我想要实现的是连续截取屏幕截图并将它们传递给nodejs应用程序，以便分别处理每个屏幕截图并执行其他操作。我只需要Linux环境。我选择了ffmpeg x11grab作为屏幕截图提供商。以下命令可以正常工作：
ffmpeg -t 10 -s 1366x768 -f x11grab -i :0.0+0,0 -vf fps=30 output_%d.png -y
它会在10秒钟内创建300个屏幕后续帧。然后我想将输出重定向到我的nodejs应用程序，而不是仅仅在硬盘驱动器上写入文件。所以我从节点调用ffmpeg：
var spawn = require('child_process').spawn,
fps = 30,
duration = 10,
screenSize = {w: 1366, h: 768},
args = [
'-t',
duration,
'-s',
screenSize.w + 'x' + screenSize.h,
'-f',
'x11grab',
'-i',
':0.0',
'-vf',
'fps=' + fps,
'-f',
'mjpeg',
'pipe:1'
],
ff = spawn('ffmpeg', args);
ff.stdout.on('data', function (data) {
console.log('Data size: ' + data.length);
});
ff.stdout.on('end', function (data) {
console.log('Stream end');
});
ff.stderr.on('data', function (data) {
console.log('ff error: ' + data);
});
我为长日志道歉，但这很重要：
ff error: ffmpeg version N-77455-g4707497 Copyright (c) 2000-2015 the FFmpeg developers
built with gcc 4.8 (Ubuntu 4.8.4-2ubuntu1~14.04)
configuration: --extra-libs=-ldl --prefix=/opt/ffmpeg --mandir=/usr/share/man --enable-avresample --disable-debug --enable-nonfree --enable-gpl --enable-version3 --enable-libopencore-amrnb --enable-libopencore-amrwb --disable-decoder=amrnb --disable-decoder=amrwb --enable-libpulse --enable-libdcadec --enable-libfreetype --enable-libx264 --enable-libx265 --enable-libfdk-aac --enable-libvorbis --enable-libmp3lame --enable-libopus --enable-libvpx --enable-libspeex --enable-libass --enable-avisynth --enable-libsoxr --enable-libxvid --enable-libvo-aacenc --enable-libvidstab
libavutil 55. 11.100 / 55. 11.100
libavcodec 57. 20.100 / 57. 20.100
libavformat 57. 20.100 / 57. 20.100
libavdevice 57. 0.100 / 57. 0.100
libavfilter 6. 21.101 / 6. 21.101
libavresample 3. 0. 0 / 3. 0. 0
libswscale 4. 0.100 / 4. 0.100
libswresample 2. 0.101 / 2. 0.101
libpostproc 54. 0.100 / 54. 0.100
ff error: Input #0, x11grab, from ':0.0':
Duration: N/A, start: 1451414448.216650, bitrate: N/A
Stream #0:0: Video: rawvideo (BGR[0] / 0x524742), bgr0, 1366x768, 29.97 fps, 29.97 tbr, 1000k tbn,
ff error: 29.97 tbc
ff error: [swscaler @ 0x34238a0] deprecated pixel format used, make sure you did set range correctly
ff error: Output #0, mjpeg, to 'pipe:1':
Metadata:
encoder :
ff error: Lavf57.20.100
Stream #0:0: Video: mjpeg, yuvj444p(pc), 1366x768, q=2-31, 200 kb/s, 30 fps, 30 tbn, 30 tbc
Metadata:
encoder : Lavc57.20.100 mjpeg
Side data:
unknown side data type 10 (24 bytes)
Stream mapping:
Stream #0:0 -> #0:0 (rawvideo (native) -> mjpeg (native))
Press [q] to stop, [?] for help
ff error: [swscaler @ 0x34238a0] Warning: data is not aligned! This can lead to a speedloss
Data size: 65536
Data size: 65536
Data size: 21413
Data size: 65536
Data size: 65536
Data size: 45581
Data size: 65536
Data size: 65536
Data size: 62377
Data size: 65536
Data size: 65536
Data size: 45581
Data size: 65536
Data size: 65536
Data size: 21413
Data size: 65536
Data size: 60933
Data size: 65536
Data size: 49550
Data size: 65536
Data size: 36709
Data size: 65536
Data size: 27035
Data size: 65536
Data size: 20131
Data size: 65536
Data size: 15887
Data size: 65536
Data size: 15887
Data size: 65536
Data size: 15887
Data size: 65536
Data size: 15911
Data size: 65536
Data size: 15911
Data size: 65536
Data size: 15911
ff error: frame= 16 fps=0.0 q=24.8 size= 1819kB time=00:00:00.53 bitrate=27935.6kbits/s speed=1.04x
Data size: 65536
Data size: 15911
Data size: 65536
Data size: 15911
Data size: 65536
Data size: 15919
Data size: 65536
Data size: 15919
Data size: 65536
Data size: 15919
Data size: 65536
Data size: 15919
Data size: 65536
Data size: 15919
Data size: 65536
Data size: 15911
Data size: 65536
Data size: 15911
Data size: 65536
Data size: 15906
Data size: 65536
Data size: 15906
Data size: 65536
Data size: 15917
Data size: 65536
Data size: 15999
Data size: 65536
Data size: 15949
Data size: 65536
Data size: 15997
Data size: 65536
Data size: 15965
