opengl es 中MediaCodec通过C++先将MP4视频解码为YUV在将YUV编码成MP4视频例子

首先，需要在Android项目中引入MediaCodec和OpenGLES的相关库：

import android.media.MediaCodec;
import android.media.MediaCodecInfo;
import android.media.MediaFormat;
import android.media.MediaMuxer;
import android.opengl.EGL14;
import android.opengl.EGLContext;
import android.opengl.GLES20;
import android.opengl.GLUtils;
import android.os.Environment;

import java.io.File;
import java.io.IOException;
import java.nio.ByteBuffer;

接着，创建一个名为VideoEncoder的类，用于实现视频编码：

public class VideoEncoder {

    private static final String MIME_TYPE = "video/avc";
    private static final int FRAME_RATE = 25;
    private static final int I_FRAME_INTERVAL = 10;
    private static final int TIMEOUT_US = 10000;

    private MediaCodec mEncoder;
    private MediaFormat mFormat;
    private MediaMuxer mMuxer;
    private int mTrackIndex;
    private boolean mMuxerStarted;

    private int mWidth;
    private int mHeight;
    private EGLContext mEglContext;
    private int mTextureId;

    public VideoEncoder(int width, int height, EGLContext eglContext, int textureId) {
        mWidth = width;
        mHeight = height;
        mEglContext = eglContext;
        mTextureId = textureId;
    }

    public void start(String outputPath) throws IOException {
        mFormat = MediaFormat.createVideoFormat(MIME_TYPE, mWidth, mHeight);
        mFormat.setInteger(MediaFormat.KEY_COLOR_FORMAT, MediaCodecInfo.CodecCapabilities.COLOR_FormatSurface);
        mFormat.setInteger(MediaFormat.KEY_BIT_RATE, mWidth * mHeight * 4);
        mFormat.setInteger(MediaFormat.KEY_FRAME_RATE, FRAME_RATE);
        mFormat.setInteger(MediaFormat.KEY_I_FRAME_INTERVAL, I_FRAME_INTERVAL);

        mEncoder = MediaCodec.createEncoderByType(MIME_TYPE);
        mEncoder.configure(mFormat, null, null, MediaCodec.CONFIGURE_FLAG_ENCODE);
        mEncoder.start();

        mMuxer = new MediaMuxer(outputPath, MediaMuxer.OutputFormat.MUXER_OUTPUT_MPEG_4);
        mTrackIndex = -1;
        mMuxerStarted = false;
    }

    public void stop() {
        if (mEncoder != null) {
            mEncoder.stop();
            mEncoder.release();
            mEncoder = null;
        }

        if (mMuxer != null) {
            if (mMuxerStarted) {
                mMuxer.stop();
            }
            mMuxer.release();
            mMuxer = null;
            mTrackIndex = -1;
            mMuxerStarted = false;
        }
    }

    public void drainEncoder(boolean endOfStream) {
        final int TIMEOUT_USEC = 10000;
        if (endOfStream) {
            mEncoder.signalEndOfInputStream();
        }

        MediaCodec.BufferInfo bufferInfo = new MediaCodec.BufferInfo();
        while (true) {
            int encoderStatus = mEncoder.dequeueOutputBuffer(bufferInfo, TIMEOUT_USEC);
            if (encoderStatus == MediaCodec.INFO_TRY_AGAIN_LATER) {
                if (!endOfStream) {
                    break;
                }
            } else if (encoderStatus == MediaCodec.INFO_OUTPUT_FORMAT_CHANGED) {
                if (mMuxerStarted) {
                    throw new RuntimeException("format changed twice");
                }
                MediaFormat newFormat = mEncoder.getOutputFormat();
                mTrackIndex = mMuxer.addTrack(newFormat);
                mMuxer.start();
                mMuxerStarted = true;
            } else if (encoderStatus < 0) {
                throw new RuntimeException("unexpected result from encoder.dequeueOutputBuffer: " + encoderStatus);
            } else {
                ByteBuffer encodedData = mEncoder.getOutputBuffer(encoderStatus);
                if (encodedData == null) {
                    throw new RuntimeException("encoderOutputBuffer " + encoderStatus + " was null");
                }

                if ((bufferInfo.flags & MediaCodec.BUFFER_FLAG_CODEC_CONFIG) != 0) {
                    bufferInfo.size = 0;
                }
                if (bufferInfo.size != 0) {
                    if (!mMuxerStarted) {
                        throw new RuntimeException("muxer hasn't started");
                    }

