
 
-  try {
-    parser.ParseCLI(argc, argv);
+args::Group probgen_group {
+  parser, "params for feature probability generator", args::Group::Validators::DontCare
+};
+args::ValueFlag<double> probgen_false_positive {
+  probgen_group,
+  "0<=double<=1",
+  "false positive ratio in feature probability generation",
+  {"probgen-false-positive"},
+  0
+};
+args::ValueFlag<double> probgen_false_negative {
+  probgen_group,
+  "0<=double<=1",
+  "false negative ratio in feature probability generation",
+  {"probgen-false-negative"},
+  0
+};
+args::ValueFlag<uint64_t> probgen_seed {
+  probgen_group,
+  "int>0",
+  "random seed",
+  {"probgen-seed"},
+  1
+};
+args::Flag probgen_normalize {
+  probgen_group,
+  "probgen-normalize",
+  "normalize probabilities",
+  {"probgen-normalize"},
+};
 
-    std::vector<uint8_t>  bytes;
-    std::vector<uint32_t> features;
-    std::vector<double>   feature_probs;
 
-    // read input
+std::vector<uint8_t>        bytes;
+std::vector<uint32_t>       features;
+std::vector<double>         feature_probs;
+std::optional<VideoDecoder> decoder;
+
+int main(int argc, char** argv)
+try {
+  parser.ParseCLI(argc, argv);
+
+  // read input
+  switch (args::get(from)) {
+  case kBytes:
+    if (src_stdin) {
+      std::string temp;
+      std::cin >> temp;
+      bytes = {temp.begin(), temp.end()};
+    } else if (src_stdin_hex) {
+      for (;;) {
+        char buf[2];
+        std::cin >> buf[0] >> buf[1];
+        if (std::cin.eof()) break;
+        bytes.push_back(ToHex(buf[0]) << 4 | ToHex(buf[1]));
+      }
+    } else {
+      throw std::runtime_error {"invalid source format for bytes"};
+    }
+    break;
+
+  case kFeatures:
+    if (src_stdin) {
+      features = ReadAll<uint32_t>(std::cin);
+    } else {
+      throw std::runtime_error {"invalid source format for features"};
+    }
+    break;
+
+  case kFeatureProbs:
+    if (src_stdin) {
+      feature_probs = ReadAll<double>(std::cin);
+    } else {
+      throw std::runtime_error {"invalid source format for feature probs"};
+    }
+    break;
+
+  case kVideo:
+    if (src_video) {
+      decoder.emplace(args::get(src_video));
+    } else {
+      throw std::runtime_error {"invalid source format for video"};
+    }
+  }
+
+  if (args::get(from) < args::get(to)) {
+    // execute encoding
     switch (args::get(from)) {
     case kBytes:
-      if (src_stdin) {
-        std::string temp;
-        std::cin >> temp;
-        bytes = {temp.begin(), temp.end()};
-      } else if (src_stdin_hex) {
-        for (;;) {
-          char buf[2];
-          std::cin >> buf[0] >> buf[1];
-          if (std::cin.eof()) break;
-          bytes.push_back(ToHex(buf[0]) << 4 | ToHex(buf[1]));
-        }
-      } else {
-        throw std::runtime_error {"invalid source format for bytes"};
-      }
-      break;
+      if (args::get(to) == kBytes) break;
+      features = BytesEncoder(
+          bytes,
+          args::get(param_block_num),
+          args::get(param_feat_bits),
+          args::get(param_block_first),
+          args::get(param_seed));
+      /* fallthrough */
 
