Android平台MediaCodec避坑指北
2018-02-14 本文已影响2825人
我心若氺
最近使用MediaCodec做编解码H264，写一点东西以免自己再次掉坑。
先说一下具体环境，使用的是，Windows10 + AndroidStudio3.0 + CMake
既然都用了AndroidStudio3.0，就不用android.mk文件啦，直接上CMake。
先上一个，谷歌Android官方链接，可以看到MediaCodec的架构，状态转移图，全部API等。
https://developer.android.com/reference/android/media/MediaCodec.html
这个是Android源码media部分，内含MediaCodec，分别在jni和java目录下。
https://android.googlesource.com/platform/frameworks/base/+/master/media
这个是Android源码ndk c++ media部分，内含MediaCodec，分别在libmedia，libstagefright，ndk目录下。
https://android.googlesource.com/platform/frameworks/av/+/master/media/
这些都要认真上网才能看。
这里有个MediaCodec 中文API文档，文档虽然不是最新版，但是对我们理解还是有帮助的，感谢译者的奉献。
https://www.cnblogs.com/roger-yu/p/5635494.html
一、架构介绍
MediaCodec有输入和输入的缓冲区和队列。
MediaCodec的架构
在MediaCodec的生命周期中，存在三种状态：
Stopped：包含Uninitialized、Configured、Error三种子状态。
Executing：包含Flushed、Running、End-of-Stream三种子状态。
Released。
MediaCodec的状态循环
这些在MediaCodec官方文档都有，很多人翻译的很好。我就不多说了。
二、调用流程简介
Android从API 16开始提供java层的MediaCodec视频硬解码接口；
从API 21，也就是Android 5.0开始提供native层的MediaCodec的接口。
Android调用MediaCodec简单流程的是：
SDK : JavaApi---->JNI---->C++
NDK：NdkApi----->C++
这个博客非常详细的讲了这个流程。我就不赘述了。
http://blog.csdn.net/hejjunlin/article/details/53386117
http://blog.csdn.net/hejjunlin/article/details/53573819
http://blog.csdn.net/hejjunlin/article/details/72859142
最后文章有个最后疑惑点
最后疑惑点
在阅读时，还发现有NdkMediaCodec及NdkMediaCodec.cpp这些个class, 和上面几个class的区别是什么？有什么关系？为什么要这么设计？
frameworks\av\include\ndk\NdkMediaCodec.h
其实这俩就是NDK的Api接口类，这个类最终还是调用的stagefright内的MediaCodec。
三、实现方式
Talk is cheap，show me the code
首先要选择的是：使用SDK还是NDK？
SDK用Java，API16（即Android 4.1）以上的设备都可用。
NDK用C++，API21（集Android 5.0）以上的设备可支持。
据谷歌2018年1月的Android版本统计数据：
Android4.1及以后的版本，占99.1%，我们可以理解为支持所有在市面上的Android设备。
Android4.1~4.4的版本，占18.4%，Android5.0及以后的版本，占80.7%。
特别是Android4.4，占有率达12.8%，不能忽视。
简单计算一下，Android4.4及以上版本，占93.5%，可以视为支持了绝大多数Android设备。
如果使用NDK的话，可能需要考虑下Android4.4的兼容问题，幸好有人从Android4.4源码里抽取了libnative_codec19.so并封装了和NdkMediaCodec一样接口，感谢他的贡献精神。
具体这两种方式，我都要讲讲。
MediaCodec接入的准备工作
从/etc/media_codecs.xml文件里可以看出支持的各种格式，一般来说，又能编码又能解码只有H264。
那么我们就以H264来举例，从MediaCodec reference页面得知，H264使用"video/avc"。
### createDecoderByType
added in [API level 16](https://developer.android.com/guide/topics/manifest/uses-sdk-element.html#ApiLevels)
Instantiate the preferred decoder supporting input data of the given mime type.
