FFmpeg input与output 函数流程

重要结构体

AVFormatContext


AVCodecContext
AVCodec

AVPacket
AVFrame

0.公共部分

av_register_all();

avfilter_register_all();

avformat_network_init();

avdevice_register_all();

av_log_set_level(AV_LOG_ERROR);

1.input部分

avformat_alloc_context

av_find_input_format

avformat_open_input

avformat_find_stream_info

avcodec_find_decoder

avcodec_open2

av_read_frame

avcodec_decode_video2

avformat_close_input

2.output部分

avformat_alloc_output_context2

avio_open2

avcodec_find_encoder

avcodec_alloc_context3

avcodec_open2

avformat_new_stream

avcodec_copy_context ---> avcodec_find_decoder + avcodec_alloc_context3
　　　　　　　　　　　　　　 + avcodec_parameters_to_context + avcodec_parameters_from_context

avformat_write_header

avcodec_encode_video2

av_interleaved_write_frame

av_write_trailer

avcodec_close

avformat_close_input

3.裁剪视频代码

  1 #include <stdlib.h>

  2 #include <libavutil/timestamp.h>

  3 #include <libavformat/avformat.h>

  4

  5 static void log_packet(const AVFormatContext *fmt_ctx, const AVPacket *pkt, const char *tag)

  6 {

  7     AVRational *time_base = &fmt_ctx->streams[pkt->stream_index]->time_base;

  8

  9     printf("%s: pts:%s pts_time:%s dts:%s dts_time:%s duration:%s duration_time:%s stream_index:%d\n",

 10            tag,

 11            av_ts2str(pkt->pts), av_ts2timestr(pkt->pts, time_base),

 12            av_ts2str(pkt->dts), av_ts2timestr(pkt->dts, time_base),

 13            av_ts2str(pkt->duration), av_ts2timestr(pkt->duration, time_base),

 14            pkt->stream_index);

 15 }

 16

 17 int cut_video(double from_seconds, double end_seconds, const char* in_filename, const char* out_filename) {

 18     AVOutputFormat *ofmt = NULL;

 19     AVFormatContext *ifmt_ctx = NULL, *ofmt_ctx = NULL;

 20     AVPacket pkt;

 21     int ret, i;

 22

 23     av_register_all();

 24

 25     if ((ret = avformat_open_input(&ifmt_ctx, in_filename, 0, 0)) < 0) {

 26         fprintf(stderr, "Could not open input file '%s'", in_filename);

 27         goto end;

 28     }

 29

 30     if ((ret = avformat_find_stream_info(ifmt_ctx, 0)) < 0) {

 31         fprintf(stderr, "Failed to retrieve input stream information");

 32         goto end;

 33     }

 34

 35     av_dump_format(ifmt_ctx, 0, in_filename, 0);

 36

 37     avformat_alloc_output_context2(&ofmt_ctx, NULL, NULL, out_filename);

 38     if (!ofmt_ctx) {

 39         fprintf(stderr, "Could not create output context\n");

 40         ret = AVERROR_UNKNOWN;

 41         goto end;

 42     }

 43

 44     ofmt = ofmt_ctx->oformat;

 45

 46     for (i = 0; i < ifmt_ctx->nb_streams; i++) {

 47 /*        AVStream *in_stream = ifmt_ctx->streams[i];

 48         AVStream *out_stream = avformat_new_stream(ofmt_ctx, in_stream->codec->codec);

 49         if (!out_stream) {

 50             fprintf(stderr, "Failed allocating output stream\n");

 51             ret = AVERROR_UNKNOWN;

 52             goto end;

 53         }

 54

 55         ret = avcodec_copy_context(out_stream->codec, in_stream->codec);

 56         if (ret < 0) {

 57             fprintf(stderr, "Failed to copy context from input to output stream codec context\n");

 58             goto end;

 59         }

 60         out_stream->codec->codec_tag = 0;

 61         if (ofmt_ctx->oformat->flags & AVFMT_GLOBALHEADER)

 62             out_stream->codec->flags |= AV_CODEC_FLAG_GLOBAL_HEADER;

 63 */

 64         AVStream *in_stream = ifmt_ctx->streams[i];

 65         AVCodec * codec = avcodec_find_decoder(in_stream->codecpar->codec_id);

 66         AVStream *out_stream = avformat_new_stream(ofmt_ctx, codec);

 67         if (!out_stream) {

 68             fprintf(stderr, "Failed allocating output stream\n");

 69             ret = AVERROR_UNKNOWN;

 70             goto end;

 71         }

 72

 73         AVCodecContext * codec_ctx = avcodec_alloc_context3(codec);

 74         ret = avcodec_parameters_to_context(codec_ctx, in_stream->codecpar);

 75         if (ret < 0) {

 76             fprintf(stderr, "Failed to copy in_stream codecpar to codec context\n");

 77             goto end;

 78         }

 79

 80         codec_ctx->codec_tag = 0;

 81         if (ofmt_ctx->oformat->flags & AVFMT_GLOBALHEADER)

 82             codec_ctx->flags |= AV_CODEC_FLAG_GLOBAL_HEADER;

 83

 84         ret = avcodec_parameters_from_context(out_stream->codecpar, codec_ctx);

 85         if (ret < 0) {

 86             fprintf(stderr, "Failed to copy codec context to out_stream codecpar context\n");

 87             goto end;

 88         }

 89     }

 90     av_dump_format(ofmt_ctx, 0, out_filename, 1);

