I’m trying to transcode a video on my iOS app using FFMpeg/LibAv. What I’m trying to accomplish is to transcode a video in order to resize each frame and possibly lower the bitrate in order to save valuable MB in the device.
The resulting video must be playable on all iPhone5+ devices.
After reading the documentation I found out that:
- I do not need to encode/decode the audio stream -> I’ll copy as-is to the output file
- I need to encode the video using the h264 codec (LibX264) with a profile supported by iOS (baseline profile with level 3.0 - https://trac.ffmpeg.org/wiki/Encode/H.264#Compatibility)
- I’m also setting the picture format to YUV planar since it’s the only one supported by iOS
- For the sake of testing I’m not using any filter (I’m using a dummy/passthrough) at all or even trying to lower the bitrate, I’m just trying to decode the video stream and encode it again
- Most of the code is based on the transcoding.c and filtering.c available on the FFMpeg examples directory
FFMpeg-wise what I’m trying to achieve with LibAv is:
ffmpeg -i INPUT.MOV -c:v libx264 -preset ultrafast -profile:v baseline -level 3.0 -c:a copy output.MOV
(the resulting file - which can be found below - is playable on QuickTime if it’s generated by FFMpeg through the command line)
The original video was generated with a regular iPhone using iOS 8.2 but the problem is not device specific or iOS specific, it occurs on all videos generated with LibAv.
Although both resulting files are playable by VideoLan (VLC) the one I generated through LibAv is not playable by QuickTime even though I can’t find anything wrong with it.
As you can see below, I create the video stream with the proper video codec on the call to avformat_new_stream:
AVStream *out_stream; // output stream
AVStream *in_stream; // input stream
AVCodecContext *dec_ctx, *enc_ctx; // codec context for the stream
AVCodec *encoder; // codec used
int ret;
unsigned int i;
ofmt_ctx = NULL;
// Allocate an AVFormatContext for an output format. This will be the file header (similar to avformat_open_input but with an zero'ed memory)
avformat_alloc_output_context2(&ofmt_ctx, NULL, NULL, filename);
if (!ofmt_ctx) {
av_log(NULL, AV_LOG_ERROR, "Could not create output context\n");
[self errorWith:kErrorCreatingOutputContext and:@"Could not create output context"];
return AVERROR_UNKNOWN;
}
// we must not use the AVCodecContext from the video stream directly! So we have to use avcodec_copy_context() to copy the context to a new location (after allocating memory for it, of course).
// iterate over all input streams
for (i = 0; i < ifmt_ctx->nb_streams; i++) {
in_stream = ifmt_ctx->streams[i]; // input stream
dec_ctx = in_stream->codec; // get the codec context for the decoder
if (dec_ctx->codec_type == AVMEDIA_TYPE_VIDEO) {
// lets use h264
encoder = avcodec_find_encoder(AV_CODEC_ID_H264);
if (!encoder) {
[self errorWith:kErrorCodecNotFound and:@"H264 Codec Not Found"];
return AVERROR_UNKNOWN;
}
out_stream = avformat_new_stream(ofmt_ctx, encoder); // create a new stream with h264 codec
if (!out_stream) {
av_log(NULL, AV_LOG_ERROR, "Failed allocating output stream\n");
[self errorWith:kErrorAllocateOutputStream and:@"Failed allocating output stream"];
return AVERROR_UNKNOWN;
}
enc_ctx = out_stream->codec; // pointer to the stream codec context
/* we transcode to same properties (picture size,
* sample rate etc.). These properties can be changed for output