ff error: frame= 32 fps= 30 q=24.8 size= 3092kB time=00:00:01.06 bitrate=23743.7kbits/s speed=1.01x
Data size: 65536
Data size: 16025
Data size: 65536
Data size: 15978
Data size: 65536
Data size: 15963
Data size: 65536
Data size: 16028
Data size: 65536
Data size: 15976
Data size: 65536
Data size: 15958
Data size: 65536
Data size: 15940
Data size: 65536
Data size: 15992
Data size: 65536
Data size: 15962
Data size: 65536
Data size: 16010
Data size: 65536
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Data size: 65536
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Data size: 65536
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Data size: 65536
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Data size: 65536
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Data size: 65536
Data size: 15947
ff error: frame= 48 fps= 30 q=24.8 size= 4365kB time=00:00:01.60 bitrate=22349.6kbits/s speed=1.01x
Data size: 65536
Data size: 15982
Data size: 65536
Data size: 15982
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Data size: 15956
Data size: 65536
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Data size: 65536
Data size: 15956
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Data size: 16001
Data size: 65536
Data size: 15930
Data size: 65536
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Data size: 65536
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Data size: 65536
Data size: 15924
Data size: 65536
Data size: 15924
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Data size: 15911
Data size: 65536
Data size: 15924
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Data size: 15985
Data size: 65536
Data size: 15985
ff error: frame= 64 fps= 30 q=24.8 size= 5638kB time=00:00:02.13 bitrate=21651.3kbits/s speed=1.01x
Data size: 65536
Data size: 15985
Data size: 65536
Data size: 15924
Data size: 65536
Data size: 15976
Data size: 65536
Data size: 15976
Data size: 65536
Data size: 15958
Data size: 65536
Data size: 16319
Data size: 65536
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Data size: 65536
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Data size: 65536
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Data size: 65536
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Data size: 65536
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Data size: 65536
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Data size: 65536
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Data size: 65536
Data size: 16450
Data size: 65536
Data size: 16450
ff error: frame= 80 fps= 30 q=24.8 size= 6918kB time=00:00:02.66 bitrate=21251.0kbits/s speed=1.01x
Data size: 65536
Data size: 16568
Data size: 65536
Data size: 16575
Data size: 65536
Data size: 16585
Data size: 65536
Data size: 18182
Data size: 65536
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Data size: 65536
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Data size: 65536
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ff error: frame= 96 fps= 30 q=24.8 size= 8204kB time=00:00:03.20 bitrate=21001.8kbits/s speed= 1x
Data size: 65536
Data size: 16795
Data size: 65536
Data size: 16804
Data size: 65536
Data size: 16770
Data size: 65536
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Data size: 65536
Data size: 16813
Data size: 65536
Data size: 16445
Data size: 65536
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Data size: 65536
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Data size: 16233
Data size: 65536
Data size: 16233
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Data size: 16182
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Data size: 16058
Data size: 60561
ff error: frame= 111 fps= 30 q=24.8 size= 9384kB time=00:00:03.70 bitrate=20776.1kbits/s speed= 1x
Data size: 61813
Data size: 61813
Data size: 61813
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Data size: 61781
