文章来源51cto，作者：killads
https://blog.51cto.com/u_14731/6959356
一、前言
我们用手机录制一个视频之后，点开相册等APP，在不点开播放的时候也可以看到视频的第一帧，这就是视频的缩略图。
安卓中具体怎么获取视频的缩略图呢？本文将一一揭晓。
二、视频缩略图获取过程
1）首先我们看看APP怎么调用的。
APP的调用很简单，直接用createVideoThumbnail函数即可。
下面看看这个函数的具体实现。
frameworks/base/media/java/android/media/ThumbnailUtils.java
// 输入为视频的文件名和kind，输出为bitmap
public static Bitmap createVideoThumbnail(String filePath, int kind) {
Bitmap bitmap = null;
// 首先创建一个MediaMetadataRetriever
MediaMetadataRetriever retriever = new MediaMetadataRetriever();
try {
retriever.setDataSource(filePath); // setDataSource
bitmap = retriever.getFrameAtTime(-1); // 获取封面
// 此处有两种方式获取封面，一种是getFrameAtTime， 一种是getImageAtIndex
// 这两个的区别是一个是通过时间戳解析封面，另一个是通过index解析封面
} catch (IllegalArgumentException ex) {
// Assume this is a corrupt video file
} catch (RuntimeException ex) {
// Assume this is a corrupt video file.
} finally {
try {
retriever.release();
} catch (RuntimeException ex) {
// Ignore failures while cleaning up.
}
}
if (bitmap == null) return null;
if (kind == Images.Thumbnails.MINI_KIND) { // 如果是缩略图需要做缩放
// Scale down the bitmap if it's too large.
int width = bitmap.getWidth();
int height = bitmap.getHeight();
int max = Math.max(width, height);
if (max > 512) {
float scale = 512f / max;
int w = Math.round(scale * width);
int h = Math.round(scale * height);
bitmap = Bitmap.createScaledBitmap(bitmap, w, h, true);
}
} else if (kind == Images.Thumbnails.MICRO_KIND) {
bitmap = extractThumbnail(bitmap,
TARGET_SIZE_MICRO_THUMBNAIL,
TARGET_SIZE_MICRO_THUMBNAIL,
OPTIONS_RECYCLE_INPUT);
}
return bitmap;
}
上面的MediaMetadataRetriever位于/frameworks/base/media/java/android/media/MediaMetadataRetriever.java中。
其通过jni调用的最终函数都是/frameworks/av/media/libmedia/mediametadataretriever.cpp中的函数。mediametadataretriever.cpp实际是一个bp，具体实现都在mRetriever中。
2）下面来看看怎么获取的mRetriever:
frameworks/av/media/libmedia/mediametadataretriever.cpp
MediaMetadataRetriever::MediaMetadataRetriever()
{
ALOGV("constructor");
const sp<IMediaPlayerService> service(getService());
if (service == 0) {
ALOGE("failed to obtain MediaMetadataRetrieverService");
return;
}
// 从service创建MediaMetadataRetriever
sp<IMediaMetadataRetriever> retriever(service->createMetadataRetriever());
if (retriever == 0) {
ALOGE("failed to create IMediaMetadataRetriever object from server");
}
mRetriever = retriever;
}
/frameworks/av/media/libmediaplayerservice/MediaPlayerService.cpp
sp<IMediaMetadataRetriever> MediaPlayerService::createMetadataRetriever()
{
pid_t pid = IPCThreadState::self()->getCallingPid();
sp<MetadataRetrieverClient> retriever = new MetadataRetrieverClient(pid);
ALOGV("Create new media retriever from pid %d", pid);
return retriever;
}
综合上面可以看到实际MediaMetadataRetriever这个bp对应bn就是MetadataRetrieverClient，
具体函数的实现就是在MetadataRetrieverClient中进行的。
3) 接下来我们就忽略jni、bp等繁琐的过程，直接讲下MetadataRetrieverClient中的实现。
frameworks/av/media/libmediaplayerservice/MetadataRetrieverClient.cpp
status_t MetadataRetrieverClient::setDataSource(int fd, int64_t offset, int64_t length)
{
ALOGV("setDataSource fd=%d, offset=%" PRId64 ", length=%" PRId64 "", fd, offset, length);
Mutex::Autolock lock(mLock);
struct stat sb;
int ret = fstat(fd, &sb);
if (ret != 0) {
ALOGE("fstat(%d) failed: %d, %s", fd, ret, strerror(errno));