The following is a partial list of defined mime types and their semantics:
* "video/x-vnd.on2.vp8" - VP8 video (i.e. video in .webm)
* "video/x-vnd.on2.vp9" - VP9 video (i.e. video in .webm)
* "video/avc" - H.264/AVC video
* "video/hevc" - H.265/HEVC video
* "video/mp4v-es" - MPEG4 video
* "video/3gpp" - H.263 video
* "audio/3gpp" - AMR narrowband audio
* "audio/amr-wb" - AMR wideband audio
* "audio/mpeg" - MPEG1/2 audio layer III
* "audio/mp4a-latm" - AAC audio (note, this is raw AAC packets, not packaged in LATM!)
* "audio/vorbis" - vorbis audio
* "audio/g711-alaw" - G.711 alaw audio
* "audio/g711-mlaw" - G.711 ulaw audio
Note: It is preferred to use findDecoderForFormat(MediaFormat) and createByCodecName(String) to ensure that the resulting codec can handle a given format.
①MediaCodec格式支持
上一段检测MediaCodec格式支持的代码。
private int getSupportColorFormat() {
int numCodecs = MediaCodecList.getCodecCount();
MediaCodecInfo codecInfo = null;
for (int i = 0; i < numCodecs && codecInfo == null; i++) {
MediaCodecInfo info = MediaCodecList.getCodecInfoAt(i);
if (!info.isEncoder()) {
continue;
}
String[] types = info.getSupportedTypes();
boolean found = false;
for (int j = 0; j < types.length && !found; j++) {
if (types[j].equals("video/avc")) {
System.out.println("found");
found = true;
}
}
if (!found)
continue;
codecInfo = info;
}
Log.e("AvcEncoder", "Found " + codecInfo.getName() + " supporting " + "video/avc");
// Find a color profile that the codec supports
MediaCodecInfo.CodecCapabilities capabilities = codecInfo.getCapabilitiesForType("video/avc");
Log.e("AvcEncoder",
"length-" + capabilities.colorFormats.length + "==" + Arrays.toString(capabilities.colorFormats));
for (int i = 0; i < capabilities.colorFormats.length; i++) {
switch (capabilities.colorFormats[i]) {
case MediaCodecInfo.CodecCapabilities.COLOR_FormatYUV420SemiPlanar:
case MediaCodecInfo.CodecCapabilities.COLOR_FormatYUV420Planar:
case MediaCodecInfo.CodecCapabilities.COLOR_FormatYUV420Flexible:
Log.e("AvcEncoder", "supported color format::" + capabilities.colorFormats[i]);
break;
default:
Log.e("AvcEncoder", "other color format " + capabilities.colorFormats[i]);
break;
}
}
//return capabilities.colorFormats[i];
return 0;
}
可以由此得知Android设备具体支持的格式数量和格式枚举值。
这个MediaCodecInfo和MediaCodecList在NDKC++并未暴露出接口，
所以NDKC++代码不能调用，无法获得Android设备具体支持的格式数量和格式枚举值。
（如果有办法调用，或者得知具体支持的格式枚举值，请不吝赐教。）
②Codec输入编码格式
得知Android设备具体支持的格式枚举值后，大多数设备都会支持这几种格式。
public static final class CodecCapabilities {
/** @deprecated */
@Deprecated
public static final int COLOR_FormatYUV420Planar = 19;
/** @deprecated */
@Deprecated
public static final int COLOR_FormatYUV420SemiPlanar = 21;
public static final int COLOR_FormatYUV422Flexible = 2135042184;
public class ImageFormat {
public static final int DEPTH16 = 1144402265;
public static final int DEPTH_POINT_CLOUD = 257;
