c 调用ffmpeg 进行视频抽帧

要在 C 语言中调用 FFmpeg 进行视频抽帧，可以使用 FFmpeg 的 C API。以下是一个简单的示例代码：

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

#include <libavformat/avformat.h>
#include <libavutil/imgutils.h>
#include <libswscale/swscale.h>

#define INBUF_SIZE 4096

int main(int argc, char **argv)
{
    AVFormatContext *fmt_ctx = NULL;
    AVCodecContext *dec_ctx = NULL;
    AVCodec *dec = NULL;
    AVPacket pkt;
    int ret, i;

    if (argc != 2) {
        fprintf(stderr, "Usage: %s <input_file>\n", argv[0]);
        exit(1);
    }

    av_register_all();

    if (avformat_open_input(&fmt_ctx, argv[1], NULL, NULL) < 0) {
        fprintf(stderr, "Could not open source file %s\n", argv[1]);
        exit(1);
    }

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

    for (i = 0; i < fmt_ctx->nb_streams; i++) {
        AVStream *stream = fmt_ctx->streams[i];
        dec_ctx = stream->codec;
        dec = avcodec_find_decoder(dec_ctx->codec_id);
        if (!dec) {
            fprintf(stderr, "Failed to find decoder for stream #%d\n", i);
            exit(1);
        }
        if ((ret = avcodec_open2(dec_ctx, dec, NULL)) < 0) {
            fprintf(stderr, "Failed to open decoder for stream #%d\n", i);
            exit(1);
        }
    }

    av_init_packet(&pkt);
    pkt.data = NULL;
    pkt.size = 0;

    while (av_read_frame(fmt_ctx, &pkt) >= 0) {
        if (pkt.stream_index == i) {
            AVFrame *frame = av_frame_alloc();
            int got_frame;

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

            ret = avcodec_decode_video2(dec_ctx, frame, &got_frame, &pkt);
            if (ret < 0) {
                fprintf(stderr, "Error decoding video frame\n");
                exit(1);
            }

            if (got_frame) {
                char buf[1024];
                snprintf(buf, sizeof(buf), "frame-%d.pgm", dec_ctx->frame_number);
                FILE *f = fopen(buf, "wb");
                if (!f) {
                    fprintf(stderr, "Could not open output file\n");
                    exit(1);
                }

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

                int w = dec_ctx->width;
                int h = dec_ctx->height;
                av_image_alloc(rgb->data, rgb->linesize, w, h, AV_PIX_FMT_RGB24, 1);
                struct SwsContext *sws_ctx = sws_getContext(w, h, dec_ctx->pix_fmt, w, h, AV_PIX_FMT_RGB24, 0, NULL, NULL, NULL);
                if (!sws_ctx) {
                    fprintf(stderr, "Could not initialize conversion context\n");
                    exit(1);
                }

                sws_scale(sws_ctx, (const uint8_t * const *)frame->data, frame->linesize, 0, h, rgb->data, rgb->linesize);

                fprintf(f, "P6\n%d %d\n255\n", w, h);
                fwrite(rgb->data[0], 1, w * h * 3, f);

                fclose(f);

                av_freep(&rgb->data[0]);
                av_frame_free(&rgb);
            }

            av_frame_free(&frame);
        }
        av_packet_unref(&pkt);
    }

    avcodec_free_context(&dec_ctx);
    avformat_close_input(&fmt_ctx);

    return 0;
}

上面的代码会将输入文件的视频流抽帧，并将每一帧转换为 PGM 图像格式并保存到磁盘上。你可以根据自己的需求修改代码来实现更复杂的功能。