* streams easily using filters */
if (dec_ctx->codec_type == AVMEDIA_TYPE_VIDEO) {
enc_ctx->width = dec_ctx->width;
enc_ctx->height = dec_ctx->height;
enc_ctx->sample_aspect_ratio = dec_ctx->sample_aspect_ratio;
enc_ctx->pix_fmt = AV_PIX_FMT_YUV420P;
enc_ctx->time_base = dec_ctx->time_base;
av_opt_set(enc_ctx->priv_data, "preset", "ultrafast", 0);
av_opt_set(enc_ctx->priv_data, "profile", "baseline", 0);
av_opt_set(enc_ctx->priv_data, "level", "3.0", 0);
}
out_stream->time_base = in_stream->time_base;
AVDictionaryEntry *tag = NULL;
while ((tag = av_dict_get(in_stream->metadata, "", tag, AV_DICT_IGNORE_SUFFIX))) {
printf("%s=%s\n", tag->key, tag->value);
char *k = av_strdup(tag->key); // if your strings are already allocated,
char *v = av_strdup(tag->value); // you can avoid copying them like this
av_dict_set(&out_stream->metadata, k, v, 0);
}
ret = avcodec_open2(enc_ctx, encoder, NULL);
if (ret < 0) {
av_log(NULL, AV_LOG_ERROR, "Cannot open video encoder for stream #%u\n", i);
[self errorWith:kErrorCantOpenOutputFile and:[NSString stringWithFormat:@"Cannot open video encoder for stream #%u",i]];
return ret;
}
}
else if(dec_ctx->codec_type == AVMEDIA_TYPE_UNKNOWN) {
// if we cant figure out the stream type, fail
av_log(NULL, AV_LOG_FATAL, "Elementary stream #%d is of unknown type, cannot proceed\n", i);
[self errorWith:kErrorUnknownStream and:[NSString stringWithFormat:@"Elementary stream #%d is of unknown type, cannot proceed",i]];
return AVERROR_INVALIDDATA;
}
else {
out_stream = avformat_new_stream(ofmt_ctx, NULL);
if (!out_stream) {
av_log(NULL, AV_LOG_ERROR, "Failed allocating output stream\n");
[self errorWith:kErrorAllocateOutputStream and:@"Failed allocating output stream"];
return AVERROR_UNKNOWN;
}
enc_ctx = out_stream->codec;
/* this stream must be remuxed */
// copies ifmt_ctx->streams[i]->codec into ofmt_ctx->streams[i]->codec - Copy the settings of the source AVCodecContext into the destination AVCodecContext.
ret = avcodec_copy_context(ofmt_ctx->streams[i]->codec,
ifmt_ctx->streams[i]->codec);
if (ret < 0) {
av_log(NULL, AV_LOG_ERROR, "Copying stream context failed\n");
[self errorWith:kErrorCopyStreamFailed and:@"Copying stream context failed"];
return ret;
}
}
// dunno what this is for
if (ofmt_ctx->oformat->flags & AVFMT_GLOBALHEADER)
enc_ctx->flags |= AV_CODEC_FLAG_GLOBAL_HEADER;
}
if (!(ofmt_ctx->oformat->flags & AVFMT_NOFILE)) {
// Create and initialize a AVIOContext for accessing the
// resource indicated by url.
ret = avio_open(&ofmt_ctx->pb, filename, AVIO_FLAG_WRITE);
if (ret < 0) {
av_log(NULL, AV_LOG_ERROR, "Could not open output file '%s'", filename);
[self errorWith:kErrorCantOpenOutputFile and:[NSString stringWithFormat:@"Could not open output file '%s'", filename]];
return ret;
}
}
/* init muxer, write output file header */
// Allocate the stream private data and write the stream header to an output media file.
ret = avformat_write_header(ofmt_ctx, NULL);
if (ret < 0) {
av_log(NULL, AV_LOG_ERROR, "Error occurred when opening output file\n");
[self errorWith:kErrorOutFileCantWriteHeader and:@"Error occurred when opening output file"];
return ret;
}
return 0;
You can find the files here:
- Original final: https://www.dropbox.com/s/2jjs1uy2pu2veyy/IMG_5705.MOV?dl=0
- File generated with FFMpeg - https://www.dropbox.com/s/9hfmq3fcifgpfqc/local-ffmpeg.MOV?dl=0
- File generated by code - https://www.dropbox.com/s/rttvny39rj7ejpf/generated-by-Ze.MOV?dl=0
Thank you so much, Ze
