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"。

### createDecoderByTypeadded 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 WIN32const 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 WIN32ssize_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 WIN32if (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 SHAREDlooper.cppnative-codec-jni.cpp)# Include libraries needed for native-codec-jni lib
target_link_libraries(native-codec-jniandroidlog 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