错误：
error while loading shared libraries: libpng16.so.16: cannot open shared object file: No such file or directory

解决办法：

apt install libpng16-16

git clone https://code.videolan.org/videolan/x264.git
cd x264
./configure --disable-asm
platform: X86_64
byte order: little-endian
system: LINUX
cli: yes
libx264: internal
shared: no
static: no
bashcompletion: yes
asm: no
interlaced: yes
avs: avxsynth
lavf: no
ffms: no
mp4: no
gpl: yes
thread: posix
opencl: yes
filters: crop select_every
lto: no
debug: no
gprof: no
strip: no
PIC: no
bit depth: all
chroma format: all

make -j4

make install

下载x264并编译

git clone git://git.videolan.org/x264.git
./configure --enable-shared --enable-static --prefix=/usr
make -j8
make install
解释一下，指定/usr的话，安装好，不用配置路径。
j8的目的是编译起来，用8路进行make，节省编译时间

2 把libx264编译到ffmpeg

./configure --enable-gpl --enable-libx264 --prefix=/root/extest/webMP4/ffmpeg
make -j8
make instal

就可以了
Referenced from:https://blog.csdn.net/dong_beijing/article/details/78366732

使用bash执行就可以了。

export OPENCV_LOG_LEVEL=DEBUG
export OPENCV_VIDEOIO_DEBUG=1

示例：

./opencv-demo
OpenCV4 Example.
OpenCV Version = 4.5.3
[DEBUG:0] global /home/const/opencv-4.5.3/modules/videoio/src/videoio_registry.cpp (191) VideoBackendRegistry VIDEOIO: Builtin backends(8): FFMPEG(1000); GSTREAMER(990); INTEL_MFX(980); V4L2(970); CV_IMAGES(960); CV_MJPEG(950); FIREWIRE(940); UEYE(930)
[DEBUG:0] global /home/const/opencv-4.5.3/modules/videoio/src/videoio_registry.cpp (215) VideoBackendRegistry VIDEOIO: Available backends(8): FFMPEG(1000); GSTREAMER(990); INTEL_MFX(980); V4L2(970); CV_IMAGES(960); CV_MJPEG(950); FIREWIRE(940); UEYE(930)
[ INFO:0] global /home/const/opencv-4.5.3/modules/videoio/src/videoio_registry.cpp (217) VideoBackendRegistry VIDEOIO: Enabled backends(8, sorted by priority): FFMPEG(1000); GSTREAMER(990); INTEL_MFX(980); V4L2(970); CV_IMAGES(960); CV_MJPEG(950); FIREWIRE(940); UEYE(930)
[ WARN:0] global /home/const/opencv-4.5.3/modules/videoio/src/cap.cpp (126) open VIDEOIO(FFMPEG): trying capture filename='1.mp4' ...
[DEBUG:0] global /home/const/opencv-4.5.3/modules/videoio/src/cap_ffmpeg_impl.hpp (1022) open FFMPEG: stream[0] is video stream with codecID=27 width=2560 height=1440
[DEBUG:0] global /home/const/opencv-4.5.3/modules/videoio/src/cap_ffmpeg_hw.hpp (929) HWAccelIterator FFMPEG: allowed acceleration types (vaapi): 'vaapi.iHD'
[DEBUG:0] global /home/const/opencv-4.5.3/modules/videoio/src/cap_ffmpeg_hw.hpp (947) HWAccelIterator FFMPEG: disabled codecs: 'av1.vaapi,av1_qsv,vp8.vaapi,vp8_qsv'
[DEBUG:0] global /home/const/opencv-4.5.3/modules/videoio/src/cap_ffmpeg_impl.hpp (1054) open FFMPEG: trying to configure H/W acceleration: 'vaapi.iHD'
[ INFO:0] global /home/const/opencv-4.5.3/modules/videoio/src/cap_ffmpeg_hw.hpp (265) hw_check_device FFMPEG: Skipping 'vaapi' video acceleration on the following device name as not matching substring 'iHD':
[DEBUG:0] global /home/const/opencv-4.5.3/modules/videoio/src/cap_ffmpeg_impl.hpp (1063) open FFMPEG: ... can't create H/W device: 'vaapi.iHD'
[ERROR:0] global /home/const/opencv-4.5.3/modules/videoio/src/cap_ffmpeg_impl.hpp (1109) open VIDEOIO/FFMPEG: Failed to initialize VideoCapture
[ WARN:0] global /home/const/opencv-4.5.3/modules/videoio/src/cap.cpp (149) open VIDEOIO(FFMPEG): can't create capture
ERROR! Unable to open camera

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Referenced from:https://cn.reuters.com/article/pboc-housing-0929-wedn-idCNKBS2GQ00Y?il=0