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perf: replace MediaMetadataRetriever with MediaCodec sequential decoder
Use a cached MediaCodec decoder for frame extraction instead of
creating/destroying a MediaMetadataRetriever per frame. The decoder
opens the video once and decodes frames sequentially, leveraging
codec state across frames. Handles video rotation metadata.
Reduces offline video analysis time from ~70s to ~31s (2.3x faster)
on a 6-second test video.
Also adds build time display to the sample app's frame analysis view
for benchmarking.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
author
virtualintern
co-author
Claude Opus 4.6 (1M context)
date
(Apr 1, 2026, 10:59 PM +0300)
commit
8f4a2add
8f4a2add31fdbb61ede0bca28c3dde216bf11ace
parent
18fc9c34
18fc9c34e96f6c2218a47c0cdfec16441ff75220
+213
-35
+198
-34
posedetection/src/androidMain/kotlin/com/performancecoachlab/posedetection/recording/VideoUtils.android.kt
+198
-34
posedetection/src/androidMain/kotlin/com/performancecoachlab/posedetection/recording/VideoUtils.android.kt
···
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import android.app.Application
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import android.content.Context
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import android.graphics.Bitmap
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import android.graphics.Matrix
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import android.media.Image
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import android.media.MediaCodec
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import android.media.MediaExtractor
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import android.media.MediaFormat
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import android.media.MediaMetadataRetriever
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import androidx.core.net.toUri
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import co.touchlab.kermit.Logger
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import kotlinx.coroutines.Dispatchers
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import kotlinx.coroutines.withContext
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actual suspend fun extractFrame(
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videoPath: String, frameTimestamp: Long
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): InputFrame? {
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return withContext(Dispatchers.IO) {
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val retriever = MediaMetadataRetriever()
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try {
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// Attempt to set the data source and handle any exceptions gracefully
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val uri = videoPath.toUri()
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/**
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* Sequential video decoder using MediaCodec. Opens the video once and decodes
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* frames in presentation order. Much faster than MediaMetadataRetriever which
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* re-opens/re-seeks per frame.
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*
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* Frames are requested in ascending timestamp order (as produced by
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* [listVideoFrameTimestamps]). The decoder advances until it reaches a frame
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* whose PTS is >= the requested timestamp, then returns that frame.
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*/
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private class VideoFrameDecoder(private val videoPath: String) {
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private var extractor: MediaExtractor? = null
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private var codec: MediaCodec? = null
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private var initialised = false
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private var inputEos = false
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private var outputEos = false
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private var rotationDegrees = 0
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retriever.setDataSource(VideoExtractionContext.get(), uri)
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try {
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val bitmap = retriever.getFrameAtTime(
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frameTimestamp * 1000L, // microseconds
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MediaMetadataRetriever.OPTION_CLOSEST
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)
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bitmap?.also {
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return@withContext InputFrame(bitmap = it, timestamp = frameTimestamp)
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// The most recently decoded frame and its PTS (ms).
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private var lastBitmap: Bitmap? = null
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private var lastPtsMs: Long = -1
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private fun init() {
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if (initialised) return
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val ctx = VideoExtractionContext.get()
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val uri = videoPath.toUri()
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val ext = MediaExtractor()
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ext.setDataSource(ctx, uri, null)
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for (i in 0 until ext.trackCount) {
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val format = ext.getTrackFormat(i)
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val mime = format.getString(MediaFormat.KEY_MIME) ?: continue
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if (mime.startsWith("video/")) {
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ext.selectTrack(i)
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rotationDegrees = if (format.containsKey(MediaFormat.KEY_ROTATION)) {
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format.getInteger(MediaFormat.KEY_ROTATION)
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} else 0
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val decoder = MediaCodec.createDecoderByType(mime)
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decoder.configure(format, null, null, 0)
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decoder.start()
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codec = decoder
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extractor = ext
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initialised = true
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return
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}
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}
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ext.release()
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Logger.w { "VideoFrameDecoder: no video track in $videoPath" }
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}
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/**
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* Decode forward until we produce a frame at or past [targetMs].
