AV_CODEC_ID_NONE

AV_CODEC_ID_NONE是FFmpeg库中的一个特殊的AVCodecID枚举值。它表示没有与之对应的有效的编解码器。

当调用avcodec_find_encoder_by_name或avcodec_find_decoder_by_name函数时，如果未找到指定名称的编解码器，这些函数将返回NULL，并且AVCodec的id字段将被设置为AV_CODEC_ID_NONE。

因此，当初始化AVCodecID时，如果返回的值为AV_CODEC_ID_NONE，表示未找到对应的有效编解码器。这可能是由于名称拼写错误、编解码器不可用或未正确安装FFmpeg库等原因导致的。

在编写代码时，可以通过检查AVCodecID是否等于AV_CODEC_ID_NONE来判断是否成功初始化AVCodecID，并根据需要进行错误处理或其他操作。

相关问题

av_find_input_format怎样才能支持NV21格式

av_find_input_format函数是FFmpeg中用于查找指定输入格式的函数。要支持NV21格式，需要先定义一个AVInputFormat结构体，然后将其添加到FFmpeg的输入格式列表中。具体做法是：

1. 定义一个AVInputFormat结构体：

AVInputFormat nv21_demuxer = {
    .name = "nv21",
    .long_name = "NV21",
    .priv_data_size = 0,
    .read_probe = nv21_probe,
    .read_header = nv21_read_header,
    .read_packet = nv21_read_packet,
    .read_close = nv21_read_close,
    .extensions = "nv21",
    .raw_codec_id = AV_CODEC_ID_NONE
};

2. 实现相应的回调函数：

static int nv21_probe(AVProbeData *p)
{
    // TODO: 实现探测函数
    return 0;
}

static int nv21_read_header(AVFormatContext *s)
{
    // TODO: 实现读取头部函数
    return 0;
}

static int nv21_read_packet(AVFormatContext *s, AVPacket *pkt)
{
    // TODO: 实现读取数据包函数
    return 0;
}

static int nv21_read_close(AVFormatContext *s)
{
    // TODO: 实现关闭函数
    return 0;
}

3. 将AVInputFormat结构体添加到输入格式列表中：

av_register_input_format(&nv21_demuxer);

这样，当你调用av_find_input_format("nv21")时，FFmpeg就可以找到相应的输入格式了。注意，这里的实现代码仅供参考，具体实现需要根据实际需求进行调整。

C#H264转BMP

根据提供的引用内容，您可以使用FFmpeg API在C#中编写程序，以实现将H264关键帧转换为BMP的功能。以下是大致的流程原理：

1.使用FFmpeg API解复用mp4文件，得到一个视频流。
2.将视频流解码为yuv序列。
3.将yuv序列中指定的一帧图片内容转换为rgb。
4.将rgb数据存储为bmp位图。

以下是一个C#的示例代码：

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

namespace H264ToBmp
{
    class Program
    {
        static void Main(string[] args)
        {
            // 初始化FFmpeg
            FFmpeg.av_register_all();
            FFmpeg.avcodec_register_all();

            // 打开输入文件
            string inputFileName = "input.mp4";
            AVFormatContext* inputFormatContext = null;
            if (FFmpeg.avformat_open_input(&inputFormatContext, inputFileName, null, null) != 0)
            {
                Console.WriteLine("无法打开输入文件");
                return;
            }

            // 查找视频流
            int videoStreamIndex = -1;
            for (int i = 0; i < inputFormatContext->nb_streams; i++)
            {
                if (inputFormatContext->streams[i]->codecpar->codec_type == AVMediaType.AVMEDIA_TYPE_VIDEO)
                {
                    videoStreamIndex = i;
                    break;
                }
            }
            if (videoStreamIndex == -1)
            {
                Console.WriteLine("无法找到视频流");
                return;
            }

            // 打开视频解码器
            AVCodec* videoCodec = FFmpeg.avcodec_find_decoder(inputFormatContext->streams[videoStreamIndex]->codecpar->codec_id);
            if (videoCodec == null)
            {
                Console.WriteLine("无法找到视频解码器");
                return;
            }
            AVCodecContext* videoCodecContext = FFmpeg.avcodec_alloc_context3(videoCodec);
            if (FFmpeg.avcodec_parameters_to_context(videoCodecContext, inputFormatContext->streams[videoStreamIndex]->codecpar) < 0)
            {
                Console.WriteLine("无法初始化视频解码器上下文");
                return;
            }
            if (FFmpeg.avcodec_open2(videoCodecContext, videoCodec, null) < 0)
            {
                Console.WriteLine("无法打开视频解码器");
                return;
            }

