跨平台xamarin.Android 开发之 ：适配各架构(X86_64 、 X86、arm64-v8a、 armeabi-v7a )FFmpeg 解码

此代码的编写花费了脑细胞：在每次解码开启解码器到只需要一次解码器的开启优化

前提：编译好FFMpeg 的各平台的动态库

Windows 、Android (X86_64 、 X86、arm64-v8a、 armeabi-v7a ) 解码 相对编码要简单一些，因为不涉及到AVFrame 取指转换

解码包括：创建解码器、解码、释放解码器

using System;
using System.Drawing;
using System.Runtime.InteropServices;

namespace FFmpegAnalyzer
{
    /// <summary>
    /// 解码器
    /// </summary>
    internal unsafe class FFmpegDecoder
    {
        /// <param name="decodedFrameSize">解码后一帧数据的大小</param>
        /// <param name="isRgb">Rgb数据</param>
        public FFmpegDecoder(Size decodedFrameSize, bool isRgb )
        {
            _decodedFrameSize = decodedFrameSize;
            _isRgb = isRgb;
        }

        /// <summary>
        /// 创建解码器
        /// </summary>
        /// <param name="codecFormat">解码格式</param>
        public void CreateDecoder(AVCodecID codecFormat)
        {
            var originPixelFormat = AVPixelFormat.AV_PIX_FMT_YUV420P;
            var destinationPixelFormat = _isRgb ? AVPixelFormat.AV_PIX_FMT_RGB24 : AVPixelFormat.AV_PIX_FMT_BGRA; 

            //获取解码器
            _pDecodec = FFmpeg.avcodec_find_decoder(codecFormat);
            if (_pDecodec == null) throw new InvalidOperationException("Codec not found.");

            _pDecodecContext = FFmpeg.avcodec_alloc_context3(_pDecodec);
            _pDecodecContext->width = _decodedFrameSize.Width;
            _pDecodecContext->height = _decodedFrameSize.Height;
            _pDecodecContext->time_base = new AVRational { num = 1, den = 30 };
            _pDecodecContext->pix_fmt = originPixelFormat;
            _pDecodecContext->framerate = new AVRational { num = 30, den = 1 };                        
            _pDecodecContext->gop_size = 30;
            // 设置预测算法
            _pDecodecContext->flags |= FFmpeg.AV_CODEC_FLAG_PSNR;
            _pDecodecContext->flags2 |= FFmpeg.AV_CODEC_FLAG2_FAST;
            _pDecodecContext->max_b_frames = 0;
            FFmpeg.av_opt_set(_pDecodecContext->priv_data, "preset", "veryfast", 0);
            FFmpeg.av_opt_set(_pDecodecContext->priv_data, "tune", "zerolatency", 0);
            //打开解码器
            FFmpeg.avcodec_open2(_pDecodecContext, _pDecodec, null).ThrowExceptionIfError();
            _pConvertContext = FFmpeg.sws_getContext(_decodedFrameSize.Width, _decodedFrameSize.Height,
                originPixelFormat, _decodedFrameSize.Width, _decodedFrameSize.Height,
                destinationPixelFormat,
               FFmpeg.SWS_FAST_BILINEAR,
                null, null, null);
            if (_pConvertContext == null)
                throw new ApplicationException("Could not initialize the conversion context.");

            var convertedFrameBufferSize = FFmpeg.av_image_get_buffer_size(destinationPixelFormat, _decodedFrameSize.Width, _decodedFrameSize.Height, 4);
            _convertedFrameBufferPtr = Marshal.AllocHGlobal(convertedFrameBufferSize);
            _dstData = new BytePtr4();
            _dstLineSize = new Int4();

            FFmpeg.av_image_fill_arrays(ref _dstData, ref _dstLineSize, (byte*)_convertedFrameBufferPtr, destinationPixelFormat,
                _decodedFrameSize.Width, _decodedFrameSize.Height, 4);
            _isCodecRunning = true;
        }

        /// <summary>
        /// 解码
        /// </summary>
        /// <param name="frameBytes"></param>
        /// <returns></returns>
        public  byte[] DecodeFrames(byte[] frameBytes)
        {
            if (!_isCodecRunning)
            {
                throw new InvalidOperationException("解码器未运行!");
            }
            var waitDecodePacket = FFmpeg.av_packet_alloc();
            var waitDecoderFrame = FFmpeg.av_frame_alloc();
            FFmpeg.av_frame_unref(waitDecoderFrame);
            fixed (byte* waitDecodeData = frameBytes)
            {
                waitDecodePacket->data = waitDecodeData;
                waitDecodePacket->size = frameBytes.Length;
                FFmpeg.av_frame_unref(waitDecoderFrame);
                try
                {
                    int error;
                    do
                    {
                        FFmpeg.avcodec_send_packet(_pDecodecContext, waitDecodePacket);
                        error = FFmpeg.avcodec_receive_frame(_pDecodecContext, waitDecoderFrame);
                    } while (error == FFmpeg.AVERROR(FFmpeg.EAGAIN));
                }
                finally
                {
                    FFmpeg.av_packet_unref(waitDecodePacket);
                }

                var decodeAfterFrame = ConvertToRgb(waitDecoderFrame, _decodedFrameSize);

                var length = _isRgb ? decodeAfterFrame.height * decodeAfterFrame.width * 3
                    : decodeAfterFrame.height * decodeAfterFrame.width * 4;

                byte[] buffer = new byte[length];
                Marshal.Copy((IntPtr)decodeAfterFrame.data[0], buffer, 0, buffer.Length);
                return buffer;
            }
        }

        /// <summary>
        /// 释放
        /// </summary>
        public  void Dispose()
        {
            _isCodecRunning = false;
            //释放解码器
            FFmpeg.avcodec_close(_pDecodecContext);
            FFmpeg.av_free(_pDecodecContext);
            //释放转换器
            Marshal.FreeHGlobal(_convertedFrameBufferPtr);
            FFmpeg.sws_freeContext(_pConvertContext);
        }

        /// <summary>
        /// 转换成Rgb
        /// </summary>
        /// <param name="waitDecoderFrame"></param>
        /// <param name="detSize">变化后目标大小</param>
        /// <returns></returns>
        private AVFrame ConvertToRgb(AVFrame* waitDecoderFrame, Size detSize)
        {
            FFmpeg.sws_scale(_pConvertContext, waitDecoderFrame->data, waitDecoderFrame->linesize, 0, waitDecoderFrame->height, _dstData, _dstLineSize).ThrowExceptionIfError(); 
            var decodeAfterData = new BytePtr8();
            decodeAfterData.UpdateFrom(_dstData);
            var lineSize = new Int8();
            lineSize.UpdateFrom(_dstLineSize);

            FFmpeg.av_frame_unref(waitDecoderFrame);
            return new AVFrame
            {
                data = decodeAfterData,
                linesize = lineSize,
                width = detSize.Width,
                height = detSize.Height
            };
        }

        //解码器
        private AVCodec* _pDecodec;
        private AVCodecContext* _pDecodecContext;
        //转换缓存区
        private IntPtr _convertedFrameBufferPtr;
        private BytePtr4 _dstData;
        private Int4 _dstLineSize;
        //格式转换
        private SwsContext* _pConvertContext;

        //解码后一帧数据的大小
        private Size _decodedFrameSize;

        //三通道
        private readonly bool _isRgb;
        //解码器正在运行
        private bool _isCodecRunning;
    }
}