 91

 92

 93     if (!(ofmt->flags & AVFMT_NOFILE)) {

 94         ret = avio_open(&ofmt_ctx->pb, out_filename, AVIO_FLAG_WRITE);

 95         if (ret < 0) {

 96             fprintf(stderr, "Could not open output file '%s'", out_filename);

 97             goto end;

 98         }

 99     }

100

101     ret = avformat_write_header(ofmt_ctx, NULL);

102     if (ret < 0) {

103         fprintf(stderr, "Error occurred when opening output file\n");

104         goto end;

105     }

106

107     // int64_t start_from = 8*AV_TIME_BASE;

108     ret = av_seek_frame(ifmt_ctx, -1, from_seconds*AV_TIME_BASE, AVSEEK_FLAG_ANY);

109     if (ret < 0) {

110         fprintf(stderr, "Error seek\n");

111         goto end;

112     }

113

114     int64_t *dts_start_from = malloc(sizeof(int64_t) * ifmt_ctx->nb_streams);

115     memset(dts_start_from, 0, sizeof(int64_t) * ifmt_ctx->nb_streams);

116     int64_t *pts_start_from = malloc(sizeof(int64_t) * ifmt_ctx->nb_streams);

117     memset(pts_start_from, 0, sizeof(int64_t) * ifmt_ctx->nb_streams);

118

119     while (1) {

120         AVStream *in_stream, *out_stream;

121

122         ret = av_read_frame(ifmt_ctx, &pkt);

123         if (ret < 0)

124             break;

125

126         in_stream  = ifmt_ctx->streams[pkt.stream_index];

127         out_stream = ofmt_ctx->streams[pkt.stream_index];

128

129         log_packet(ifmt_ctx, &pkt, "in");

130

131         if (av_q2d(in_stream->time_base) * pkt.pts > end_seconds) {

132             av_free_packet(&pkt);

133             break;

134         }

135

136         if (dts_start_from[pkt.stream_index] == 0) {

137             dts_start_from[pkt.stream_index] = pkt.dts;

138             printf("dts_start_from: %s\n", av_ts2str(dts_start_from[pkt.stream_index]));

139         }

140         if (pts_start_from[pkt.stream_index] == 0) {

141             pts_start_from[pkt.stream_index] = pkt.pts;

142             printf("pts_start_from: %s\n", av_ts2str(pts_start_from[pkt.stream_index]));

143         }

144

145         /* copy packet */

146         pkt.pts = av_rescale_q_rnd(pkt.pts - pts_start_from[pkt.stream_index], in_stream->time_base, out_stream->time_base, AV_ROUND_NEAR_INF|AV_ROUND_PASS_MINMAX);

147         pkt.dts = av_rescale_q_rnd(pkt.dts - dts_start_from[pkt.stream_index], in_stream->time_base, out_stream->time_base, AV_ROUND_NEAR_INF|AV_ROUND_PASS_MINMAX);

148         if (pkt.pts < 0) {

149             pkt.pts = 0;

150         }

151         if (pkt.dts < 0) {

152             pkt.dts = 0;

153         }

154         pkt.duration = (int)av_rescale_q((int64_t)pkt.duration, in_stream->time_base, out_stream->time_base);

155         pkt.pos = -1;

156         log_packet(ofmt_ctx, &pkt, "out");

157         printf("\n");

158

159         ret = av_interleaved_write_frame(ofmt_ctx, &pkt);

160         if (ret < 0) {

161             fprintf(stderr, "Error muxing packet\n");

162             //break;    // 注释后，解决 错误 pts (6000) < dts (12000) in stream 0

163         }

164         av_free_packet(&pkt);

165     }

166     free(dts_start_from);

167     free(pts_start_from);

168

169     av_write_trailer(ofmt_ctx);

170 end:

171

172     avformat_close_input(&ifmt_ctx);

173

174     /* close output */

175     if (ofmt_ctx && !(ofmt->flags & AVFMT_NOFILE))

176         avio_closep(&ofmt_ctx->pb);

177     avformat_free_context(ofmt_ctx);

178

179     if (ret < 0 && ret != AVERROR_EOF) {

180         fprintf(stderr, "Error occurred: %s\n", av_err2str(ret));

181         return 1;

182     }

183

184     return 0;

185 }

186

187 int main(int argc, char *argv[]){

188     if(argc < 5){

189         fprintf(stderr, "Usage: \

190                 command startime, endtime, srcfile, outfile");

191         return -1;

192     }

193

194     double startime = atoi(argv[1]);

195     double endtime = atoi(argv[2]);

196     cut_video(startime, endtime, argv[3], argv[4]);

197

198     return 0;

199 }

4.问题

　　问题分析：

　　出现这种错误是由于视频pts大于dts。pts是视频播放时间，dts是送入解码器解码时间。所以一帧视频播放时间必须在解码时间点之后。

　　产生错误的原因一般是对dts，pts操作不当。比如在进行视频分割时，常用的方法是视频截取后半段视频pts与dts减去前半段pts和dts。前半段pts可能比dts大（当解码的视频帧不是I帧时）后半段刚开始视频pts和dts刚好相等（当前帧为I帧时），两个一相减就会出现dts小于pts的情况。

　　解决方法：

　　1.进行判断：在av_interleaved_write_frame之前添加 if(packet.pts < packet.dts) continue; 把异常的帧简单跳过，异常帧只是极少数简单跳过不会有什么影响。这样会是的播放裁剪后的视频起始有黑屏。

　　2.获取截取起始时间后的第一个I帧，从这个I帧开始。

　　最好使用下面的方法。

巴特西