return BAD_VALUE;
}
if (offset >= sb.st_size) {
ALOGE("offset (%" PRId64 ") bigger than file size (%" PRIu64 ")", offset, sb.st_size);
return BAD_VALUE;
}
if (offset + length > sb.st_size) {
length = sb.st_size - offset;
ALOGV("calculated length = %" PRId64 "", length);
}
// 首先需要根据文件内容获取player type
player_type playerType =
MediaPlayerFactory::getPlayerType(NULL /* client */,
fd,
offset,
length);
ALOGV("player type = %d", playerType);
// 然后根据playerType创建合适的Retriever,此处一般创建的就是StagefrightMetadataRetriever
sp<MediaMetadataRetrieverBase> p = createRetriever(playerType);
if (p == NULL) {
return NO_INIT;
}
// 调用StagefrightMetadataRetriever的setDataSource
status_t status = p->setDataSource(fd, offset, length);
if (status == NO_ERROR) mRetriever = p; // 赋值mRetriever = p
return status;
}
sp<IMemory> MetadataRetrieverClient::getFrameAtTime(
int64_t timeUs, int option, int colorFormat, bool metaOnly)
{
ALOGV("getFrameAtTime: time(%" PRId64 "us) option(%d) colorFormat(%d), metaOnly(%d)",
timeUs, option, colorFormat, metaOnly);
Mutex::Autolock lock(mLock);
Mutex::Autolock glock(sLock);
if (mRetriever == NULL) {
ALOGE("retriever is not initialized");
return NULL;
}
// mRetriever就是StagefrightMetadataRetriever
sp<IMemory> frame = mRetriever->getFrameAtTime(timeUs, option, colorFormat, metaOnly);
if (frame == NULL) {
ALOGE("failed to capture a video frame");
return NULL;
}
return frame;
}
4) 看完我们就发现MetadataRetrieverClient函数的实现在StagefrightMetadataRetriever中……真是层层封装啊……
好，接下来继续看StagefrightMetadataRetriever……
/frameworks/av/media/libstagefright/StagefrightMetadataRetriever.cpp
status_t StagefrightMetadataRetriever::setDataSource(
int fd, int64_t offset, int64_t length) {
fd = dup(fd);
ALOGV("setDataSource(%d, %" PRId64 ", %" PRId64 ")", fd, offset, length);
AVUtils::get()->printFileName(fd);
clearMetadata();
mSource = new FileSource(fd, offset, length); // 创建mSource
status_t err;
if ((err = mSource->initCheck()) != OK) {
mSource.clear();
return err;
}
mExtractor = MediaExtractorFactory::Create(mSource); // 创建extractor
if (mExtractor == NULL) {
mSource.clear();
return UNKNOWN_ERROR;
}
return OK;
}
sp<IMemory> StagefrightMetadataRetriever::getFrameAtTime(
int64_t timeUs, int option, int colorFormat, bool metaOnly) {
ALOGV("getFrameAtTime: %" PRId64 " us option: %d colorFormat: %d, metaOnly: %d",
timeUs, option, colorFormat, metaOnly);
sp<IMemory> frame;
status_t err = getFrameInternal( // 调用getFrameInternal
timeUs, 1, option, colorFormat, metaOnly, &frame, NULL /*outFrames*/);
return (err == OK) ? frame : NULL;
}
status_t StagefrightMetadataRetriever::getFrameInternal(
int64_t timeUs, int numFrames, int option, int colorFormat, bool metaOnly,
sp<IMemory>* outFrame, std::vector<sp<IMemory> >* outFrames) {
if (mExtractor.get() == NULL) {
ALOGE("no extractor.");
return NO_INIT;
}
sp<MetaData> fileMeta = mExtractor->getMetaData(); // 获取metadata
if (fileMeta == NULL) {
ALOGE("extractor doesn't publish metadata, failed to initialize?");
return NO_INIT;
}
int32_t drm = 0;
if (fileMeta->findInt32(kKeyIsDRM, &drm) && drm != 0) {
ALOGE("frame grab not allowed.");
return ERROR_DRM_UNKNOWN;
}
size_t n = mExtractor->countTracks(); // 一共有几个track
size_t i;
for (i = 0; i < n; ++i) {