public static final int FLEX_RGBA_8888 = 42;
public static final int FLEX_RGB_888 = 41;
public static final int JPEG = 256;
public static final int NV16 = 16;
public static final int NV21 = 17;
public static final int PRIVATE = 34;
public static final int RAW10 = 37;
public static final int RAW12 = 38;
public static final int RAW_PRIVATE = 36;
public static final int RAW_SENSOR = 32;
public static final int RGB_565 = 4;
public static final int UNKNOWN = 0;
public static final int YUV_420_888 = 35;
public static final int YUV_422_888 = 39;
public static final int YUV_444_888 = 40;
public static final int YUY2 = 20;
public static final int YV12 = 842094169;
其中:
CodecCapabilities.COLOR_FormatYUV420Planar，相当于ImageFormat.YV12
CodecCapabilities.COLOR_FormatYUV420SemiPlanar，相当于ImageFormat.NV21
（我有一篇写YUV格式的文章，详细描述各种格式。）
但在有些Android设备上，却是UV反过来的:
CodecCapabilities.COLOR_FormatYUV420Planar是 I420,
CodecCapabilities.COLOR_FormatYUV420SemiPlanar是 NV12,
（暂不清楚原因，望不吝赐教）
MediaCodec SDK接入
①创建Encoder，设置各种参数。
public AvcEncoder(int width, int height, int framerate, int bitrate) {
Log.d("Codec", "AvcEncoder IN");
m_width = width;
m_height = height;
yuv420 = new byte[width*height*3/2];
try {
mediaCodec = MediaCodec.createEncoderByType("video/avc");
} catch (IOException e) {
e.printStackTrace();
}
MediaFormat mediaFormat = MediaFormat.createVideoFormat("video/avc", width, height);
mediaFormat.setInteger(MediaFormat.KEY_BIT_RATE, bitrate);
mediaFormat.setInteger(MediaFormat.KEY_FRAME_RATE, framerate);
mediaFormat.setInteger(MediaFormat.KEY_COLOR_FORMAT, MediaCodecInfo.CodecCapabilities.COLOR_FormatYUV420SemiPlanar);
mediaFormat.setInteger(MediaFormat.KEY_I_FRAME_INTERVAL, 30);
mediaCodec.configure(mediaFormat, null, null, MediaCodec.CONFIGURE_FLAG_ENCODE);
mediaCodec.start();
Log.d("Codec", "AvcEncoder OUT");
}
②创建Decoder，设置各种参数。
public AvcDecoder(int width, int height, SurfaceHolder surfaceHolder) {
Log.d("Codec", "AvcDecoder IN");
try {
mediaCodec = MediaCodec.createDecoderByType("video/avc");
MediaFormat mediaFormat = MediaFormat.createVideoFormat("video/avc", width, height);
nv12 = new byte[width * height * 3 / 2];
mediaFormat.setInteger(MediaFormat.KEY_COLOR_FORMAT, MediaCodecInfo.CodecCapabilities.COLOR_FormatYUV420SemiPlanar);
//mediaCodec.configure(mediaFormat, surfaceHolder.getSurface(), null, 0);
mediaCodec.configure(mediaFormat, null, null, 0);
mediaCodec.start();
Log.d("Codec", "AvcDecoder OUT");
} catch (IOException e) {
e.printStackTrace();
}
}
③获取Encoder InputBuffer，输入数据，获取Encoder OutputBuffer，获取输出数据
public int offerEncoder(byte[] input, byte[] output)
{
Log.d("......................................................Codec", "Encoder in");
int pos = 0;
yuv420= input;
try {
ByteBuffer[] inputBuffers = mediaCodec.getInputBuffers();
ByteBuffer[] outputBuffers = mediaCodec.getOutputBuffers();
