ffmpeg实战将视频转换为图片

视频可以是文件或者从服务器拉取的流，流程如下：打开封装->解码->yuv->rgb->写入文件
在yuv->rgb时是用的ffmpeg的像素格式转换函数：

sws_scale(img_convert_ctx, frame->data, frame->linesize,
          0, h, pFrameRGB->data, pFrameRGB->linesize);

具体实现如下：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <libavformat/avformat.h>
#include <libavcodec/avcodec.h>
#include <libswscale/swscale.h>

#define INBUF_SIZE 4096

#define WORD uint16_t
#define DWORD uint32_t
#define LONG int32_t

#pragma pack(2)
typedef struct tagBITMAPFILEHEADER {
  WORD  bfType;
  DWORD bfSize;
  WORD  bfReserved1;
  WORD  bfReserved2;
  DWORD bfOffBits;
} BITMAPFILEHEADER, *PBITMAPFILEHEADER;


typedef struct tagBITMAPINFOHEADER {
  DWORD biSize;
  LONG  biWidth;
  LONG  biHeight;
  WORD  biPlanes;
  WORD  biBitCount;
  DWORD biCompression;
  DWORD biSizeImage;
  LONG  biXPelsPerMeter;
  LONG  biYPelsPerMeter;
  DWORD biClrUsed;
  DWORD biClrImportant;
} BITMAPINFOHEADER, *PBITMAPINFOHEADER;

void saveBMP(struct SwsContext *img_convert_ctx, AVFrame *frame, char *filename)
{
    //1 先进行转换,  YUV420=>RGB24:
    int w = frame->width;
    int h = frame->height;


    int numBytes=avpicture_get_size(AV_PIX_FMT_BGR24, w, h);
    uint8_t *buffer=(uint8_t *)av_malloc(numBytes*sizeof(uint8_t));


    AVFrame *pFrameRGB = av_frame_alloc();
     /* buffer is going to be written to rawvideo file, no alignment */
    /*
    if (av_image_alloc(pFrameRGB->data, pFrameRGB->linesize,
                              w, h, AV_PIX_FMT_BGR24, pix_fmt, 1) < 0) {
        fprintf(stderr, "Could not allocate destination image\n");
        exit(1);
    }
    */
    avpicture_fill((AVPicture *)pFrameRGB, buffer, AV_PIX_FMT_BGR24, w, h);

    sws_scale(img_convert_ctx, frame->data, frame->linesize,
              0, h, pFrameRGB->data, pFrameRGB->linesize);

    //2 构造 BITMAPINFOHEADER
    BITMAPINFOHEADER header;
    header.biSize = sizeof(BITMAPINFOHEADER);


    header.biWidth = w;
    header.biHeight = h*(-1);
    header.biBitCount = 24;
    header.biCompression = 0;
    header.biSizeImage = 0;
    header.biClrImportant = 0;
    header.biClrUsed = 0;
    header.biXPelsPerMeter = 0;
    header.biYPelsPerMeter = 0;
    header.biPlanes = 1;

    //3 构造文件头
    BITMAPFILEHEADER bmpFileHeader = {0,};
    //HANDLE hFile = NULL;
    DWORD dwTotalWriten = 0;
    DWORD dwWriten;

    bmpFileHeader.bfType = 0x4d42; //'BM';
    bmpFileHeader.bfSize = sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER)+ numBytes;
    bmpFileHeader.bfOffBits=sizeof(BITMAPFILEHEADER)+sizeof(BITMAPINFOHEADER);

    FILE* pf = fopen(filename, "wb");
    fwrite(&bmpFileHeader, sizeof(BITMAPFILEHEADER), 1, pf);
    fwrite(&header, sizeof(BITMAPINFOHEADER), 1, pf);
    fwrite(pFrameRGB->data[0], 1, numBytes, pf);
    fclose(pf);


    //释放资源
    //av_free(buffer);
    av_freep(&pFrameRGB[0]);
    av_free(pFrameRGB);
}

static void pgm_save(unsigned char *buf, int wrap, int xsize, int ysize,
                     char *filename)
{
    FILE *f;
    int i;

    f = fopen(filename,"w");
    fprintf(f, "P5\n%d %d\n%d\n", xsize, ysize, 255);
    for (i = 0; i < ysize; i++)
        fwrite(buf + i * wrap, 1, xsize, f);
    fclose(f);
}

static int decode_write_frame(const char *outfilename, AVCodecContext *avctx,
                              struct SwsContext *img_convert_ctx, AVFrame *frame, int *frame_count, AVPacket *pkt, int last)
{
    int len, got_frame;
    char buf[1024];

    len = avcodec_decode_video2(avctx, frame, &got_frame, pkt);
    if (len < 0) {
        fprintf(stderr, "Error while decoding frame %d\n", *frame_count);
        return len;
    }
    if (got_frame) {
        printf("Saving %sframe %3d\n", last ? "last " : "", *frame_count);
        fflush(stdout);

        /* the picture is allocated by the decoder, no need to free it */
        snprintf(buf, sizeof(buf), "%s-%d.bmp", outfilename, *frame_count);
        
        /*
        pgm_save(frame->data[0], frame->linesize[0],
                 frame->width, frame->height, buf);
        */
        
        saveBMP(img_convert_ctx, frame, buf);
        
        (*frame_count)++;
    }
    if (pkt->data) {
        pkt->size -= len;
        pkt->data += len;
    }
    return 0;
}

int main(int argc, char **argv)
{
    int ret;

    FILE *f;

    const char *filename, *outfilename;

    AVFormatContext *fmt_ctx = NULL;

    const AVCodec *codec;
    AVCodecContext *c= NULL;

