Add keyframe subsampler to FrameSubsampler (#322)

* Add keyframe subsampler to FrameSubsampler

- Fix encode_formats bug #311
- Fix bug `dictionary keys changed during iteration` in FrameSubsampler

* Format code in frame_subsampler

video2dataset/subsamplers/frame_subsampler.py

@@ -1,6 +1,7 @@
 """
 frame subsampler adjusts the fps of the videos to some constant value
 """
+
 import tempfile
 import os
 import copy
@@ -36,11 +37,10 @@ def __init__(self, frame_rate, downsample_method="fps", encode_format="mp4"):
         self.frame_rate = frame_rate
         self.downsample_method = downsample_method
         self.output_modality = "video" if downsample_method == "fps" else "jpg"
-        self.encode_format = encode_format
+        self.encode_formats = {"video": encode_format}
 
     def __call__(self, streams, metadata=None):
-        # TODO: you might not want to pop it (f.e. in case of other subsamplers)
-        video_bytes = streams.pop("video")
+        video_bytes = streams["video"]
         subsampled_bytes, subsampled_metas = [], []
         for i, vid_bytes in enumerate(video_bytes):
             with tempfile.TemporaryDirectory() as tmpdir:
@@ -52,6 +52,11 @@ def __call__(self, streams, metadata=None):
                         _ = ffmpeg.input(f"{tmpdir}/input.mp4")
                         _ = _.filter("fps", fps=self.frame_rate)
                         _ = _.output(f"{tmpdir}/output.mp4", reset_timestamps=1).run(capture_stdout=True, quiet=True)
+                    elif self.downsample_method == "keyframe":
+                        _ = ffmpeg.input(f"{tmpdir}/input.mp4", discard="nokey")
+                        # _ = _.filter("select", "eq(pict_type,I)")
+                        _ = _.output(f"{tmpdir}/output.mp4", **{"c:s": "copy", "c": "copy", "copyts": None})
+                        _ = _.run(capture_stdout=True, quiet=True)
                     elif "frame" in self.downsample_method:
                         _ = ffmpeg.input(f"{tmpdir}/input.mp4")
                         _ = _.filter("select", "eq(n,0)")