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Avoid seeking checks when decoding frames sequentially #1028

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Merged
NicolasHug merged 7 commits into from
Nov 12, 2025
Merged

Avoid seeking checks when decoding frames sequentially #1028

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3e9ae6f
Avoid seeking checks when decoding frames sequentially
NicolasHug Nov 7, 2025
86ec99e
Simplify seek skipping logic
NicolasHug Nov 11, 2025
e68d7ce
Merge branch 'main' of github.com:pytorch/torchcodec into avoid_seeki…
NicolasHug Nov 11, 2025
d98c78a
Merge branch 'simplify_skip' into avoid_seeking_checks
NicolasHug Nov 11, 2025
42409a9
comment
NicolasHug Nov 11, 2025
9acc148
Merge branch 'main' of github.com:pytorch/torchcodec into avoid_seeki…
NicolasHug Nov 12, 2025
2295241
Fix
NicolasHug Nov 12, 2025
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23 changes: 16 additions & 7 deletions src/torchcodec/_core/SingleStreamDecoder.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -626,9 +626,17 @@ FrameOutput SingleStreamDecoder::getFrameAtIndexInternal(
}
validateFrameIndex(streamMetadata, frameIndex);

int64_t pts = getPts(frameIndex);
setCursorPtsInSeconds(ptsToSeconds(pts, streamInfo.timeBase));
return getNextFrameInternal(preAllocatedOutputTensor);
// Only set cursor if we're not decoding sequentially: when decoding
// sequentially, we don't need to seek anywhere, so by *not* setting the
// cursor we allow canWeAvoidSeeking() to return true early.
if (frameIndex != lastDecodedFrameIndex_ + 1) {
int64_t pts = getPts(frameIndex);
setCursorPtsInSeconds(ptsToSeconds(pts, streamInfo.timeBase));
}
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@NicolasHug NicolasHug Nov 11, 2025

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I included the changes from #1039 in this PR.

@scotts , does the comment make sense now? What the comment is not saying is why returning early in canWeAvoidSeeking() is important. It's important because it avoids the calls to av_index_search_timestamp which are potentially slow. I don't know if we want to get to that level of detail in the comment.

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@scotts scotts Nov 11, 2025

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Yes, this greatly helps with understanding.

I do think we should explain somewhere that canWeAvoidSeeking() is itself expensive in some circumstances. . That's deeply counter-intuitive, that the function we're calling as an optimization to avoid the slow thing is itself also a slow thing. (But hopefully less slow.) Perhaps that should belong at the top of canWeAvoidSeeking().

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@NicolasHug NicolasHug Nov 12, 2025

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Will address that in an immediate follow-up


auto result = getNextFrameInternal(preAllocatedOutputTensor);
lastDecodedFrameIndex_ = frameIndex;
return result;
}

FrameBatchOutput SingleStreamDecoder::getFramesAtIndices(
Expand Down Expand Up @@ -1100,6 +1108,9 @@ I P P P I P P P I P P I P P I P
*/
bool SingleStreamDecoder::canWeAvoidSeeking() const {
const StreamInfo& streamInfo = streamInfos_.at(activeStreamIndex_);
if (!cursorWasJustSet_) {
return true;
}
if (streamInfo.avMediaType == AVMEDIA_TYPE_AUDIO) {
// For audio, we only need to seek if a backwards seek was requested
// within getFramesPlayedInRangeAudio(), when setCursorPtsInSeconds() was
Expand Down Expand Up @@ -1181,10 +1192,8 @@ UniqueAVFrame SingleStreamDecoder::decodeAVFrame(

resetDecodeStats();

if (cursorWasJustSet_) {
maybeSeekToBeforeDesiredPts();
cursorWasJustSet_ = false;
}
maybeSeekToBeforeDesiredPts();
cursorWasJustSet_ = false;

UniqueAVFrame avFrame(av_frame_alloc());
AutoAVPacket autoAVPacket;
Expand Down
1 change: 1 addition & 0 deletions src/torchcodec/_core/SingleStreamDecoder.h
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Expand Up @@ -346,6 +346,7 @@ class SingleStreamDecoder {
bool cursorWasJustSet_ = false;
int64_t lastDecodedAvFramePts_ = 0;
int64_t lastDecodedAvFrameDuration_ = 0;
int64_t lastDecodedFrameIndex_ = INT64_MIN;

// Stores various internal decoding stats.
DecodeStats decodeStats_;
Expand Down
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