            // 查找关键帧
            AVPacket packet = new AVPacket();
            AVFrame* frame = FFmpeg.av_frame_alloc();
            int gotPicture = 0;
            while (FFmpeg.av_read_frame(inputFormatContext, &packet) >= 0)
            {
                if (packet.stream_index == videoStreamIndex)
                {
                    if (FFmpeg.avcodec_decode_video2(videoCodecContext, frame, &gotPicture, &packet) < 0)
                    {
                        Console.WriteLine("无法解码视频帧");
                        return;
                    }
                    if (gotPicture != 0 && (frame->key_frame != 0 || frame->pict_type == AVPictureType.AV_PICTURE_TYPE_I))
                    {
                        break;
                    }
                }
                FFmpeg.av_packet_unref(&packet);
            }

            // 将yuv序列转换为rgb
            SwsContext* swsContext = FFmpeg.sws_getContext(videoCodecContext->width, videoCodecContext->height, videoCodecContext->pix_fmt,
                videoCodecContext->width, videoCodecContext->height, AVPixelFormat.AV_PIX_FMT_RGB24, 0, null, null, null);
            AVFrame* rgbFrame = FFmpeg.av_frame_alloc();
            byte_ptrArray4 rgbData = new byte_ptrArray4();
            int rgbDataSize = FFmpeg.av_image_alloc(rgbData, null, videoCodecContext->width, videoCodecContext->height, AVPixelFormat.AV_PIX_FMT_RGB24, 1);
            FFmpeg.sws_scale(swsContext, frame->data, frame->linesize, 0, videoCodecContext->height, rgbData, rgbFrame->linesize);

            // 将rgb数据存储为bmp位图
            Bitmap bmp = new Bitmap(videoCodecContext->width, videoCodecContext->height, videoCodecContext->width * 3, PixelFormat.Format24bppRgb, new IntPtr(rgbData[0]));
            bmp.Save("output.bmp", ImageFormat.Bmp);

            // 释放资源
            FFmpeg.avformat_close_input(&inputFormatContext);
            FFmpeg.avcodec_free_context(&videoCodecContext);
            FFmpeg.av_frame_free(&frame);
            FFmpeg.sws_freeContext(swsContext);
            FFmpeg.av_frame_free(&rgbFrame);
            FFmpeg.av_freep(&rgbData[0]);
        }
    }

    public unsafe static class FFmpeg
    {
        private const string DllName = "ffmpeg.dll";

        [DllImport(DllName)]
        public static extern void av_register_all();

        [DllImport(DllName)]
        public static extern void avcodec_register_all();

        [DllImport(DllName)]
        public static extern int avformat_open_input(AVFormatContext** ps, string url, AVInputFormat* fmt, AVDictionary** options);

        [DllImport(DllName)]
        public static extern int avcodec_decode_video2(AVCodecContext* avctx, AVFrame* picture, int* got_picture_ptr, AVPacket* avpkt);

        [DllImport(DllName)]
        public static extern AVCodec* avcodec_find_decoder(AVCodecID id);

        [DllImport(DllName)]
        public static extern AVCodecContext* avcodec_alloc_context3(AVCodec* codec);

        [DllImport(DllName)]
        public static extern int avcodec_parameters_to_context(AVCodecContext* codec, AVCodecParameters* par);

        [DllImport(DllName)]
        public static extern int avcodec_open2(AVCodecContext* avctx, AVCodec* codec, AVDictionary** options);

        [DllImport(DllName)]
        public static extern int av_read_frame(AVFormatContext* s, AVPacket* pkt);

        [DllImport(DllName)]
        public static extern void av_packet_unref(AVPacket* pkt);

        [DllImport(DllName)]
        public static extern AVFrame* av_frame_alloc();

        [DllImport(DllName)]
        public static extern SwsContext* sws_getContext(int srcW, int srcH, AVPixelFormat srcFormat,
            int dstW, int dstH, AVPixelFormat dstFormat, int flags, SwsFilter* srcFilter, SwsFilter* dstFilter, double* param);

        [DllImport(DllName)]
        public static extern int av_image_alloc(byte_ptrArray4 pointers, int_array4 linesizes, int w, int h, AVPixelFormat pix_fmt, int align);

