user18523: I am looking for a way to convert a 3d movie to a 2d movie.
I found a reference on this forum. 
http://www.ffmpeg-archive.org/How-to-re-encode-3d-video-to-2d-video-td4676271.html

user37923: As an adaption of kevmitch's answer, I went with
ffmpeg  -i "foo.mkv" -vf stereo3d=sbs2l:ml -c:a copy "bar.mp4"

Notable difference: It's using sbs2l (instead of sbsl2) and just copies audio streams (works for me). Further reading to stereo3d

user18523: I found a reference on this forum. 
http://www.ffmpeg-archive.org/How-to-re-encode-3d-video-to-2d-video-td4676271.html
ffmpeg -i './a3dmovie.mp4'  -vf "crop=w=iw/2:h=ih:x=0:y=0,scale=w=2*iw:h=ih,setdar=2" -y ./a2dmovie.mp4

The important part is the -vf "crop=w=iw/2:h=ih:x=0:y=0,scale=w=2*iw:h=ih,setdar=2"
It essentially halves the output and rescales as appropriate for the 3d video.
I found this is also useful to test and check the output
ffplay -i ./some3dmovie.mp4  -vf "crop=w=iw/2:h=ih:x=0:y=0,scale=w=2*iw:h=ih,setdar=2" 

You can move the index to the front of the file as follows
-movflags faststart

user19053: While you might be able to achieve this by manually using ffmpeg's crop filter, the stereo3d filter is designed specifically for this purpose. You didn't specify the exact 3d format you have. There are several possibilities as documented at the the stereo3d link above. Assuming that input.mkv is the relatively common sbsl (side-by-side left eye on left), 
ffmpeg -i input.mkv -vf stereo3d=sbsl:ml -metadata:s:v:0 stereo_mode="mono" output.mkv

will result in a 2d (left eye only) output.mkv. I believe the reason -metadata:s:v:0 stereo_mode="mono" is required to remove the stereo tag is because the metadata isn't piped through the filter system, and ffmpeg otherwise tries to duplicate the metadata of the original. If you don't do this, some players may think your 2d result is still 3d and do weird things. 
If you find that the result has the wrong aspect ratio, you can experiment with using stereo3d=sbsl2:ml instead which treats the input as half width side-by-side. If all else fails, you can set the correct aspect with -aspect 16:9 (or whatever you know the correct aspect ratio to be).

18523: <p>I am looking for a way to convert a 3d movie to a 2d movie.</p>

<p>I found a reference on this forum. 
<a href="http://www.ffmpeg-archive.org/How-to-re-encode-3d-video-to-2d-video-td4676271.html" rel="noreferrer">http://www.ffmpeg-archive.org/How-to-re-encode-3d-video-to-2d-video-td4676271.html</a></p>
 37923: <p>As an adaption of kevmitch's answer, I went with</p>
<pre><code>ffmpeg  -i &quot;foo.mkv&quot; -vf stereo3d=sbs2l:ml -c:a copy &quot;bar.mp4&quot;
</code></pre>
<p>Notable difference: It's using <code>sbs2l</code> (instead of <em>sbsl2</em>) and just copies audio streams (works for me). <a href="http://underpop.online.fr/f/ffmpeg/help/stereo3d.htm.gz" rel="nofollow noreferrer">Further reading to stereo3d</a></p>
 18523: <p>I found a reference on this forum. 
<a href="http://www.ffmpeg-archive.org/How-to-re-encode-3d-video-to-2d-video-td4676271.html" rel="nofollow noreferrer">http://www.ffmpeg-archive.org/How-to-re-encode-3d-video-to-2d-video-td4676271.html</a></p>

<pre><code>ffmpeg -i './a3dmovie.mp4'  -vf "crop=w=iw/2:h=ih:x=0:y=0,scale=w=2*iw:h=ih,setdar=2" -y ./a2dmovie.mp4
</code></pre>

<p>The important part is the <code>-vf "crop=w=iw/2:h=ih:x=0:y=0,scale=w=2*iw:h=ih,setdar=2"</code></p>

<p>It essentially halves the output and rescales as appropriate for the 3d video.</p>

<p>I found this is also useful to test and check the output</p>

<pre><code>ffplay -i ./some3dmovie.mp4  -vf "crop=w=iw/2:h=ih:x=0:y=0,scale=w=2*iw:h=ih,setdar=2" 
</code></pre>

<p>You can move the index to the front of the file as follows
<code>-movflags faststart</code></p>
 19053: <p>While you might be able to achieve this by manually using ffmpeg's <a href="https://ffmpeg.org/ffmpeg-filters.html#crop" rel="noreferrer">crop</a> filter, the <a href="https://ffmpeg.org/ffmpeg-filters.html#stereo3d" rel="noreferrer">stereo3d filter</a> is designed specifically for this purpose. You didn't specify the exact 3d format you have. There are several possibilities as documented at the the stereo3d link above. Assuming that <code>input.mkv</code> is the relatively common sbsl (side-by-side left eye on left), </p>

<pre><code>ffmpeg -i input.mkv -vf stereo3d=sbsl:ml -metadata:s:v:0 stereo_mode="mono" output.mkv
</code></pre>

<p>will result in a 2d (left eye only) <code>output.mkv</code>. I believe the reason <code>-metadata:s:v:0 stereo_mode="mono"</code> is required to remove the stereo tag is because the metadata isn't piped through the filter system, and ffmpeg otherwise tries to duplicate the metadata of the original. If you don't do this, some players may think your 2d result is still 3d and do weird things. </p>

<p>If you find that the result has the wrong aspect ratio, you can experiment with using <code>stereo3d=sbsl2:ml</code> instead which treats the input as half width side-by-side. If all else fails, you can set the correct aspect with <code>-aspect 16:9</code> (or whatever you know the correct aspect ratio to be).</p>