sp<MetaData> meta = mExtractor->getTrackMetaData(i); // 循环获取每个track的metadata
if (meta == NULL) {
continue;
}
const char *mime;
CHECK(meta->findCString(kKeyMIMEType, &mime)); // 获取当前track的mime
if (!strncasecmp(mime, "video/", 6)) {
break; // 如果是video就直接break，video track对应的id就是i
}
}
if (i == n) { // 未找到video track
ALOGE("no video track found.");
return INVALID_OPERATION;
}
// 获取vidoe track的metadata
sp<MetaData> trackMeta = mExtractor->getTrackMetaData(
i, MediaExtractor::kIncludeExtensiveMetaData);
if (metaOnly) { // 一般是false
if (outFrame != NULL) {
*outFrame = FrameDecoder::getMetadataOnly(trackMeta, colorFormat);
if (*outFrame != NULL) {
return OK;
}
}
return UNKNOWN_ERROR;
}
sp<IMediaSource> source = mExtractor->getTrack(i); // 获取video track
if (source.get() == NULL) {
ALOGV("unable to instantiate video track.");
return UNKNOWN_ERROR;
}
const void *data;
uint32_t type;
size_t dataSize;
if (fileMeta->findData(kKeyAlbumArt, &type, &data, &dataSize)
&& mAlbumArt == NULL) {
mAlbumArt = MediaAlbumArt::fromData(dataSize, data);
}
const char *mime;
CHECK(trackMeta->findCString(kKeyMIMEType, &mime));
Vector<AString> matchingCodecs;
MediaCodecList::findMatchingCodecs( // 从MediaCodecList中找到可用的media decoders
mime,
false, /* encoder */
0/*MediaCodecList::kPreferSoftwareCodecs*/,
&matchingCodecs);
for (size_t i = 0; i < matchingCodecs.size(); ++i) { // 循环尝试每个可用的decoedr
const AString &componentName = matchingCodecs[i];
VideoFrameDecoder decoder(componentName, trackMeta, source);
// 创建decoder，然后初始化decoder，然后用extractFrame获取一帧
if (decoder.init(timeUs, numFrames, option, colorFormat) == OK) {
if (outFrame != NULL) {
*outFrame = decoder.extractFrame();
if (*outFrame != NULL) {
return OK;
}
} else if (outFrames != NULL) {
status_t err = decoder.extractFrames(outFrames);
if (err == OK) {
return OK;
}
}
}
ALOGV("%s failed to extract frame, trying next decoder.", componentName.c_str());
}
ALOGE("all codecs failed to extract frame.");
return UNKNOWN_ERROR;
}
StagefrightMetadataRetriever中首先在setdatasource中创建了extractor，然后在getFrameInternal中提取filemetadata，循环file所有track扎到video track，并分析track metadata找到了所有可用的decoders。然后在循环尝试每个可用decoder去解析一帧图片。
具体解析过程见: FrameDecoder::extractFrame。
5) 接下来就看看这个函数的具体实现
/frameworks/av/media/libstagefright/FrameDecoder.cpp
status_t FrameDecoder::init(
int64_t frameTimeUs, size_t numFrames, int option, int colorFormat) {
if (!getDstColorFormat(
(android_pixel_format_t)colorFormat, &mDstFormat, &mDstBpp)) {
return ERROR_UNSUPPORTED;
}
sp<AMessage> videoFormat = onGetFormatAndSeekOptions(
frameTimeUs, numFrames, option, &mReadOptions);
if (videoFormat == NULL) {
ALOGE("video format or seek mode not supported");
return ERROR_UNSUPPORTED;
}
status_t err;
sp<ALooper> looper = new ALooper;
looper->start();
// 根据ComponentName创建decoder，decoder就是MediaCodec
sp<MediaCodec> decoder = MediaCodec::CreateByComponentName(
looper, mComponentName, &err);
if (decoder.get() == NULL || err != OK) {
ALOGW("Failed to instantiate decoder [%s]", mComponentName.c_str());
return (decoder.get() == NULL) ? NO_MEMORY : err;
}
err = decoder->configure( // 首先调用decoder的configure
videoFormat, NULL /* surface */, NULL /* crypto */, 0 /* flags */);
if (err != OK) {
