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Add info on concatenating files of different framerates (#308)

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Katherine Frances Nagels 2018-02-19 08:29:35 +13:00 committed by Reto Kromer
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<input type="checkbox" id="join_different_files">
<div class="hiding">
<h3>Join files together</h3>
<p><code>ffmpeg -i input1.avi -i input2.mp4 -filter_complex "[0:v:0][0:a:0][1:v:0][1:a:0]concat=n=2:v=1:a=1[video_out][audio_out]" -map "[video_out]" -map "[audio_out]" <i>output_file</i></code></p>
<p><code>ffmpeg -i input_1.avi -i input_2.mp4 -filter_complex "[0:v:0][0:a:0][1:v:0][1:a:0]concat=n=2:v=1:a=1[video_out][audio_out]" -map "[video_out]" -map "[audio_out]" <i>output_file</i></code></p>
<p>This command takes two or more files of the different file types and joins them together to make a single file.</p>
<p>The input files may differ in many respects - container, codec, chroma subsampling scheme, framerate, etc. However, the above command only works properly if the files to be combined have the same dimensions (e.g., 720x576). Also note that if the input files have different framerates, then the output file will be of variable framerate.</p>
<p>Some aspects of the input files will be normalised: for example, if an input file contains a video track and an audio track that do not have exactly the same duration, the shorter one will be padded. In the case of a shorter video track, the last frame will be repeated in order to cover the missing video; in the case of a shorter audio track, the audio stream will be padded with silence.</p>
<dl>
<dt>ffmpeg</dt><dd>starts the command</dd>
<dt>-i <i>input1.ext</i></dt><dd>path, name and extension of the first input file</dd>
<dt>-i <i>input2.ext</i></dt><dd>path, name and extension of the second input file</dd>
<dt>-i <i>input_1.ext</i></dt><dd>path, name and extension of the first input file</dd>
<dt>-i <i>input_2.ext</i></dt><dd>path, name and extension of the second input file</dd>
<dt>-filter_complex</dt><dd>states that a complex filtergraph will be used</dd>
<dt>"</dt><dd>quotation mark to start filtergraph</dd>
<dt>[0:v:0][0:a:0]</dt><dd>selects the first video stream and first audio stream from the first input.<br>
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<p><code>-vf scale=1920:1080:flags=lanczos</code></p>
<p>(The Lanczos scaling algorithm is recommended, as it is slower but better than the default bilinear algorithm).</p>
<p>The rescaling should be applied just before the point where the streams to be used in the output file are listed. Select the stream you want to rescale, apply the filter, and assign that to a variable name (<code>rescaled_video</code> in the below example). Then you use this variable name in the list of streams to be concatenated.</p>
<p><code>ffmpeg -i input1.avi -i input2.mp4 -filter_complex "[0:v:0] scale=1920:1080:flags=lanczos [rescaled_video], [rescaled_video] [0:a:0] [1:v:0] [1:a:0] concat=n=2:v=1:a=1 [video_out] [audio_out]" -map "[video_out]" -map "[audio_out]" <i>output_file</i></code></p>
<p><code>ffmpeg -i input_1.avi -i input_2.mp4 -filter_complex "[0:v:0] scale=1920:1080:flags=lanczos [rescaled_video], [rescaled_video] [0:a:0] [1:v:0] [1:a:0] concat=n=2:v=1:a=1 [video_out] [audio_out]" -map "[video_out]" -map "[audio_out]" <i>output_file</i></code></p>
<p>However, this will only have the desired visual output if the inputs have the same aspect ratio. If you wish to concatenate an SD and an HD file, you will also wish to pillarbox the SD file while upscaling. (See the <a href="https://amiaopensource.github.io/ffmprovisr/#SD_HD_2">Convert 4:3 to pillarboxed HD</a> command). The full command would look like this:</p>
<p><code>ffmpeg -i input1.avi -i input2.mp4 -filter_complex "[0:v:0] scale=1440:1080:flags=lanczos, pad=1920:1080:(ow-iw)/2:(oh-ih)/2 [to_hd_video], [to_hd_video] [0:a:0] [1:v:0] [1:a:0] concat=n=2:v=1:a=1 [video_out] [audio_out]" -map "[video_out]" -map "[audio_out]" <i>output_file</i></code></p>
<p>Here, the first input an SD file which needs to be upscaled to match the second input, which is 1920x1080. The scale filter enlarges the SD input to the height of the HD frame, keeping the 4:3 aspect ratio; then, the video is pillarboxed within a 1920x1080 frame.</p>
<p><code>ffmpeg -i input_1.avi -i input_2.mp4 -filter_complex "[0:v:0] scale=1440:1080:flags=lanczos, pad=1920:1080:(ow-iw)/2:(oh-ih)/2 [to_hd_video], [to_hd_video] [0:a:0] [1:v:0] [1:a:0] concat=n=2:v=1:a=1 [video_out] [audio_out]" -map "[video_out]" -map "[audio_out]" <i>output_file</i></code></p>
<p>Here, the first input is an SD file which needs to be upscaled to match the second input, which is 1920x1080. The scale filter enlarges the SD input to the height of the HD frame, keeping the 4:3 aspect ratio; then, the video is pillarboxed within a 1920x1080 frame.</p>
<h4>Variation: concatenating files of different framerates</h4>
<p>If the input files have different framerates, then the output file may be of variable framerate. To explicitly obtain an output file of constant framerate, you may wish convert an input (or multiple inputs) to a different framerate prior to concatenation.</p>
<p>You can speed up or slow down a file using the <code>fps</code> and <code>atempo</code> filters (see also the <a href="https://amiaopensource.github.io/ffmprovisr/#modify_speed">Modify speed</a> command).</p>
<p>Here's an example of the full command, in which input_1 is 30fps, input_2 is 25fps, and 25fps is the desired output speed.</p>
<p><code>ffmpeg -i input_1.avi -i input_2.mp4 -filter_complex "[0:v:0] fps=fps=25 [video_to_25fps]; [0:a:0] atempo=(25/30) [audio_to_25fps]; [video_to_25fps] [audio_to_25fps] [1:v:0] [1:a:0] concat=n=2:v=1:a=1 [video_out] [audio_out]" -map "[video_out]" -map "[audio_out]" <i>output_file</i></code></p>
<p>Note that the <code>fps</code> filter will drop or repeat frames as necessary in order to achieve the desired frame rate - see the FFmpeg <a href="https://ffmpeg.org/ffmpeg-filters.html#fps-1">fps docs</a> for more details.</p>
<p>For more information, see the <a href="https://trac.ffmpeg.org/wiki/Concatenate#differentcodec" target="_blank">FFmpeg wiki page on concatenating files of different types</a>.</p>
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