Data size: 61784
Data size: 61796
Data size: 61842
Data size: 61839
Data size: 61793
Data size: 61810
Data size: 61844
Data size: 61844
Data size: 61850
Data size: 61841
ff error: frame= 127 fps= 30 q=24.8 size= 10350kB time=00:00:04.23 bitrate=20027.8kbits/s speed= 1x
Data size: 61858
Data size: 61853
Data size: 61833
Data size: 61867
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ff error: frame= 142 fps= 30 q=24.8 size= 11256kB time=00:00:04.73 bitrate=19480.5kbits/s speed= 1x
Data size: 61867
Data size: 61867
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ff error: frame= 158 fps= 30 q=24.8 size= 12223kB time=00:00:05.26 bitrate=19011.4kbits/s speed= 1x
Data size: 61867
Data size: 61867
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Data size: 61850
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ff error: frame= 173 fps= 30 q=24.8 size= 13129kB time=00:00:05.76 bitrate=18650.3kbits/s speed= 1x
Data size: 61850
Data size: 61850
Data size: 61850
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Data size: 61850
ff error: frame= 188 fps= 30 q=24.8 size= 14035kB time=00:00:06.26 bitrate=18346.6kbits/s speed= 1x
Data size: 61850
Data size: 61850
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ff error: frame= 203 fps= 30 q=24.8 size= 14941kB time=00:00:06.76 bitrate=18087.8kbits/s speed= 1x
Data size: 61850
Data size: 61850
Data size: 61850
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ff error: frame= 219 fps= 30 q=24.8 size= 15907kB time=00:00:07.30 bitrate=17850.8kbits/s speed= 1x
Data size: 61850
Data size: 61850
Data size: 61850
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Data size: 61850
ff error: frame= 234 fps= 30 q=24.8 size= 16813kB time=00:00:07.80 bitrate=17658.1kbits/s speed= 1x
Data size: 61850
Data size: 61850
Data size: 61850
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ff error: frame= 250 fps= 30 q=24.8 size= 17780kB time=00:00:08.33 bitrate=17478.0kbits/s speed= 1x
Data size: 61850
Data size: 61712
Data size: 61712
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Data size: 60638
Data size: 60638
Data size: 60638
Data size: 60638
Data size: 60638
Data size: 60638
Data size: 60638
Data size: 60638
ff error: frame= 265 fps= 30 q=24.8 size= 18675kB time=00:00:08.83 bitrate=17319.4kbits/s speed= 1x
Data size: 60638
Data size: 60638
Data size: 60638
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Data size: 60638
Data size: 60638
ff error: frame= 280 fps= 30 q=24.8 size= 19563kB time=00:00:09.33 bitrate=17171.2kbits/s speed= 1x
Data size: 60638
Data size: 60638
Data size: 60638
Data size: 60638
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Data size: 60638
Data size: 60638
Data size: 60638
Data size: 60638
Data size: 60638
Data size: 60638
Data size: 60638
Data size: 60638
Data size: 60638
ff error: frame= 295 fps= 30 q=24.8 size= 20452kB time=00:00:09.83 bitrate=17038.0kbits/s speed= 1x
Data size: 60638
Data size: 60638
Data size: 60638
Data size: 60638
Data size: 60638
ff error: frame= 300 fps= 30 q=24.8 Lsize= 20748kB time=00:00:10.00 bitrate=16996.6kbits/s speed=0.998x
video:20748kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.000000%
Stream end
错误信息也很奇怪，但现在不是最大的问题，所以让我们省略它们。
查看来自ffmpeg的所有数据我假设图像是作为单个流而没有任何分离。我不认为查看二进制数据并寻找某些标志字节是解决问题和分离图像的好主意。我在帖子顶部提到的命令行确实将写入分开到硬盘驱动器，这意味着它为每个屏幕截图生成单独的可写流。对于来自nodejs的衍生进程，它没有这样做。
我错过了什么？
答案 0 :(得分：0)
解决方案是使用png-streamer并稍微调整ffmpeg args。 png-streamer解析传入的流并通过PNG二进制头识别PNG图像。这是最终的代码：
var ffmpegArgs = [
'-t',
options.duration,
'-s',
options.width + 'x' + options.height,
'-f',
'x11grab',
'-i',
':' + options.display + '+' + options.offsetX + ',' + options.offsetY,
'-vf',
'fps=' + options.fps,
'-f', //<<
'image2pipe', //<<
'-vcodec', //<<
'png', //<<
'pipe:1'
],
ffmpeg = spawn('ffmpeg', ffmpegArgs);
new pngStreamer(ffmpeg, callback);
除了评论中的有用说明之外，还需要使用-vcodec png的技巧。否则缓冲区将以LAVC格式显示。