ALOGW("configure returned error %d (%s)", err, asString(err));
decoder->release(); // 如果出错就relaese
return err;
}
err = decoder->start(); // 调用decoder的start
if (err != OK) {
ALOGW("start returned error %d (%s)", err, asString(err));
decoder->release();// 如果出错就relaese
return err;
}
err = mSource->start(); // mSource就是video track的metadata
if (err != OK) {
ALOGW("source failed to start: %d (%s)", err, asString(err));
decoder->release();
return err;
}
mDecoder = decoder; // 把decoder赋值给mDecoder
return OK;
}
sp<IMemory> FrameDecoder::extractFrame(FrameRect *rect) {
status_t err = onExtractRect(rect); // 先调用onExtractRect
if (err == OK) {
err = extractInternal(); // 先调用extractInternal
}
if (err != OK) {
return NULL;
}
return mFrames.size() > 0 ? mFrames[0] : NULL; // 实际输出的图片
}
status_t ImageDecoder::onExtractRect(FrameRect *rect) {
if (rect == NULL) {
if (mTilesDecoded > 0) {
return ERROR_UNSUPPORTED;
}
mTargetTiles = mGridRows * mGridCols;
return OK;
}
if (mTileWidth <= 0 || mTileHeight <=0) {
return ERROR_UNSUPPORTED;
}
int32_t row = mTilesDecoded / mGridCols;
int32_t expectedTop = row * mTileHeight;
int32_t expectedBot = (row + 1) * mTileHeight;
if (expectedBot > mHeight) {
expectedBot = mHeight;
}
if (rect->left != 0 || rect->top != expectedTop
|| rect->right != mWidth || rect->bottom != expectedBot) {
ALOGE("currently only support sequential decoding of slices");
return ERROR_UNSUPPORTED;
}
// advance one row
mTargetTiles = mTilesDecoded + mGridCols;
return OK;
}
status_t FrameDecoder::extractInternal() {
status_t err = OK;
bool done = false;
size_t retriesLeft = kRetryCount;
do {
size_t index;
int64_t ptsUs = 0ll;
uint32_t flags = 0;
while (mHaveMoreInputs) {
// // 先从decoder dequeue一个inputbuffer，获取inout buffer的index
err = mDecoder->dequeueInputBuffer(&index, 0);
if (err != OK) {
ALOGV("Timed out waiting for input");
if (retriesLeft) {
err = OK;
}
break;
}
sp<MediaCodecBuffer> codecBuffer;
// 然后调用decoder getInputBuffer通过index拿到codecbuffer
err = mDecoder->getInputBuffer(index, &codecBuffer); //
if (err != OK) {
ALOGE("failed to get input buffer %zu", index);
break;
}
MediaBufferBase *mediaBuffer = NULL;
// 从video track中读取一帧码流数据
err = mSource->read(&mediaBuffer, &mReadOptions);
mReadOptions.clearSeekTo();
if (err != OK) {
ALOGW("Input Error or EOS");
mHaveMoreInputs = false;
if (!mFirstSample && err == ERROR_END_OF_STREAM) {
err = OK;
flags |= MediaCodec::BUFFER_FLAG_EOS;
mHaveMoreInputs = true;
}
break;
}
// 接下来判断codecBuffer是否够用，如果够用就把mediaBuffer的数据给codecBuffer
if (mediaBuffer->range_length() > codecBuffer->capacity()) {
ALOGE("buffer size (%zu) too large for codec input size (%zu)",
mediaBuffer->range_length(), codecBuffer->capacity());
mHaveMoreInputs = false;
err = BAD_VALUE;
} else {
codecBuffer->setRange(0, mediaBuffer->range_length());
CHECK(mediaBuffer->meta_data().findInt64(kKeyTime, &ptsUs));
memcpy(codecBuffer->data(),
(const uint8_t*)mediaBuffer->data() + mediaBuffer->range_offset(),
mediaBuffer->range_length());
onInputReceived(codecBuffer, mediaBuffer->meta_data(), mFirstSample, &flags);
mFirstSample = false;
}
mediaBuffer->release(); // release mediabuffer
if (mHaveMoreInputs) {
ALOGV("QueueInput: size=%zu ts=%" PRId64 " us flags=%x",