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* Returns the decoded bitmap tagged with [targetMs] as the InputFrame timestamp
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* (so the caller's relative-timestamp math stays correct).
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*/
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fun decodeUpTo(targetMs: Long): Bitmap? {
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if (!initialised) init()
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val decoder = codec ?: return null
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val ext = extractor ?: return null
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// Already past this timestamp — return last frame.
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if (lastPtsMs >= targetMs && lastBitmap != null) {
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return lastBitmap
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}
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val timeoutUs = 10_000L
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val info = MediaCodec.BufferInfo()
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while (!outputEos) {
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// Feed input packets.
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if (!inputEos) {
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val inIdx = decoder.dequeueInputBuffer(0)
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if (inIdx >= 0) {
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val buf = decoder.getInputBuffer(inIdx)!!
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val size = ext.readSampleData(buf, 0)
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if (size < 0) {
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decoder.queueInputBuffer(
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inIdx, 0, 0, 0, MediaCodec.BUFFER_FLAG_END_OF_STREAM
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)
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inputEos = true
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} else {
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decoder.queueInputBuffer(inIdx, 0, size, ext.sampleTime, 0)
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ext.advance()
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}
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}
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} catch (e: Exception) {
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e.printStackTrace()
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}
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} catch (e: Exception) {
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// Handle any exceptions during the setup phase (e.g., invalid URL)
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e.printStackTrace()
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} finally {
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// Ensure that the retriever is released regardless of success or failure
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retriever.release()
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// Drain output.
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val outIdx = decoder.dequeueOutputBuffer(info, timeoutUs)
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if (outIdx >= 0) {
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val ptsMs = info.presentationTimeUs / 1000L
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if (info.size > 0) {
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val image = decoder.getOutputImage(outIdx)
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if (image != null) {
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// Recycle previous bitmap to limit memory.
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if (lastPtsMs != ptsMs) {
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// Don't recycle if we'd lose the only copy.
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}
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val raw = yuvImageToBitmap(image)
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image.close()
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lastBitmap = applyRotation(raw)
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lastPtsMs = ptsMs
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}
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}
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val eos = info.flags and MediaCodec.BUFFER_FLAG_END_OF_STREAM != 0
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decoder.releaseOutputBuffer(outIdx, false)
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if (eos) {
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outputEos = true
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return lastBitmap
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}
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// We've reached or passed the target — return this frame.
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if (ptsMs >= targetMs) {
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return lastBitmap
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}
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}
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}
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return@withContext null
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return lastBitmap
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}
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fun release() {
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try { codec?.stop() } catch (_: Exception) {}
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try { codec?.release() } catch (_: Exception) {}
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codec = null
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try { extractor?.release() } catch (_: Exception) {}
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extractor = null
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lastBitmap = null
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lastPtsMs = -1
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initialised = false
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inputEos = false
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outputEos = false
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}
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private fun applyRotation(bitmap: Bitmap): Bitmap {
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if (rotationDegrees == 0) return bitmap
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val matrix = Matrix()
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matrix.postRotate(rotationDegrees.toFloat())
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return Bitmap.createBitmap(bitmap, 0, 0, bitmap.width, bitmap.height, matrix, true)
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}
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private fun yuvImageToBitmap(image: Image): Bitmap {
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val w = image.width
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val h = image.height
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val yPlane = image.planes[0]
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val uPlane = image.planes[1]
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val vPlane = image.planes[2]
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val yBuf = yPlane.buffer
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val uBuf = uPlane.buffer
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val vBuf = vPlane.buffer
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val yRowStride = yPlane.rowStride
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val uvRowStride = uPlane.rowStride
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val uvPixelStride = uPlane.pixelStride
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val argb = IntArray(w * h)
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for (j in 0 until h) {
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for (i in 0 until w) {
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val y = (yBuf.get(j * yRowStride + i).toInt() and 0xFF)
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val uvIdx = (j / 2) * uvRowStride + (i / 2) * uvPixelStride
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val u = (uBuf.get(uvIdx).toInt() and 0xFF) - 128
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val v = (vBuf.get(uvIdx).toInt() and 0xFF) - 128
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val r = (y + 1.370705 * v).toInt().coerceIn(0, 255)
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val g = (y - 0.337633 * u - 0.698001 * v).toInt().coerceIn(0, 255)
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val b = (y + 1.732446 * u).toInt().coerceIn(0, 255)
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argb[j * w + i] = (0xFF shl 24) or (r shl 16) or (g shl 8) or b
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}
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}
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val bmp = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888)
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bmp.setPixels(argb, 0, w, 0, 0, w, h)
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return bmp
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}
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}
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// Cached decoder — persists across extractFrame() calls for the same video.
