Fix two ffmpeg 7.1 regressions: lost blends and an overlay deadlock

backend/app/render/ffmpeg.py

@@ -1044,7 +1044,11 @@ def build_stitch_command(
         if hi is not None:
             bounds.append(f"end_frame={hi}")
             samples.append(f"end_sample={round(hi * 48000 / fps)}")
-        chains.append(f"[{i}:v]trim={':'.join(bounds)},setpts=PTS-STARTPTS,settb=AVTB[s{i}]")
+        # ffmpeg 7 leaves a trimmed stream with no frame rate (1/0), and `xfade`
+        # refuses one, so it is set back: the frames are already at it, so `fps`
+        # neither drops nor repeats any.
+        chains.append(f"[{i}:v]trim={':'.join(bounds)},setpts=PTS-STARTPTS,"
+                      f"fps={fps},settb=AVTB[s{i}]")
         chains.append(f"[{i}:a]atrim={':'.join(samples)},asetpts=PTS-STARTPTS[st{i}]")
         sounds.append(f"st{i}")
     video, audio = "s0", sounds[0]
@@ -1249,25 +1253,35 @@ def build_overlay_pass(
     So this is one re-encode of the whole picture after the stitch, done only when a
     video has timed overlays at all.
 
-    Each layer is a looped still that only exists for its window: `-itsoffset`
-    starts it at the layer's start and `-t` ends it there, so its fades are alpha
-    fades at absolute times on the video's own clock. Looping every layer for the
-    whole video instead decoded each one on every frame, which made this pass's
-    memory and time grow with the overlay count — thirty layers took 560 MB and
-    twenty times as long. `enable` still keeps it off outside its window, and
-    `eof_action=pass` lets the picture through once it has ended.
+    Each layer is its PNG read once, as a single frame, and repeated inside the
+    graph for its window only: `loop` repeats it, `trim` stops it after the
+    window's frames and `setpts` places them from the layer's start. Its fades are
+    then alpha fades at absolute times on the video's own clock, `enable` keeps it
+    off outside its window, and `eof_action=pass` lets the picture through after.
+
+    Two ways of doing this have failed. Looping each PNG as an input for the whole
+    video decoded it on every frame, which made this pass's memory and time grow
+    with the overlay count: thirty layers took 560 MB and twenty times as long. An
+    input per layer that starts late (`-itsoffset`) deadlocked Debian's ffmpeg
+    7.1.5 at the first layer's start: idle, never finishing, until the timeout. A
+    one-frame input is finished before the picture starts, and from then on the
+    overlay only pulls the layer's frames through the graph as it needs them, so
+    nothing is left waiting on ffmpeg's scheduling of inputs.
     """
-    fps = options.fps
+    fps = max(1, options.fps)
     total = max(0.1, duration)
     argv = ["ffmpeg", "-y", "-hide_banner", "-loglevel", "error", "-i", source]
     chains: List[str] = []
     stage = "0:v"
     for position, layer in enumerate(layers, start=1):
         start = max(0.0, min(layer.start, total))
-        length = max(1.0 / max(1, fps), min(layer.end, total) - start)
-        argv += ["-threads", "1", "-itsoffset", f"{start:.3f}", "-loop", "1",
-                 "-framerate", str(fps), "-t", f"{length:.3f}", "-i", layer.path]
-        chain = f"[{position}:v]format=rgba"
+        # Every frame that starts inside the window, the last one included.
+        frames = max(1, math.ceil((min(layer.end, total) - start) * fps - 1e-6))
+        argv += ["-i", layer.path]
+        # `settb` first: a lone PNG comes in on the image demuxer's 1/25 timebase,
+        # which would round every frame's time, and with it the fades, to 40 ms.
+        chain = (f"[{position}:v]format=rgba,loop=loop=-1:size=1,trim=end_frame={frames},settb=AVTB,"
+                 f"setpts=N/({fps}*TB)+{start:.3f}/TB")
         if layer.fade > 0:
             chain += (f",fade=t=in:st={layer.start:.3f}:d={layer.fade:.3f}:alpha=1"
                       f",fade=t=out:st={layer.end - layer.fade:.3f}:d={layer.fade:.3f}:alpha=1")

backend/tests/test_render_ffmpeg.py

@@ -1342,18 +1342,21 @@ def test_a_stitch_run_trims_its_ends_by_frame_and_sample_and_decodes_single_thre
         options=RenderOptions(), preview=True, start_frame=30, end_frame=60)
     graph = _graph(argv)
     assert argv.count("-threads") == 3
-    assert "[0:v]trim=start_frame=30,setpts=PTS-STARTPTS,settb=AVTB[s0]" in graph
+    assert "[0:v]trim=start_frame=30,setpts=PTS-STARTPTS,fps=30,settb=AVTB[s0]" in graph
     assert "[0:a]atrim=start_sample=48000,asetpts=PTS-STARTPTS[st0]" in graph
     assert "[2:v]trim=end_frame=60," in graph and "atrim=end_sample=96000" in graph
     # The first shot is 3 s long once trimmed, so the first blend starts at 2.5 s.
     assert "offset=2.500" in graph
     assert "[at2][st2]concat" in graph
 
 
-def test_each_overlay_layer_exists_only_for_its_window():
+def test_each_overlay_layer_is_one_frame_repeated_for_its_window_only():
+    """A layer looped as an input for the whole video was decoded on every frame,
+    and one started late with `-itsoffset` deadlocked ffmpeg 7.1.5."""
     argv = F.build_overlay_pass(
         "stitched.mp4", [F.TimedLayer("a.png", 1.0, 4.5, 0.3)], "out.mp4",
         options=RenderOptions(), preview=True, duration=10.0)
-    layer = argv[argv.index("a.png") - 12:argv.index("a.png")]
-    assert layer[layer.index("-itsoffset") + 1] == "1.000"
-    assert layer[layer.index("-t") + 1] == "3.500"
+    assert argv[argv.index("a.png") - 1] == "-i"
+    assert "-itsoffset" not in argv and "-loop" not in argv
+    assert ("[1:v]format=rgba,loop=loop=-1:size=1,trim=end_frame=105,settb=AVTB,"
+            "setpts=N/(30*TB)+1.000/TB,") in _graph(argv)