int inputBufferIndex = mediaCodec.dequeueInputBuffer(-1);
Log.d("......................................................Codec", "inputBufferIndex = " +inputBufferIndex);
if (inputBufferIndex >= 0)
{
ByteBuffer inputBuffer = inputBuffers[inputBufferIndex];
inputBuffer.clear();
inputBuffer.put(yuv420);
mediaCodec.queueInputBuffer(inputBufferIndex, 0, input.length, 0, 0);
}
MediaCodec.BufferInfo bufferInfo = new MediaCodec.BufferInfo();
int outputBufferIndex = mediaCodec.dequeueOutputBuffer(bufferInfo,0);
Log.d("......................................................Codec", "outputBufferIndex = " +outputBufferIndex);
while (outputBufferIndex >= 0)
{
ByteBuffer outputBuffer = outputBuffers[outputBufferIndex];
byte[] outData = new byte[bufferInfo.size];
outputBuffer.get(outData);
if(m_info != null)
{
System.arraycopy(outData, 0, output, 0, outData.length);
pos += outData.length;
Log.d("Encoder", "m_info: " + pos);
}
else
{
ByteBuffer spsPpsBuffer = ByteBuffer.wrap(outData);
//if (spsPpsBuffer.getInt() == 0x00000001)
if(bufferInfo.flags == 2)
{
m_info = new byte[outData.length];
System.arraycopy(outData, 0, m_info, 0, outData.length);
System.arraycopy(outData, 0, output, pos, outData.length);
pos+=outData.length;
}
else
{
Log.d("Encoder", "errrrr: ");
return -1;
}
Log.d("Encoder", "m_info: " + Arrays.toString(m_info));
}
mediaCodec.releaseOutputBuffer(outputBufferIndex, false);
outputBufferIndex = mediaCodec.dequeueOutputBuffer(bufferInfo, 0);
}
if(bufferInfo.flags == 1)// if( nv12[4] == 0x65) //key frame
{
Log.d("Encoder", "Key frame");
System.arraycopy(output, 0, yuv420, 0, pos);
System.arraycopy(m_info, 0, output, 0, m_info.length);
System.arraycopy(yuv420, 0, output, m_info.length, pos);
pos += m_info.length;
}
} catch (Throwable t) {
t.printStackTrace();
}
//Log.d("......................................................Codec", "Encoder out");
return pos;
}
④获取Decoder InputBuffer，输入数据，获取Decoder OutputBuffer，获取输出数据
public void onFrame(byte[] buf, int length) {
ByteBuffer[] inputBuffers = mediaCodec.getInputBuffers();
ByteBuffer[] outputBuffers = mediaCodec.getOutputBuffers();
int inputBufferIndex = mediaCodec.dequeueInputBuffer(-1);
Log.d("Decoder", "inputBufferIndex: " + inputBufferIndex);
if (inputBufferIndex >= 0) {
ByteBuffer inputBuffer = inputBuffers[inputBufferIndex];
inputBuffer.clear();
inputBuffer.put(buf, 0, length);
mediaCodec.queueInputBuffer(inputBufferIndex, 0, length, mCount * 1000000, 0);
mCount++;
}
MediaCodec.BufferInfo bufferInfo = new MediaCodec.BufferInfo();
int outputBufferIndex = mediaCodec.dequeueOutputBuffer(bufferInfo, 0);
while (outputBufferIndex >= 0) {
outputBuffers[outputBufferIndex].get(nv12 , 0 , nv12.length);
CallbackAdapt.UpdateH264Decode(nv12, outputBufferIndex);
mediaCodec.releaseOutputBuffer(outputBufferIndex, false);