     case kFeatures:
-      if (src_stdin) {
-        features = ReadAll<uint32_t>(std::cin);
-      } else {
-        throw std::runtime_error {"invalid source format for features"};
-      }
-      break;
-
-    case kFeatureProbs:
-      if (src_stdin) {
-        feature_probs = ReadAll<double>(std::cin);
-      } else {
-        throw std::runtime_error {"invalid source format for feature probs"};
-      }
-      break;
-
-    case kVideo:
-      assert(false);
-    }
-
-    if (args::get(from) < args::get(to)) {
-      // execute encoding
-      switch (args::get(from)) {
-      case kBytes:
-        if (args::get(to) == kBytes) break;
-        features = BytesEncoder(
-            bytes,
-            args::get(param_block_num),
-            args::get(param_feat_bits),
-            args::get(param_block_first),
-            args::get(param_seed));
-        /* fallthrough */
-
-      case kFeatures:
-        if (args::get(to) == kFeatures) break;
-        if (args::get(to) == kFeatureProbs) {
-          feature_probs = GenerateFeatureProbs(
-              features,
-              args::get(param_block_num),
-              args::get(probgen_false_positive),
-              args::get(probgen_false_negative),
-              args::get(probgen_seed),
-              probgen_normalize);
-          break;
-        }
-        // TODO embed into video
-        assert(false);
-        /* fallthrough */
-
-      case kVideo:
-        if (args::get(to) == kVideo) break;
-        assert(false);
-
-      case kFeatureProbs:
-        throw std::runtime_error("couldn't start flow from the data");
-      }
-
-    } else if (args::get(from) > args::get(to)) {
-      // execute decoding
-      switch (args::get(from)) {
-      case kVideo:
-        if (args::get(to) == kVideo) break;
-        // TODO extract feature probs // features = XX
-        assert(false);
-
-      case kFeatureProbs:
-        if (args::get(to) == kFeatureProbs) break;
-        features = PathfindFeatures(
-            feature_probs,
-            args::get(param_block_num),
-            args::get(param_feat_bits),
-            args::get(param_seed));
-        /* fallthrough */
-
-      case kFeatures:
-        if (args::get(to) == kFeatures) break;
-        bytes = BytesDecoder(
+      if (args::get(to) == kFeatures) break;
+      if (args::get(to) == kFeatureProbs) {
+        feature_probs = GenerateFeatureProbs(
             features,
             args::get(param_block_num),
-            args::get(param_feat_bits),
-            args::get(param_seed));
-        /* fallthrough */
-
-      case kBytes:
-        if (args::get(to) == kBytes) break;
-        assert(false);
+            args::get(probgen_false_positive),
+            args::get(probgen_false_negative),
+            args::get(probgen_seed),
+            probgen_normalize);
+        break;
       }
-    }
-
-    // output
-    switch (args::get(to)) {
-    case kBytes:
-      if (dst_stdout) {
-        std::cout << std::string {bytes.begin(), bytes.end()} << std::endl;
-      } else if (dst_stdout_hex) {
-        for (auto c : bytes) {
-          std::cout << std::hex << (int) c;
-        }
-        std::cout << std::endl;
-      } else {
-        throw std::runtime_error {"invalid destination format for bytes"};
-      }
-      break;
-
-    case kFeatures:
-      if (dst_stdout) {
-        for (auto& f : features) std::cout << f << "\n";
-      } else {
-        throw std::runtime_error {"invalid destination format for features"};
-      }
-      break;
-
-    case kFeatureProbs:
-      if (dst_stdout) {
-        const auto size = args::get(param_block_num);
-        const auto cols = std::get<0>(size) * std::get<1>(size);
-        for (size_t i = 0; i < feature_probs.size();) {
-          for (size_t j = 0; i < feature_probs.size() && j < cols; ++i, ++j) {
-            std::cout << feature_probs[i] << " ";
-          }
-          std::cout << "\n";
-        }
-      } else {
-        throw std::runtime_error {"invalid destination format for feature probs"};
-      }
-      break;
+      // TODO embed into video
+      assert(false);
+      /* fallthrough */
 
     case kVideo:
-      break;
+      if (args::get(to) == kVideo) break;
+      assert(false);
+
+    case kFeatureProbs:
+      throw std::runtime_error("couldn't start flow from the data");
     }
 
-  } catch (const args::Help&) {
-    std::cout << parser << std::endl;
-    return 0;
+  } else if (args::get(from) > args::get(to)) {
+    // execute decoding
+    switch (args::get(from)) {
+    case kVideo:
+      if (args::get(to) == kVideo) break;
+      // TODO extract feature probs // features = XX
+      assert(false);
 
