程式碼實現視訊轉圖片主要是使用了FFmpeg視訊編解碼相關的知識，所以首先了解下FFmpeg中的編解碼相關函式以及流程，後面再看程式碼就會比較輕鬆了。
視訊解碼
- 開啟輸入檔案
avformat_open_input - 找到視訊流
av_find_best_stream - 找到對應的解碼器
avcodec_find_decoder - 初始化一個編解碼上下文
avcodec_alloc_context3 - 拷貝流引數到編解碼上下文中
avcodec_parameters_to_context - 開啟解碼器
avcodec_open2 - 讀取視訊幀
av_read_frame - 傳送等待解碼幀
avcodec_send_packet - 接收解碼後frame資料
avcodec_receive_frame
int main(int argc, char *argv[]) {
int ret;
const char *in_filename, *out_filename;
AVFormatContext *fmt_ctx = NULL;
const AVCodec *codec;
AVCodecContext *codeCtx = NULL;
AVStream *stream = NULL;
int stream_index;
AVPacket avpkt;
int frame_count;
AVFrame *frame;
if (argc <= 2) {
printf("Usage: %s <input file> <output file>\n", argv[0]);
exit(0);
}
in_filename = argv[1];
out_filename = argv[2];
// 1
if (avformat_open_input(&fmt_ctx, in_filename, NULL, NULL) < 0) {
printf("Could not open source file %s\n", in_filename);
exit(1);
}
if (avformat_find_stream_info(fmt_ctx, NULL) < 0) {
printf("Could not find stream information\n");
exit(1);
}
av_dump_format(fmt_ctx, 0, in_filename, 0);
av_init_packet(&avpkt);
avpkt.data = NULL;
avpkt.size = 0;
// 2
stream_index = 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), in_filename);
return ret;
}
stream = fmt_ctx->streams[stream_index];
// 3
codec = avcodec_find_decoder(stream->codecpar->codec_id);
if (codec == NULL) {
return -1;
}
// 4
codeCtx = avcodec_alloc_context3(NULL);
if (!codeCtx) {
fprintf(stderr, "Could not allocate video codec context\n");
exit(1);
}
// 5
if ((ret = avcodec_parameters_to_context(codeCtx, stream->codecpar)) < 0) {
fprintf(stderr, "Failed to copy %s codec parameters to decoder context\n",
av_get_media_type_string(AVMEDIA_TYPE_VIDEO));
return ret;
}
// 6
avcodec_open2(codeCtx, codec, NULL);
//初始化frame，解碼後資料
frame = av_frame_alloc();
if (!frame) {
fprintf(stderr, "Could not allocate video frame\n");
exit(1);
}
frame_count = 0;
char buf[1024];
// 7
while (av_read_frame(fmt_ctx, &avpkt) >= 0) {
if (avpkt.stream_index == stream_index) {
// 8
int re = avcodec_send_packet(codeCtx, &avpkt);
if (re < 0) {
continue;
}
// 9 這裡必須用while()，因為一次avcodec_receive_frame可能無法接收到所有資料
while (avcodec_receive_frame(codeCtx, frame) == 0) {
// 拼接圖片路徑、名稱
snprintf(buf, sizeof(buf), "%s/Demo-%d.jpg", out_filename, frame_count);
saveJpg(frame, buf); //儲存為jpg圖片
}
frame_count++;
}
av_packet_unref(&avpkt);
}
}
複製程式碼視訊編碼（儲存為圖片）
編碼整體在流程上和解碼一致
int saveJpg(AVFrame *pFrame, char *out_name) {
int width = pFrame->width;
int height = pFrame->height;
AVCodecContext *pCodeCtx = NULL;
AVFormatContext *pFormatCtx = avformat_alloc_context();
// 設定輸出檔案格式
pFormatCtx->oformat = av_guess_format("mjpeg", NULL, NULL);
// 建立並初始化輸出AVIOContext
if (avio_open(&pFormatCtx->pb, out_name, AVIO_FLAG_READ_WRITE) < 0) {
printf("Couldn't open output file.");
return -1;
}
// 構建一個新stream
AVStream *pAVStream = avformat_new_stream(pFormatCtx, 0);
if (pAVStream == NULL) {
return -1;
}
AVCodecParameters *parameters = pAVStream->codecpar;
parameters->codec_id = pFormatCtx->oformat->video_codec;
parameters->codec_type = AVMEDIA_TYPE_VIDEO;
parameters->format = AV_PIX_FMT_YUVJ420P;
parameters->width = pFrame->width;
parameters->height = pFrame->height;
AVCodec *pCodec = avcodec_find_encoder(pAVStream->codecpar->codec_id);
if (!pCodec) {
printf("Could not find encoder\n");
return -1;
}
pCodeCtx = avcodec_alloc_context3(pCodec);
if (!pCodeCtx) {
fprintf(stderr, "Could not allocate video codec context\n");
exit(1);
}
if ((avcodec_parameters_to_context(pCodeCtx, pAVStream->codecpar)) < 0) {
fprintf(stderr, "Failed to copy %s codec parameters to decoder context\n",
av_get_media_type_string(AVMEDIA_TYPE_VIDEO));
return -1;
}
pCodeCtx->time_base = (AVRational) {1, 25};
if (avcodec_open2(pCodeCtx, pCodec, NULL) < 0) {
printf("Could not open codec.");
return -1;
}
int ret = avformat_write_header(pFormatCtx, NULL);
if (ret < 0) {
printf("write_header fail\n");
return -1;
}
int y_size = width * height;
//Encode
// 給AVPacket分配足夠大的空間
AVPacket pkt;
av_new_packet(&pkt, y_size * 3);
// 編碼資料
ret = avcodec_send_frame(pCodeCtx, pFrame);
if (ret < 0) {
printf("Could not avcodec_send_frame.");
return -1;
}
// 得到編碼後資料
ret = avcodec_receive_packet(pCodeCtx, &pkt);
if (ret < 0) {
printf("Could not avcodec_receive_packet");
return -1;
}
ret = av_write_frame(pFormatCtx, &pkt);
if (ret < 0) {
printf("Could not av_write_frame");
return -1;
}
av_packet_unref(&pkt);
//Write Trailer
av_write_trailer(pFormatCtx);
avcodec_close(pCodeCtx);
avio_close(pFormatCtx->pb);
avformat_free_context(pFormatCtx);
return 0;
}
複製程式碼