Adolfo Reyna
63 lines
3.7 KiB
JavaScript
63 lines
3.7 KiB
JavaScript
"use strict";
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// For seamless AAC concatenation, we need to capture a bit of extra audio on both sides
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// to later align the audio correctly. This function calculates the exact frames to capture.
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Object.defineProperty(exports, "__esModule", { value: true });
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exports.getExtraFramesToCapture = void 0;
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const combine_audio_1 = require("./combine-audio");
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const sample_rate_1 = require("./sample-rate");
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// Inspired by https://github.com/wistia/seamless-aac-split-and-stitch-demo
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// We can seamlessly concatenate AAC files if we capture a bit of extra audio on both sides in each chunk and then align the audio correctly.
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// This function calculates which extra frames should be evaluated for their audio content.
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const getExtraFramesToCapture = ({ compositionStart, realFrameRange, fps, forSeamlessAacConcatenation, }) => {
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// If the feature is disabled, don't capture extra frames.
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if (!forSeamlessAacConcatenation) {
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return {
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extraFramesToCaptureAssetsBackend: [],
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extraFramesToCaptureAssetsFrontend: [],
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chunkLengthInSeconds: (realFrameRange[1] - realFrameRange[0] + 1) / fps,
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trimLeftOffset: 0,
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trimRightOffset: 0,
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};
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}
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// If this video is just a chunk as part of a larger video,
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// We also need to know the start time of this chunk to align it correctly.
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const chunkStart = realFrameRange[0];
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// If we are only rendering a portion of the composition, we also need to account for that.
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// It cannot be that the chunk start time is earlier than the composition time.
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if (chunkStart < compositionStart) {
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throw new Error('chunkStart may not be below compositionStart');
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}
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const realLeftEnd = chunkStart - compositionStart;
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if (realLeftEnd < 0) {
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throw new Error('chunkStat - compositionStart may not be below 0');
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}
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const realRightEnd = realLeftEnd + (realFrameRange[1] - realFrameRange[0] + 1);
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// Find the closest AAC packet border and add two AAC packet padding.
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const aacAdjustedLeftEnd = Math.max(0, (0, combine_audio_1.getClosestAlignedTime)(realLeftEnd / fps) - 2 * (1024 / sample_rate_1.DEFAULT_SAMPLE_RATE));
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const aacAdjustedRightEnd = (0, combine_audio_1.getClosestAlignedTime)(realRightEnd / fps) +
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2 * (1024 / sample_rate_1.DEFAULT_SAMPLE_RATE);
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// Now find the additional frames that we need to capture to have enough audio
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const alignedStartFrameWithoutOffset = Math.floor(aacAdjustedLeftEnd * fps);
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const alignedStartFrame = alignedStartFrameWithoutOffset + compositionStart;
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const alignedEndFrame = Math.ceil(aacAdjustedRightEnd * fps) + compositionStart;
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const extraFramesToCaptureAudioOnlyFrontend = new Array(realFrameRange[0] - alignedStartFrame)
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.fill(true)
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.map((_, f) => f + alignedStartFrame);
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const extraFramesToCaptureAudioOnlyBackend = new Array(alignedEndFrame - realFrameRange[1] - 1)
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.fill(true)
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.map((_, f) => f + realFrameRange[1] + 1);
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// But now, we might have too much audio, since the extra frames only have a `1 / fps` step.
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// When creating the lossless audio for a chunk, we need to shave that extra audio off.
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const trimLeftOffset = (aacAdjustedLeftEnd * fps - alignedStartFrameWithoutOffset) / fps;
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const trimRightOffset = (aacAdjustedRightEnd * fps - Math.ceil(aacAdjustedRightEnd * fps)) / fps;
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const chunkLengthInSeconds = aacAdjustedRightEnd - aacAdjustedLeftEnd;
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return {
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extraFramesToCaptureAssetsFrontend: extraFramesToCaptureAudioOnlyFrontend,
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extraFramesToCaptureAssetsBackend: extraFramesToCaptureAudioOnlyBackend,
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chunkLengthInSeconds,
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trimLeftOffset,
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trimRightOffset,
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};
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};
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exports.getExtraFramesToCapture = getExtraFramesToCapture;
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