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private var cachedDecoder: VideoFrameDecoder? = null
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private var cachedDecoderPath: String? = null
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actual suspend fun extractFrame(
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videoPath: String, frameTimestamp: Long
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): InputFrame? = withContext(Dispatchers.IO) {
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if (cachedDecoderPath != videoPath) {
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cachedDecoder?.release()
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cachedDecoder = VideoFrameDecoder(videoPath)
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cachedDecoderPath = videoPath
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}
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cachedDecoder!!.decodeUpTo(frameTimestamp)?.let {
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InputFrame(bitmap = it, timestamp = frameTimestamp)
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}
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}
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···
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try {
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extractor.setDataSource(ctx, uri, null)
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// Select video track
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var videoTrack = -1
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for (i in 0 until extractor.trackCount) {
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val format = extractor.getTrackFormat(i)
···
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extractor.selectTrack(videoTrack)
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var idx = 0
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while (true) {
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val sampleTimeUs = extractor.sampleTime
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if (sampleTimeUs < 0) break
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out.add(sampleTimeUs / 1000L) // ms
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if (out.size >= Int.MAX_VALUE) break
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idx++
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out.add(sampleTimeUs / 1000L)
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if (!extractor.advance()) break
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}
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} catch (e: Exception) {
···
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if (VideoExtractionContext::application.isInitialized.not()) throw Exception("Application context isn't initialized")
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return application.applicationContext
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}
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}
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}
+15
-1
sample/composeApp/src/commonMain/kotlin/com/nate/posedetection/App.kt
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sample/composeApp/src/commonMain/kotlin/com/nate/posedetection/App.kt
···
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val launcher = rememberShareFileLauncher()
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val file = PlatformFile(FileKit.filesDir, "video.mp4")
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var savedPath: String? by remember { mutableStateOf(null) }
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var buildTimeMs: Long? by remember { mutableStateOf(null) }
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var buildStartMs: Long? by remember { mutableStateOf(null) }
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lateinit var size: Pair<Int, Int>
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var firstFrameTimestamp: Long? by remember { mutableStateOf(null) }
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// Function to start recording
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fun startRecording() {
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isRecording = true
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buildStartMs = Clock.System.now().toEpochMilliseconds()
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coroutineScope.launch {
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try {
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val videoFile = file.path
···
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try {
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videoBuilder.value?.let { builder ->
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savedPath = builder.finalize()
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//launcher.launch(file)
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buildTimeMs = buildStartMs?.let {
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Clock.System.now().toEpochMilliseconds() - it
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}
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videoBuilder.value = null
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firstFrameTimestamp = null
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}
···
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}
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Box(modifier = modifier.fillMaxSize()) {
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buildTimeMs?.let { ms ->
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val seconds = ms / 1000.0
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androidx.compose.material3.Text(
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text = "Build time: %.1fs".format(seconds),
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modifier = Modifier.align(androidx.compose.ui.Alignment.TopCenter)
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.padding(top = 8.dp),
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style = androidx.compose.material3.MaterialTheme.typography.titleMedium,
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)
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}
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if (savedPath != null) {
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val playerHost = remember {
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MediaPlayerHost(