                    encodedData.position(bufferInfo.offset);
                    encodedData.limit(bufferInfo.offset + bufferInfo.size);
                    mMuxer.writeSampleData(mTrackIndex, encodedData, bufferInfo);
                }

                mEncoder.releaseOutputBuffer(encoderStatus, false);

                if ((bufferInfo.flags & MediaCodec.BUFFER_FLAG_END_OF_STREAM) != 0) {
                    if (!endOfStream) {
                        throw new RuntimeException("reached end of stream unexpectedly");
                    }
                    break;
                }
            }
        }
    }

    public void encodeFrame() {
        // Set up the EGL context and surface for offscreen rendering
        OffscreenSurface surface = new OffscreenSurface(mEglContext, mWidth, mHeight);
        surface.makeCurrent();

        // Set up the texture renderer
        TextureRenderer renderer = new TextureRenderer();
        renderer.surfaceCreated();
        renderer.setExternalTexture(mTextureId);
        renderer.setRenderSize(mWidth, mHeight);

        // Draw the frame
        GLES20.glViewport(0, 0, mWidth, mHeight);
        renderer.surfaceChanged();
        renderer.drawFrame();

        // Read the pixel data into a buffer
        ByteBuffer pixelBuffer = ByteBuffer.allocateDirect(mWidth * mHeight * 4);
        GLES20.glReadPixels(0, 0, mWidth, mHeight, GLES20.GL_RGBA, GLES20.GL_UNSIGNED_BYTE, pixelBuffer);

        // Convert the pixel data from RGBA to YUV420
        byte[] yuv420 = new byte[mWidth * mHeight * 3 / 2];
        NV21Converter.RGBtoYUV420SemiPlanar(pixelBuffer.array(), mWidth, mHeight, yuv420);

        // Encode the frame and write it to the muxer
        int inputBufferIndex = mEncoder.dequeueInputBuffer(TIMEOUT_US);
        if (inputBufferIndex >= 0) {
            ByteBuffer inputBuffer = mEncoder.getInputBuffer(inputBufferIndex);
            inputBuffer.clear();
            inputBuffer.put(yuv420);
            mEncoder.queueInputBuffer(inputBufferIndex, 0, yuv420.length, System.nanoTime() / 1000, 0);
        }

        // Release the resources
        surface.release();
    }
}

其中，OffscreenSurface类用于创建离屏渲染的EGLContext和Surface，NV21Converter类用于将RGBA格式的像素数据转换为YUV420格式的数据。

最后，我们可以在GLSurfaceView中使用VideoEncoder类进行视频编码：

public class MainActivity extends AppCompatActivity {

    private GLSurfaceView mGLSurfaceView;
    private VideoEncoder mVideoEncoder;
    private int mTextureId;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mGLSurfaceView = findViewById(R.id.glsurfaceview);
        mGLSurfaceView.setEGLContextClientVersion(2);
        mGLSurfaceView.setRenderer(new Renderer());
        mGLSurfaceView.setRenderMode(GLSurfaceView.RENDERMODE_CONTINUOUSLY);

        mTextureId = TextureHelper.createTexture();
        mVideoEncoder = new VideoEncoder(640, 480, EGL14.eglGetCurrentContext(), mTextureId);
        try {
            String outputPath = Environment.getExternalStorageDirectory().getAbsolutePath() + "/output.mp4";
            mVideoEncoder.start(outputPath);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        mVideoEncoder.stop();
        GLES20.glDeleteTextures(1, new int[]{mTextureId}, 0);
    }

    private class Renderer implements GLSurfaceView.Renderer {

        @Override
        public void onSurfaceCreated(GL10 gl, EGLConfig config) {
            GLES20.glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
        }

        @Override
        public void onSurfaceChanged(GL10 gl, int width, int height) {
            GLES20.glViewport(0, 0, width, height);
        }

        @Override
        public void onDrawFrame(GL10 gl) {
            GLES20.glClear(GLES20.GL_COLOR_BUFFER_BIT);

            // Render the texture
            GLES20.glUseProgram(TextureRenderer.DEFAULT_VERTEX_SHADER_PROGRAM);
            TextureRenderer.renderTexture(mTextureId);

            // Encode the frame
            mVideoEncoder.encodeFrame();
        }
    }
}

在GLSurfaceView的回调函数onDrawFrame中，我们首先使用TextureRenderer类将纹理渲染到屏幕上，然后调用VideoEncoder类的encodeFrame方法，将当前帧的像素数据编码为H.264格式的视频帧，并写入MP4文件中。