outputBufferIndex = mediaCodec.dequeueOutputBuffer(bufferInfo, 0);
}
}
MediaCodec NDK接入
①创建Encoder，Decoder
void FrameListener::InitCodec()
{
#ifndef WIN32
const char* mime = "video/avc";
//编码器
m_encoder = AMediaCodec_createEncoderByType(mime);
if (m_encoder == NULL)
{
LOGE("MediaCodecH264: could not create Encoder");
}
AMediaFormat *m_format = AMediaFormat_new();
AMediaFormat_setString(m_format, AMEDIAFORMAT_KEY_MIME, "video/avc");
AMediaFormat_setInt32(m_format, AMEDIAFORMAT_KEY_WIDTH, m_lastWidth);
AMediaFormat_setInt32(m_format, AMEDIAFORMAT_KEY_HEIGHT, m_lastHeight);
int bitrate = 500000;
int framerate = 30;
AMediaFormat_setInt32(m_format, AMEDIAFORMAT_KEY_BIT_RATE, bitrate);
AMediaFormat_setInt32(m_format, AMEDIAFORMAT_KEY_FRAME_RATE, framerate);
AMediaFormat_setInt32(m_format, AMEDIAFORMAT_KEY_I_FRAME_INTERVAL, framerate);
AMediaFormat_setInt32(m_format, AMEDIAFORMAT_KEY_COLOR_FORMAT, 21);
media_status_t status = AMediaCodec_configure(m_encoder, m_format, NULL, NULL, AMEDIACODEC_CONFIGURE_FLAG_ENCODE);
if (status != 0)
{
LOGE("AMediaCodec_configure() failed with error %i for format %u", (int)status, 21);
}
else
{
if ((status = AMediaCodec_start(m_encoder)) != AMEDIA_OK)
{
LOGE("AMediaCodec_start: Could not start encoder.");
}
else
{
LOGD("AMediaCodec_start: encoder successfully started");
}
}
AMediaFormat_delete(m_format);
//解码器
m_decoder = AMediaCodec_createDecoderByType(mime);
if (m_decoder == NULL)
{
LOGE("MediaCodecH264: could not create Decoder");
}
else
{
AMediaFormat *m_format2 = AMediaFormat_new();
AMediaFormat_setString(m_format2, AMEDIAFORMAT_KEY_MIME, "video/avc");
AMediaFormat_setInt32(m_format2, AMEDIAFORMAT_KEY_WIDTH, m_lastWidth);
AMediaFormat_setInt32(m_format2, AMEDIAFORMAT_KEY_HEIGHT, m_lastHeight);
AMediaFormat_setInt32(m_format2, AMEDIAFORMAT_KEY_COLOR_FORMAT, 21);
//AMediaFormat_setBuffer(m_format2, "csd-0",sps, sizeof(sps));
//AMediaFormat_setBuffer(m_format2, "csd-1",pps, sizeof(pps));
if ((status = AMediaCodec_configure(m_decoder, m_format2, NULL, NULL, 0)) !=
AMEDIA_OK) {
LOGD("MediaCodecH264Dec: configuration failure: %i", (int)status);
}
if ((status = AMediaCodec_start(m_decoder)) != AMEDIA_OK) {
LOGD("MediaCodecH264Dec: starting failure: %i", (int)status);
}
AMediaFormat_delete(m_format2);
}
#endif // !WIN32
}
②获取Encoder InputBuffer，然后输入数据，获取Encoder OutputBuffer，然后获取输出数据
void FrameListener::Encode()
{
#ifndef WIN32
ssize_t ibufidx, obufidx;
AMediaCodecBufferInfo info;
size_t bufsize;
/*First queue input image*/
uint8_t *buf;
ibufidx = AMediaCodec_dequeueInputBuffer(m_encoder, TIMEOUT_US);
if (ibufidx >= 0)
{
buf = AMediaCodec_getInputBuffer(m_encoder, ibufidx, &bufsize);
if (buf)
{
memcpy(buf, m_inputNV12, m_YUVSize);
auto curTime = timeGetTime();
AMediaCodec_queueInputBuffer(m_encoder, ibufidx, 0, bufsize, curTime, 0);
}
else
{
LOGD("MediaCodecH264Enc: obtained InputBuffer, but no address.");