-  } catch (const args::ParseError& e) {
-    std::cerr << e.what() << std::endl;
-    std::cerr << parser   << std::endl;
-    return 1;
+    case kFeatureProbs:
+      if (args::get(to) == kFeatureProbs) break;
+      features = PathfindFeatures(
+          feature_probs,
+          args::get(param_block_num),
+          args::get(param_feat_bits),
+          args::get(param_seed));
+      /* fallthrough */
 
-  } catch (const args::ValidationError& e) {
-    std::cerr << e.what() << std::endl;
-    std::cerr << parser   << std::endl;
-    return 1;
+    case kFeatures:
+      if (args::get(to) == kFeatures) break;
+      bytes = BytesDecoder(
+          features,
+          args::get(param_block_num),
+          args::get(param_feat_bits),
+          args::get(param_seed));
+      /* fallthrough */
 
-  } catch (const std::runtime_error& e) {
-    std::cerr << "runtime error: " << e.what() << std::endl;
-    return 1;
+    case kBytes:
+      if (args::get(to) == kBytes) break;
+      assert(false);
+    }
+  }
+
+  // output
+  switch (args::get(to)) {
+  case kBytes:
+    if (dst_stdout) {
+      std::cout << std::string {bytes.begin(), bytes.end()} << std::endl;
+    } else if (dst_stdout_hex) {
+      for (auto c : bytes) {
+        std::cout << std::hex << (int) c;
+      }
+      std::cout << std::endl;
+    } else {
+      throw std::runtime_error {"invalid destination format for bytes"};
+    }
+    break;
+
+  case kFeatures:
+    if (dst_stdout) {
+      for (auto& f : features) std::cout << f << "\n";
+    } else {
+      throw std::runtime_error {"invalid destination format for features"};
+    }
+    break;
+
+  case kFeatureProbs:
+    if (dst_stdout) {
+      const auto size = args::get(param_block_num);
+      const auto cols = std::get<0>(size) * std::get<1>(size);
+      for (size_t i = 0; i < feature_probs.size();) {
+        for (size_t j = 0; i < feature_probs.size() && j < cols; ++i, ++j) {
+          std::cout << feature_probs[i] << " ";
+        }
+        std::cout << "\n";
+      }
+    } else {
+      throw std::runtime_error {"invalid destination format for feature probs"};
+    }
+    break;
+
+  case kVideo:
+    break;
   }
   return 0;
+
+} catch (const args::Help&) {
+  std::cout << parser << std::endl;
+  return 0;
+
+} catch (const args::ParseError& e) {
+  std::cerr << e.what() << std::endl;
+  std::cerr << parser   << std::endl;
+  return 1;
+
+} catch (const args::ValidationError& e) {
+  std::cerr << e.what() << std::endl;
+  std::cerr << parser   << std::endl;
+  return 1;
+
+} catch (const std::runtime_error& e) {
+  std::cerr << "runtime error: " << e.what() << std::endl;
+  return 1;
 }
diff --git a/blocky/video_decoder.hh b/blocky/video_decoder.hh
new file mode 100644
index 0000000..762dc5f
--- /dev/null
+++ b/blocky/video_decoder.hh
@@ -0,0 +1,169 @@
+#pragma once
+
+#include <cassert>
+#include <cstdint>
+#include <cstring>
+#include <exception>
+#include <fstream>
+#include <string_view>
+#include <vector>
+
+#include <minimp4.h>
+
+#include <wels/codec_api.h>
+
+
+class VideoDecoder final {
+ public:
+  VideoDecoder() = delete;
+  VideoDecoder(const std::string& path) :
+      file_(path, std::ifstream::binary | std::ifstream::ate),
+      size_(static_cast<size_t>(file_.tellg())) {
+    // init objects
+    SDecodingParam dparam = {};