        [DllImport(DllName)]
        public static extern void sws_scale(SwsContext* c, byte_ptrArray4 srcSlice, int_array4 srcStride,
            int srcSliceY, int srcSliceH, byte_ptrArray4 dst, int_array4 dstStride);

        [DllImport(DllName)]
        public static extern void sws_freeContext(SwsContext* swsContext);

        [DllImport(DllName)]
        public static extern void av_frame_free(AVFrame** frame);

        [DllImport(DllName)]
        public static extern void avcodec_free_context(AVCodecContext** avctx);

        [DllImport(DllName)]
        public static extern void avformat_close_input(AVFormatContext** s);

        [DllImport(DllName)]
        public static extern void av_freep(void* ptr);
    }

    public enum AVMediaType
    {
        AVMEDIA_TYPE_UNKNOWN = -1,
        AVMEDIA_TYPE_VIDEO,
        AVMEDIA_TYPE_AUDIO,
        AVMEDIA_TYPE_DATA,
        AVMEDIA_TYPE_SUBTITLE,
        AVMEDIA_TYPE_ATTACHMENT,
        AVMEDIA_TYPE_NB
    }

    public enum AVCodecID
    {
        AV_CODEC_ID_NONE,

        /* video codecs */
        AV_CODEC_ID_MPEG1VIDEO,
        AV_CODEC_ID_MPEG2VIDEO, ///< preferred ID for MPEG-1/2 video decoding
        AV_CODEC_ID_H261,
        AV_CODEC_ID_H263,
        AV_CODEC_ID_RV10,
        AV_CODEC_ID_RV20,
        AV_CODEC_ID_MJPEG,
        AV_CODEC_ID_MJPEGB,
        AV_CODEC_ID_LJPEG,
        AV_CODEC_ID_SP5X,
        AV_CODEC_ID_JPEGLS,
        AV_CODEC_ID_MPEG4,
        AV_CODEC_ID_RAWVIDEO,
        AV_CODEC_ID_MSMPEG4V1,
        AV_CODEC_ID_MSMPEG4V2,
        AV_CODEC_ID_MSMPEG4V3,
        AV_CODEC_ID_WMV1,
        AV_CODEC_ID_WMV2,
        AV_CODEC_ID_H263P,
        AV_CODEC_ID_H263I,
        AV_CODEC_ID_FLV1,
        AV_CODEC_ID_SVQ1,
        AV_CODEC_ID_SVQ3,
        AV_CODEC_ID_DVVIDEO,
        AV_CODEC_ID_HUFFYUV,
        AV_CODEC_ID_CYUV,
        AV_CODEC_ID_H264,
        AV_CODEC_ID_INDEO3,
        AV_CODEC_ID_VP3,
        AV_CODEC_ID_THEORA,
        AV_CODEC_ID_ASV1,
        AV_CODEC_ID_ASV2,
        AV_CODEC_ID_FFV1,
        AV_CODEC_ID_4XM,
        AV_CODEC_ID_VCR1,
        AV_CODEC_ID_CLJR,
        AV_CODEC_ID_MDEC,
        AV_CODEC_ID_ROQ,
        AV_CODEC_ID_INTERPLAY_VIDEO,
        AV_CODEC_ID_XAN_WC3,
        AV_CODEC_ID_XAN_WC4,
        AV_CODEC_ID_RPZA,
        AV_CODEC_ID_CINEPAK,
        AV_CODEC_ID_WS_VQA,
        AV_CODEC_ID_MSRLE,
        AV_CODEC_ID_MSVIDEO1,
        AV_CODEC_ID_IDCIN,
        AV_CODEC_ID_8BPS,
        AV_CODEC_ID_SMC,
        AV_CODEC_ID_FLIC,
        AV_CODEC_ID_TRUEMOTION1,
        AV_CODEC_ID_VMDVIDEO,
        AV_CODEC_ID_MSZH,
        AV_CODEC_ID_ZLIB,
        AV_CODEC_ID_QTRLE,
        AV_CODEC_ID_TSCC,
        AV_CODEC_ID_ULTI,
        AV_CODEC_ID_QDRAW,
        AV_CODEC_ID_VIXL,
        AV_CODEC_ID_QPEG,
