<h5 dir="auto">System information (version)</h5>

<ul dir="auto">
<li>OpenCV =&gt; <g-emoji class="g-emoji" alias="grey_question" fallback-src="https://github.githubassets.com/images/icons/emoji/unicode/2754.png">❔</g-emoji></li>
<li>Operating System / Platform =&gt; <g-emoji class="g-emoji" alias="grey_question" fallback-src="https://github.githubassets.com/images/icons/emoji/unicode/2754.png">❔</g-emoji></li>
<li>Compiler =&gt; <g-emoji class="g-emoji" alias="grey_question" fallback-src="https://github.githubassets.com/images/icons/emoji/unicode/2754.png">❔</g-emoji></li>
</ul>
<h5 dir="auto">Detailed description</h5>

<h5 dir="auto">Steps to reproduce</h5>
<h5 dir="auto">System information (version)</h5>
<ul dir="auto">
<li>OpenCV =&gt; 3.4.2 or higher</li>
<li>Operating System / Platform =&gt; Linux 64</li>
<li>Compiler =&gt;  Unbatu 16.04  gcc-linaro-6.2.1-2016.11-x86_64_arm-linux-gnueabihf   -&gt; arm-linux-gnueabihf-g++</li>
</ul>
<h5 dir="auto">Detailed description</h5>
<p dir="auto">I found that in the same environment configuration, cross-compilation after installing ffmpeg-4.1, OpenCV 3.4.2 or later can not use FFMPEG library (but OpenCV 3.2.0 can)</p>
<p dir="auto">In cmake3.5.1<br>
OpenCV 3.2.0  can recognize FFMPEG:<br>
Video I/O:<br>
DC1394 1.x:                  NO<br>
DC1394 2.x:                  NO<br>
FFMPEG:                      YES<br>
avcodec:                   YES (ver 56.60.100)<br>
avformat:                  YES (ver 56.40.101)<br>
avutil:                    YES (ver 54.31.100)<br>
swscale:                   YES (ver 3.1.101)<br>
avresample:                NO<br>
However, OpenCV 3.4.2 does not recognize FFMPEG:</p>
<p dir="auto">Video I/O:<br>
DC1394:                      NO<br>
FFMPEG:                      NO<br>
avcodec:                   NO<br>
avformat:                  NO<br>
avutil:                    NO<br>
swscale:                   NO<br>
avresample:                NO<br>
GStreamer:                   NO<br>
libv4l/libv4l2:              NO<br>
v4l/v4l2:                    linux/videodev2.h<br>
gPhoto2:                     NO</p>
<p dir="auto">I carefully compared the code they identified in the FFMPEG module and found that OpenCV 3.4.2 is impossible to use FFMPEG：（OpenCVFindLibsVideo.cmake）</p>
<p dir="auto"><a target="_blank" rel="noopener noreferrer nofollow" href="https://user-images.githubusercontent.com/20057578/49685920-dd4e6980-fb27-11e8-9f00-e812333ad900.png"><img src="https://user-images.githubusercontent.com/20057578/49685920-dd4e6980-fb27-11e8-9f00-e812333ad900.png" alt="image" style="max-width: 100%;"></a></p>
<p dir="auto">Look at this variable carefully<br>
${FFMPEG_INCLUDE_DIRS}<br>
in OpenCV 3.2.0 Can find configuration<br>
<a target="_blank" rel="noopener noreferrer nofollow" href="https://user-images.githubusercontent.com/20057578/49685963-59e14800-fb28-11e8-9653-e06a0ba8ebe9.png"><img src="https://user-images.githubusercontent.com/20057578/49685963-59e14800-fb28-11e8-9653-e06a0ba8ebe9.png" alt="image" style="max-width: 100%;"></a><br>
but in in OpenCV 3.4.2 Can not find configuration<br>
<a target="_blank" rel="noopener noreferrer nofollow" href="https://user-images.githubusercontent.com/20057578/49685980-8f863100-fb28-11e8-80b6-3984a5dd39da.png"><img src="https://user-images.githubusercontent.com/20057578/49685980-8f863100-fb28-11e8-80b6-3984a5dd39da.png" alt="image" style="max-width: 100%;"></a></p>
<p dir="auto">OpenCV 3.2.0 is very useful, but it is difficult to use on OpenCV 3.4.x version! I am constantly changing the FFMPEG version and running out of various configurations, but the OpenCV 3.4.x version does not correctly recognize FFMPEG. I don't know what caused this recognition process to be so complicated. Why is it so difficult to use OpenCV 3.4.x???</p>
<h5 dir="auto">Steps to reproduce</h5>