+    dparam.sVideoProperty.eVideoBsType = VIDEO_BITSTREAM_DEFAULT;
+    dparam.eEcActiveIdc = ERROR_CON_SLICE_COPY;
+
+    WelsCreateDecoder(&decoder_);
+    decoder_->Initialize(&dparam);
+
+    int lv = WELS_LOG_DEBUG;
+    decoder_->SetOption(DECODER_OPTION_TRACE_LEVEL, &lv);
+
+    demuxer_ = {};
+    MP4D_open(&demuxer_, ReadCallback, this, static_cast<int64_t>(size_));
+
+    // find video track
+    track_ = SIZE_MAX;
+    for (size_t i = 0; i < demuxer_.track_count; ++i) {
+      if (demuxer_.track[i].handler_type == MP4D_HANDLER_TYPE_VIDE) {
+        if (track_ != SIZE_MAX) {
+          throw std::runtime_error {"there are many video tracks"};
+        }
+        track_ = i;
+      }
+    }
+    if (track_ == SIZE_MAX) {
+      throw std::runtime_error {"there is no video track"};
+    }
+
+    // setup decoder
+    std::vector<uint8_t> temp_;
+
+    for (size_t i = 0;; ++i) {
+      int size;
+      auto sps = static_cast<const uint8_t*>(MP4D_read_sps(
+              &demuxer_,
+              static_cast<unsigned int>(track_),
+              static_cast<int>(i),
+              &size));
+      if (!sps) break;
+
+      temp_.resize(static_cast<size_t>(4+size));
+      temp_[0] = 0, temp_[1] = 0, temp_[2] = 0, temp_[3] = 1;
+      std::memcpy(&temp_[4], sps, static_cast<size_t>(size));
+
+      if (decoder_->DecodeFrameNoDelay(temp_.data(), static_cast<int>(temp_.size()), yuv_, &frame_)) {
+        throw std::runtime_error {"failed to decode SPS"};
+      }
+    }
+    for (size_t i = 0;; ++i) {
+      int size;
+      auto pps = static_cast<const uint8_t*>(MP4D_read_pps(
+              &demuxer_,
+              static_cast<unsigned int>(track_),
+              static_cast<int>(i),
+              &size));
+      if (!pps) break;
+
+      temp_.resize(static_cast<size_t>(4+size));
+      temp_[0] = 0, temp_[1] = 0, temp_[2] = 0, temp_[3] = 1;
+      std::memcpy(&temp_[4], pps, static_cast<size_t>(size));
+
+      if (decoder_->DecodeFrameNoDelay(temp_.data(), static_cast<int>(temp_.size()), yuv_, &frame_)) {
+        throw std::runtime_error {"failed to decode SPS"};
+      }
+    }
+    temp_.clear();
+  }
+  ~VideoDecoder() noexcept {
+    decoder_->Uninitialize();
+    WelsDestroyDecoder(decoder_);
+
+    MP4D_close(&demuxer_);
+  }
+  VideoDecoder(const VideoDecoder&) = delete;
+  VideoDecoder(VideoDecoder&&) = delete;
+  VideoDecoder& operator=(const VideoDecoder&) = delete;
+  VideoDecoder& operator=(VideoDecoder&&) = delete;
+
+  void Decode() {
+    if (temp_consumed_ >= temp_.size()) {
+      unsigned size, time, dur;
+      const auto off = MP4D_frame_offset(
+          &demuxer_,
+          static_cast<unsigned int>(track_),
+          static_cast<unsigned int>(count_),
+          &size, &time, &dur);
+
+      assert(size > 0);
+      temp_.resize(size);
+      temp_consumed_ = 0;
+
+      file_.seekg(static_cast<std::streamoff>(off));
+      assert(file_);
+      file_.read((char*) temp_.data(), size);
+      assert(file_);
+
+      Decode();
+
+    } else {
+      auto& i = temp_consumed_;
+
+      const uint32_t nal_size = 4 +
+          static_cast<uint32_t>((temp_[i+0] << 24) |
+                                (temp_[i+1] << 16) |
+                                (temp_[i+2] <<  8) |
+                                (temp_[i+3] <<  0));
+      temp_[i  ] = 0;
+      temp_[i+1] = 0;