    AVStream *st = NULL;
    int stream_index;

    int frame_count;
    AVFrame *frame;

    struct SwsContext *img_convert_ctx;

    //uint8_t inbuf[INBUF_SIZE + AV_INPUT_BUFFER_PADDING_SIZE];
    AVPacket avpkt;

    if (argc <= 2) {
        fprintf(stderr, "Usage: %s <input file> <output file>\n", argv[0]);
        exit(0);
    }
    filename    = argv[1];
    outfilename = argv[2];

    /* register all formats and codecs */
    av_register_all();

    /* open input file, and allocate format context */
    if (avformat_open_input(&fmt_ctx, filename, NULL, NULL) < 0) {
        fprintf(stderr, "Could not open source file %s\n", filename);
        exit(1);
    }

    /* retrieve stream information */
    if (avformat_find_stream_info(fmt_ctx, NULL) < 0) {
        fprintf(stderr, "Could not find stream information\n");
        exit(1);
    }

    /* dump input information to stderr */
    av_dump_format(fmt_ctx, 0, filename, 0);

    av_init_packet(&avpkt);

    /* set end of buffer to 0 (this ensures that no overreading happens for damaged MPEG streams) */
    //memset(inbuf + INBUF_SIZE, 0, AV_INPUT_BUFFER_PADDING_SIZE);
    //

    ret = av_find_best_stream(fmt_ctx, AVMEDIA_TYPE_VIDEO, -1, -1, NULL, 0);
    if (ret < 0) {
        fprintf(stderr, "Could not find %s stream in input file '%s'\n",
                av_get_media_type_string(AVMEDIA_TYPE_VIDEO), filename);
        return ret;
    }

    stream_index = ret;
    st = fmt_ctx->streams[stream_index];

    /* find decoder for the stream */
    codec = avcodec_find_decoder(st->codecpar->codec_id);
    if (!codec) {
        fprintf(stderr, "Failed to find %s codec\n",
                av_get_media_type_string(AVMEDIA_TYPE_VIDEO));
        return AVERROR(EINVAL);
    }


    /* find the MPEG-1 video decoder */
    /*
    codec = avcodec_find_decoder(AV_CODEC_ID_MPEG1VIDEO);
    if (!codec) {
        fprintf(stderr, "Codec not found\n");
        exit(1);
    }
    */

    c = avcodec_alloc_context3(NULL);
    if (!c) {
        fprintf(stderr, "Could not allocate video codec context\n");
        exit(1);
    }

    /* Copy codec parameters from input stream to output codec context */
    if ((ret = avcodec_parameters_to_context(c, st->codecpar)) < 0) {
        fprintf(stderr, "Failed to copy %s codec parameters to decoder context\n",
                av_get_media_type_string(AVMEDIA_TYPE_VIDEO));
        return ret;
    }


    /*
    if (codec->capabilities & AV_CODEC_CAP_TRUNCATED)
        c->flags |= AV_CODEC_FLAG_TRUNCATED; // we do not send complete frames
    */

    /* For some codecs, such as msmpeg4 and mpeg4, width and height
       MUST be initialized there because this information is not
       available in the bitstream. */

    /* open it */
    if (avcodec_open2(c, codec, NULL) < 0) {
        fprintf(stderr, "Could not open codec\n");
        exit(1);
    }

    /*
    f = fopen(filename, "rb");
    if (!f) {
        fprintf(stderr, "Could not open %s\n", filename);
        exit(1);
    }
    */

    img_convert_ctx = sws_getContext(c->width, c->height,
                                     c->pix_fmt,
                                     c->width, c->height,
                                     AV_PIX_FMT_RGB24,
                                     SWS_BICUBIC, NULL, NULL, NULL);

    if (img_convert_ctx == NULL)
    {
        fprintf(stderr, "Cannot initialize the conversion context\n");
        exit(1);
    }

    frame = av_frame_alloc();
    if (!frame) {
        fprintf(stderr, "Could not allocate video frame\n");
        exit(1);
    }

    frame_count = 0;
    while (av_read_frame(fmt_ctx, &avpkt) >= 0) {
        /*
        avpkt.size = fread(inbuf, 1, INBUF_SIZE, f);
        if (avpkt.size == 0)
            break;
        */

        /* NOTE1: some codecs are stream based (mpegvideo, mpegaudio)
           and this is the only method to use them because you cannot
           know the compressed data size before analysing it.

           BUT some other codecs (msmpeg4, mpeg4) are inherently frame
           based, so you must call them with all the data for one
           frame exactly. You must also initialize 'width' and
           'height' before initializing them. */

        /* NOTE2: some codecs allow the raw parameters (frame size,
           sample rate) to be changed at any frame. We handle this, so
           you should also take care of it */

        /* here, we use a stream based decoder (mpeg1video), so we
           feed decoder and see if it could decode a frame */
        //avpkt.data = inbuf;
        //while (avpkt.size > 0)
        if(avpkt.stream_index == stream_index){
            if (decode_write_frame(outfilename, c, img_convert_ctx, frame, &frame_count, &avpkt, 0) < 0)
                exit(1);
        }

        av_packet_unref(&avpkt);
    }

    /* Some codecs, such as MPEG, transmit the I- and P-frame with a
       latency of one frame. You must do the following to have a
       chance to get the last frame of the video. */
    avpkt.data = NULL;
    avpkt.size = 0;
    decode_write_frame(outfilename, c, img_convert_ctx, frame, &frame_count, &avpkt, 1);

    fclose(f);

    avformat_close_input(&fmt_ctx);

    sws_freeContext(img_convert_ctx);
    avcodec_free_context(&c);
    av_frame_free(&frame);

    return 0;
}