        AV_CODEC_ID_PNG,
        AV_CODEC_ID_PPM,
        AV_CODEC_ID_PBM,
        AV_CODEC_ID_PGM,
        AV_CODEC_ID_PGMYUV,
        AV_CODEC_ID_PAM,
        AV_CODEC_ID_FFVHUFF,
        AV_CODEC_ID_RV30,
        AV_CODEC_ID_RV40,
        AV_CODEC_ID_VC1,
        AV_CODEC_ID_WMV3,
        AV_CODEC_ID_LOCO,
        AV_CODEC_ID_WNV1,
        AV_CODEC_ID_AASC,
        AV_CODEC_ID_INDEO2,
        AV_CODEC_ID_FRAPS,
        AV_CODEC_ID_TRUEMOTION2,
        AV_CODEC_ID_BMP,
        AV_CODEC_ID_CSCD,
        AV_CODEC_ID_MMVIDEO,
        AV_CODEC_ID_ZMBV,
        AV_CODEC_ID_AVS,
        AV_CODEC_ID_SMACKVIDEO,
        AV_CODEC_ID_NUV,
        AV_CODEC_ID_KMVC,
        AV_CODEC_ID_FLASHSV,
        AV_CODEC_ID_CAVS,
        AV_CODEC_ID_JPEG2000,
        AV_CODEC_ID_VMNC,
        AV_CODEC_ID_VP5,
        AV_CODEC_ID_VP6,
        AV_CODEC_ID_VP6F,
        AV_CODEC_ID_TARGA,
        AV_CODEC_ID_DSICINVIDEO,
        AV_CODEC_ID_TIERTEXSEQVIDEO,
        AV_CODEC_ID_TIFF,
        AV_CODEC_ID_GIF,
        AV_CODEC_ID_DXA,
        AV_CODEC_ID_DNXHD,
        AV_CODEC_ID_THP,
        AV_CODEC_ID_SGI,
        AV_CODEC_ID_C93,
        AV_CODEC_ID_BETHSOFTVID,
        AV_CODEC_ID_PTX,
        AV_CODEC_ID_TXD,
        AV_CODEC_ID_VP6A,
        AV_CODEC_ID_AMV,
        AV_CODEC_ID_VB,
        AV_CODEC_ID_PCX,
        AV_CODEC_ID_SUNRAST,
        AV_CODEC_ID_INDEO4,
        AV_CODEC_ID_INDEO5,
        AV_CODEC_ID_MIMIC,
        AV_CODEC_ID_RL2,
        AV_CODEC_ID_ESCAPE124,
        AV_CODEC_ID_DIRAC,
        AV_CODEC_ID_BFI,
        AV_CODEC_ID_CMV,
        AV_CODEC_ID_MOTIONPIXELS,
        AV_CODEC_ID_TGV,
        AV_CODEC_ID_TGQ,
        AV_CODEC_ID_TQI,
        AV_CODEC_ID_AURA,
        AV_CODEC_ID_AURA2,
        AV_CODEC_ID_V210X,
        AV_CODEC_ID_TMV,
        AV_CODEC_ID_V210,
        AV_CODEC_ID_DPX,
        AV_CODEC_ID_MAD,
        AV_CODEC_ID_FRWU,
        AV_CODEC_ID_FLASHSV2,
        AV_CODEC_ID_CDGRAPHICS,
        AV_CODEC_ID_R210,
        AV_CODEC_ID_ANM,
        AV_CODEC_ID_BINKVIDEO,
        AV_CODEC_ID_IFF_ILBM,
        AV_CODEC_ID_IFF_BYTERUN1,
        AV_CODEC_ID_KGV1,
        AV_CODEC_ID_YOP,
        AV_CODEC_ID_VP8,
        AV_CODEC_ID_PICTOR,
        AV_CODEC_ID_ANSI,
        AV_CODEC_ID_A64_MULTI,
        AV_CODEC_ID_A64_MULTI5,
        AV_CODEC_ID_R10K,
        AV_CODEC_ID_MXPEG,
        AV_CODEC_ID_LAGARITH,
        AV_CODEC_ID_PRORES,
        AV_CODEC_ID_JV,
        AV_CODEC_ID_DFA,
        AV_CODEC_ID_WMV3IMAGE,
        AV_CODEC_ID_VC1IMAGE,
        AV_CODEC_ID_UTVIDEO,
        AV_CODEC_ID_BMV_VIDEO,
        AV_CODEC_ID_VBLE,
        AV_CODEC_ID_DXTORY,
        AV_CODEC_ID_V410,
        AV_CODEC_ID_XWD,
        AV_CODEC_ID_CDXL,
        AV_CODEC_ID_XBM,
        AV_CODEC_ID_ZEROCODEC,