+      temp_[i+2] = 0;
+      temp_[i+3] = 1;
+      if (decoder_->DecodeFrameNoDelay(temp_.data(), static_cast<int>(nal_size), yuv_, &frame_)) {
+        throw std::runtime_error {"failed to decode a frame"};
+      }
+      i += nal_size;
+    }
+  }
+
+ private:
+  std::ifstream file_;
+  size_t size_;
+
+  ISVCDecoder* decoder_;
+  MP4D_demux_t demuxer_;
+
+  size_t track_;
+
+  uint8_t*    yuv_[3] = {0};
+  SBufferInfo frame_  = {};
+  size_t      count_  = 0;
+
+  size_t temp_consumed_ = 0;
+  std::vector<uint8_t> temp_;
+
+
+  static int ReadCallback(int64_t off, void* buf, size_t size, void* ptr) noexcept {
+    auto self = (VideoDecoder*) ptr;
+
+    auto n = self->size_ - static_cast<size_t>(off) - size;
+    if (size < n) n = size;
+
+    self->file_.seekg(off);
+    assert(self->file_);
+
+    self->file_.read((char*) buf, static_cast<std::streamsize>(n));
+    assert(self->file_);
+    return 0;
+  }
+};
diff --git a/blocky/video_encoder.hh b/blocky/video_encoder.hh
new file mode 100644
index 0000000..f855738
--- /dev/null
+++ b/blocky/video_encoder.hh
@@ -0,0 +1,79 @@
+#pragma once
+
+#include <cassert>
+#include <cstdint>
+#include <cstring>
+#include <exception>
+#include <fstream>
+#include <string>
+
+#include <minimp4.h>
+
+#include <wels/codec_api.h>
+
+
+class VideoEncoder final {
+ public:
+  VideoEncoder() = delete;
+  VideoEncoder(const std::string& path, const SEncParamBase& p) :
+      file_(path, std::ofstream::binary), fps_(p.fMaxFrameRate) {
+    if (WelsCreateSVCEncoder(&encoder_)) {
+      throw std::runtime_error {"failed to init openh264 encoder"};
+    }
+
+    muxer_ = MP4E_open(false, false, this, WriteCallback);
+    if (MP4E_STATUS_OK != mp4_h26x_write_init(&writer_, muxer_, p.iPicWidth, p.iPicHeight, false)) {
+      throw std::runtime_error {"failed to init h26x writer"};
+    }
+
+    encoder_->Initialize(&p);
+
+    int lv = WELS_LOG_DEBUG;
+    encoder_->SetOption(ENCODER_OPTION_TRACE_LEVEL, &lv);
+    int fmt = videoFormatI420;
+    encoder_->SetOption(ENCODER_OPTION_DATAFORMAT, &fmt);
+  }
+  ~VideoEncoder() noexcept {
+    encoder_->Uninitialize();
+    WelsDestroySVCEncoder(encoder_);
+
+    MP4E_close(muxer_);
+    mp4_h26x_write_close(&writer_);
+  }
+
+  void Encode(const SSourcePicture& pic) {
+    SFrameBSInfo info;
+    if (cmResultSuccess != encoder_->EncodeFrame(&pic, &info)) {
+      throw std::runtime_error {"failed to encode a frame"};
+    }
+    if (info.eFrameType == videoFrameTypeSkip) return;
+
+    for (size_t i = 0; i < static_cast<size_t>(info.iLayerNum); ++i) {
+      const auto& lay = info.sLayerInfo[i];
+
+      uint8_t* buf = lay.pBsBuf;
+      for (size_t j = 0; j < static_cast<size_t>(lay.iNalCount); ++j) {
+        mp4_h26x_write_nal(&writer_, buf, lay.pNalLengthInByte[j], static_cast<unsigned int>(90000./fps_));
+        buf += lay.pNalLengthInByte[j];
+      }
+    }
+  }
+
+ private:
+  std::ofstream file_;
+
+  ISVCEncoder*      encoder_;
+  MP4E_mux_t*       muxer_;
+  mp4_h26x_writer_t writer_;
+
+  double fps_;
+
+
+  static int WriteCallback(int64_t off, const void* buf, size_t size, void* ptr) noexcept {
+    auto self = (VideoEncoder*) ptr;
+    self->file_.seekp(off);
+    assert(self->file_);
+    self->file_.write(static_cast<const char*>(buf), static_cast<std::streamsize>(size));
+    return !self->file_;
+  }
+};