codecBuffer->size(), ptsUs, flags);
err = mDecoder->queueInputBuffer( // 把码流数据送给decoder
index,
codecBuffer->offset(),
codecBuffer->size(),
ptsUs,
flags);
if (flags & MediaCodec::BUFFER_FLAG_EOS) {
mHaveMoreInputs = false;
}
}
}
while (err == OK) {
size_t offset, size;
// wait for a decoded buffer
// 从decoder dequeue一个output buffer,获取到buffer的index
err = mDecoder->dequeueOutputBuffer(
&index,
&offset,
&size,
&ptsUs,
&flags,
kBufferTimeOutUs);
if (err == INFO_FORMAT_CHANGED) {
ALOGV("Received format change");
err = mDecoder->getOutputFormat(&mOutputFormat);
} else if (err == INFO_OUTPUT_BUFFERS_CHANGED) {
ALOGV("Output buffers changed");
err = OK;
} else {
if (err == -EAGAIN /* INFO_TRY_AGAIN_LATER */ && --retriesLeft > 0) {
err = OK;
} else if (err == OK) {
sp<MediaCodecBuffer> videoFrameBuffer;
// 从decoder中根据index获取videoFrameBuffer
err = mDecoder->getOutputBuffer(index, &videoFrameBuffer);
if (err != OK) {
ALOGE("failed to get output buffer %zu", index);
break;
}
// 然后就是等待decoder输出一个output buffer
err = onOutputReceived(videoFrameBuffer, mOutputFormat, ptsUs, &done);
mDecoder->releaseOutputBuffer(index); // 释放buffer
} else {
ALOGW("Received error %d (%s) instead of output", err, asString(err));
done = true;
}
break;
}
}
} while (err == OK && !done);
if (err != OK) {
ALOGE("failed to get video frame (err %d)", err);
}
return err;
}
// 接下来看看decoder解码完的处理
status_t VideoFrameDecoder::onOutputReceived(
const sp<MediaCodecBuffer> &videoFrameBuffer,
const sp<AMessage> &outputFormat,
int64_t timeUs, bool *done) {
bool shouldOutput = (mTargetTimeUs < 0ll) || (timeUs >= mTargetTimeUs);
// If this is not the target frame, skip color convert.
if (!shouldOutput) {
*done = false;
return OK;
}
*done = (++mNumFramesDecoded >= mNumFrames);
if (outputFormat == NULL) {
return ERROR_MALFORMED;
}
int32_t width, height, stride, slice_height;
CHECK(outputFormat->findInt32("width", &width));
CHECK(outputFormat->findInt32("height", &height));
CHECK(outputFormat->findInt32("stride", &stride));
CHECK(outputFormat->findInt32("slice-height", &slice_height));
int32_t crop_left, crop_top, crop_right, crop_bottom;
if (!outputFormat->findRect("crop", &crop_left, &crop_top, &crop_right, &crop_bottom)) {
crop_left = crop_top = 0;
crop_right = width - 1;
crop_bottom = height - 1;
}
sp<IMemory> frameMem = allocVideoFrame( // 创建内存
trackMeta(),
(crop_right - crop_left + 1),
(crop_bottom - crop_top + 1),
0,
0,
dstBpp());
addFrame(frameMem); // 把frameMem加入mFrames
VideoFrame* frame = static_cast<VideoFrame*>(frameMem->pointer());
int32_t srcFormat;
CHECK(outputFormat->findInt32("color-format", &srcFormat));
// 接下来会根据decoder输出的格式和app需要的输出格式判断是否需要颜色转换。此处先忽略。
ColorConverter converter((OMX_COLOR_FORMATTYPE)srcFormat, dstFormat());
if (converter.isValid()) {
converter.convert(
(const uint8_t *)videoFrameBuffer->data(),
stride, slice_height,
crop_left, crop_top, crop_right, crop_bottom,
frame->getFlattenedData(),
frame->mWidth,
frame->mHeight,
crop_left, crop_top, crop_right, crop_bottom);
return OK;
}
ALOGE("Unable to convert from format 0x%08x to 0x%08x",
srcFormat, dstFormat());
return ERROR_UNSUPPORTED;
}
以上就是安卓中整个获取视频缩略图的过程了，改天有空画个流程图就更直观了，就酱~不知道聪明的你看懂了没呢?
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