}
}
else if (ibufidx == AMEDIA_ERROR_UNKNOWN)
{
LOGD("MediaCodecH264Enc: AMediaCodec_dequeueInputBuffer() had an exception");
}
//int pos = 0;
/*Second, dequeue possibly pending encoded frames*/
while ((obufidx = AMediaCodec_dequeueOutputBuffer(m_encoder, &info, TIMEOUT_US)) >= 0)
{
auto oBuf = AMediaCodec_getOutputBuffer(m_encoder, obufidx, &bufsize);
if (oBuf)
{
if (m_info == NULL)
{
m_infoSize = info.size;
m_info = new byte[m_infoSize];
if (info.flags == 2 )
{
memcpy(m_info, oBuf, m_infoSize);
LOGD("obBuf %d %d flag:%d offest:%d size:%d", m_infoSize, bufsize, info.flags, info.offset, info.size);
char str[256] = {0};
for (int i = 0; i < m_infoSize; ++i)
{
sprintf(str, "%s %d", str, m_info[i]);
}
LOGD("obBuf %s", str);
//pos += m_infoSize;
continue;
}
else
{
LOGD("errorrr");
return;
}
}
LOGD("m_infoSize %d %d flag:%d offest:%d size:%d", m_infoSize, bufsize, info.flags, info.offset, info.size);
H264Data *data = new H264Data();
m_dataList.push_back(data);
data->flag = info.flags;
if (info.flags == 1 ) //key frame
{
data->dataPtr = new byte[bufsize + m_infoSize];
memcpy(data->dataPtr, m_info, m_infoSize);
memcpy(data->dataPtr + m_infoSize, oBuf, bufsize);
data->size = bufsize + m_infoSize;
}
else
{
data->dataPtr = new byte[bufsize];
memcpy(data->dataPtr, oBuf, bufsize);
data->size = bufsize;
}
LOGD("Out finish");
}
AMediaCodec_releaseOutputBuffer(m_encoder, obufidx, false);
}
if (obufidx == AMEDIA_ERROR_UNKNOWN)
{
LOGD("MediaCodecH264Enc: AMediaCodec_dequeueOutputBuffer() had an exception, MediaCodec is lost");
AMediaCodec_stop(m_encoder);
AMediaCodec_delete(m_encoder);
}
#endif // !WIN32
}
③获取Decoder InputBuffer，然后输入数据，获取Decoder OutputBuffer，然后获取输出数据
void FrameListener::Decode()
{
#ifndef WIN32
if (m_decoder == NULL)
{
return;
}
ssize_t oBufidx = -1;
size_t bufsize = 0;
AMediaCodecBufferInfo info;
uint8_t *buf = NULL;
ssize_t iBufidx = -1;
/*First put our H264 bitstream into the decoder*/
while (!m_dataList.empty())
{
iBufidx = AMediaCodec_dequeueInputBuffer(m_decoder, TIMEOUT_US);
LOGD("decoder iBufidx %d %d", iBufidx, m_dataList.size());
if (iBufidx >= 0)
{
buf = AMediaCodec_getInputBuffer(m_decoder, iBufidx, &bufsize);
int bufsize = 0;
auto iter = m_dataList.begin();
char str[512] = { 0 };
for (int i = 0; i < 100; ++i)
{
sprintf(str, "%s %d", str, *((*iter)->dataPtr + i));
}
LOGD("obBuf after %s", str);
if (buf)
{
bufsize = (*iter)->size;
memcpy(buf, (*iter)->dataPtr, bufsize);
}
AMediaCodec_queueInputBuffer(m_decoder, iBufidx, 0, bufsize, timeGetTime(), 0);
SAFE_DELETE_ARRAY((*iter)->dataPtr);
m_dataList.erase(iter);
}
else if (iBufidx == -1)
{
/*
* This is a problematic case because we can't wait the decoder to be ready, otherwise we'll freeze the entire
* video thread.
* We have no other option to drop the frame, and retry later, but with an I-frame of course.
**/
break;
}
}
/*secondly try to get decoded frames from the decoder, this is performed every tick*/
oBufidx = AMediaCodec_dequeueOutputBuffer(m_decoder, &info, TIMEOUT_US);
LOGD("Decoder oBufidx %d", oBufidx);
while (oBufidx >= 0)
{
AMediaFormat *format;
int color = 0;
uint8_t *buf = AMediaCodec_getOutputBuffer(m_decoder, oBufidx, &bufsize);
if (buf == NULL)
{
LOGD("MediaCodecH264Dec: AMediaCodec_getOutputBuffer() returned NULL");
//continue;
}
else
{
int width = 0, height = 0;
format = AMediaCodec_getOutputFormat(m_decoder);
if (format != NULL)
{
AMediaFormat_getInt32(format, "width", &width);
AMediaFormat_getInt32(format, "height", &height);
AMediaFormat_getInt32(format, "color-format", &color);
AMediaFormat_delete(format);
}
if (width != 0 && height != 0)
{
if (color == 21)
{
LOGD("12121212");
//NV12
byte* outNV12 = new byte[m_YUVSize];
memcpy(outNV12, buf, m_YUVSize);
m_outputNV12List.push_back(outNV12);
}
else
{
LOGD("unknown format");
}
}
else
{
LOGD("MediaCodecH264Dec: width and height are not known !");
}
}
AMediaCodec_releaseOutputBuffer(m_decoder, oBufidx, false);
oBufidx = AMediaCodec_dequeueOutputBuffer(m_decoder, &info, TIMEOUT_US);
LOGD("Decoder oBufidx %d", oBufidx);
}
if (oBufidx == AMEDIA_ERROR_UNKNOWN)
{
LOGD("MediaCodecH264Dec: AMediaCodec_dequeueOutputBuffer() had an exception");
}
#endif // !WIN32
}
MediaCodec NDK CMake
Android NDK Sample里面自带一个native-codec的demo，可以参考他的CMakeLists.txt。
cmake_minimum_required(VERSION 3.4.1)
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11 -Wall -UNDEBUG")
add_library(native-codec-jni SHARED
looper.cpp
native-codec-jni.cpp)
# Include libraries needed for native-codec-jni lib
target_link_libraries(native-codec-jni
android
log
mediandk
OpenMAXAL)
一定要加上mediandk和OpenMAXAL，才能正常使用MediaCodec。
常见坑点FAQ
假设读者知道H264帧头和NALU
1、H264编码首帧，内部存有SPS和PPS信息，需要保留起来，然后，加在每个H264关键帧的前面。
mediaCodec.dequeueOutputBuffer会返回MediaCodec.BufferInfo，这个和AMediaCodec_dequeueOutputBuffer返回AMediaCodecBufferInfo是一样的。
struct AMediaCodecBufferInfo {
int32_t offset;
int32_t size;
int64_t presentationTimeUs;
uint32_t flags;
};
具体见android reference：
https://developer.android.com/reference/android/media/MediaCodec.BufferInfo.html
其中有个字段是flags，它有几种常量情况。
flags = 4；End of Stream。
flags = 2；首帧信息帧。
flags = 1；关键帧。
flags = 0；普通帧。
见https://developer.android.com/reference/android/media/MediaCodec.html里面常量部分。
2、dequeueInputBuffer，dequeueOutputBuffer返回值非0
dequeueInputBuffer返回-1，说明没有拿到可用缓冲区，一般来说下一帧再来请求输入缓冲区。
dequeueOutputBuffer返回-1，说明没有可用数据。
然而dequeueOutputBuffer可能连续输入4、5帧，都没有输出，然后一股脑在1帧中，输出4、5个可用输出缓冲区。
其他异常情况包括INFO_OUTPUT_FORMAT_CHANGED，INFO_OUTPUT_BUFFERS_CHANGED等
见https://developer.android.com/reference/android/media/MediaCodec.html里面常量部分。
3、MediaCodec格式不一致
Android对COLOR_FormatYUV420SemiPlanar和COLOR_FormatYUV420Planar，这两种常见YUV格式并没有做进一步的规定，
那么问题就来了，420P和420SP分为I420,YV12,NV12,NV21，
I420: YYYYYYYY UU VV =>YUV420P
YV12: YYYYYYYY VV UU =>YUV420P
NV12: YYYYYYYY UVUV =>YUV420SP
NV21: YYYYYYYY VUVU =>YUV420SP
有些设备可能是I420+NV12，这两种U都是在前面的，
有些设备可能是YV12+NV12，这两种V都是在前面的。
其他的组合，我也不确定有没有。
反正这个坑，要注意的。
来个Demo
Github
待续
