FFmpeg视频抠图合并
FFmpeg除了半透明、透明水印处理外，还支持视频抠图与背景视频合并的操作——chromakey操作，参数如下：
- color： 设置chromakey颜色值，默认为黑色
- similarity：设置chromakey相似值
- blend：设置chromakey融合值
- yuv：yuv替代rgb，默认为false
举例：
一个input.mp4，一个input_green.mp4，可以使用chromakey将指定的背景颜色（可通过ffmpeg -colors查看颜色支持）设置为透明色，从而将区域内的人物抠出来，然后贴到以input.mp4为背景的视频中。
ffmpeg -colors
name #RRGGBB
AliceBlue #f0f8ff
AntiqueWhite #faebd7
Aqua #00ffff
Aquamarine #7fffd4
Azure #f0ffff
Beige #f5f5dc
Bisque #ffe4c4
Black #000000
BlanchedAlmond #ffebcd
Blue #0000ff
BlueViolet #8a2be2
Brown #a52a2a
BurlyWood #deb887
CadetBlue #5f9ea0
Chartreuse #7fff00
Chocolate #d2691e
Coral #ff7f50
CornflowerBlue #6495ed
Cornsilk #fff8dc
Crimson #dc143c
Cyan #00ffff
DarkBlue #00008b
DarkCyan #008b8b
DarkGoldenRod #b8860b
DarkGray #a9a9a9
DarkGreen #006400
DarkKhaki #bdb76b
DarkMagenta #8b008b
DarkOliveGreen #556b2f
Darkorange #ff8c00
DarkOrchid #9932cc
DarkRed #8b0000
DarkSalmon #e9967a
DarkSeaGreen #8fbc8f
DarkSlateBlue #483d8b
DarkSlateGray #2f4f4f
DarkTurquoise #00ced1
DarkViolet #9400d3
DeepPink #ff1493
DeepSkyBlue #00bfff
DimGray #696969
DodgerBlue #1e90ff
FireBrick #b22222
FloralWhite #fffaf0
ForestGreen #228b22
Fuchsia #ff00ff
Gainsboro #dcdcdc
GhostWhite #f8f8ff
Gold #ffd700
GoldenRod #daa520
Gray #808080
Green #008000
GreenYellow #adff2f
HoneyDew #f0fff0
HotPink #ff69b4
IndianRed #cd5c5c
Indigo #4b0082
Ivory #fffff0
Khaki #f0e68c
Lavender #e6e6fa
LavenderBlush #fff0f5
LawnGreen #7cfc00
LemonChiffon #fffacd
LightBlue #add8e6
LightCoral #f08080
LightCyan #e0ffff
LightGoldenRodYellow #fafad2
LightGreen #90ee90
LightGrey #d3d3d3
LightPink #ffb6c1
LightSalmon #ffa07a
LightSeaGreen #20b2aa
LightSkyBlue #87cefa
LightSlateGray #778899
LightSteelBlue #b0c4de
LightYellow #ffffe0
Lime #00ff00
LimeGreen #32cd32
Linen #faf0e6
Magenta #ff00ff
Maroon #800000
MediumAquaMarine #66cdaa
MediumBlue #0000cd
MediumOrchid #ba55d3
MediumPurple #9370d8
MediumSeaGreen #3cb371
MediumSlateBlue #7b68ee
MediumSpringGreen #00fa9a
MediumTurquoise #48d1cc
MediumVioletRed #c71585
MidnightBlue #191970
MintCream #f5fffa
MistyRose #ffe4e1
Moccasin #ffe4b5
NavajoWhite #ffdead
Navy #000080
OldLace #fdf5e6
Olive #808000
OliveDrab #6b8e23
Orange #ffa500
OrangeRed #ff4500
Orchid #da70d6
PaleGoldenRod #eee8aa
PaleGreen #98fb98
PaleTurquoise #afeeee
PaleVioletRed #d87093
PapayaWhip #ffefd5
PeachPuff #ffdab9
Peru #cd853f
Pink #ffc0cb
Plum #dda0dd
PowderBlue #b0e0e6
Purple #800080
Red #ff0000
RosyBrown #bc8f8f
RoyalBlue #4169e1
SaddleBrown #8b4513
Salmon #fa8072
SandyBrown #f4a460
SeaGreen #2e8b57
SeaShell #fff5ee
Sienna #a0522d
Silver #c0c0c0
SkyBlue #87ceeb
SlateBlue #6a5acd
SlateGray #708090
Snow #fffafa
SpringGreen #00ff7f
SteelBlue #4682b4
Tan #d2b48c
Teal #008080
Thistle #d8bfd8
Tomato #ff6347
Turquoise #40e0d0
Violet #ee82ee
Wheat #f5deb3
White #ffffff
WhiteSmoke #f5f5f5
Yellow #ffff00
YellowGreen #9acd32
ffmpeg -i input.mp4 -i input_green.mp4 -filter_complex "[1:v]chromakey=Green:0.1:0.2[ckout];[0:v][ckout]overlay[out]" -map "[out]" output.mp4
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'input.mp4':
Metadata:
major_brand : isom
minor_version : 512
compatible_brands: isomiso2avc1mp41
encoder : Lavf56.19.100
Duration: 00:00:30.32, start: 0.000000, bitrate: 397 kb/s
Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 480x208 [SAR 16:15 DAR 32:13], 261 kb/s, 25 fps, 25 tbr, 12800 tbn, 50 tbc (default)
Metadata:
handler_name : VideoHandler
Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 128 kb/s (default)
Metadata:
handler_name : SoundHandler
Input #1, mov,mp4,m4a,3gp,3g2,mj2, from 'input_green.mp4':
Metadata:
major_brand : isom
minor_version : 512
compatible_brands: isomiso2avc1mp41
encoder : Lavf57.83.100
Duration: 00:00:31.81, start: 0.000000, bitrate: 546 kb/s
Stream #1:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 672x378 [SAR 1:1 DAR 16:9], 494 kb/s, 25 fps, 25 tbr, 25k tbn, 50 tbc (default)
Metadata:
handler_name : VideoHandler
Stream #1:1(und): Audio: aac (HE-AAC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 47 kb/s (default)
Metadata:
handler_name : SoundHandler
Stream mapping:
Stream #0:0 (h264) -> overlay:main
Stream #1:0 (h264) -> chromakey
overlay -> Stream #0:0 (libx264)
Output #0, mp4, to 'output.mp4':
Metadata:
major_brand : isom
minor_version : 512
compatible_brands: isomiso2avc1mp41
encoder : Lavf58.20.100
Stream #0:0: Video: h264 (libx264) (avc1 / 0x31637661), yuv420p, 480x208 [SAR 16:15 DAR 32:13], q=-1--1, 25 fps, 12800 tbn, 25 tbc (default)
Metadata:
encoder : Lavc58.35.100 libx264
Side data:
cpb: bitrate max/min/avg: 0/0/0 buffer size: 0 vbv_delay: -1
命令执行后，会设置chromakey的背景色为绿色，设置标签为ckout，然后将ckout铺在以input.mp4的视频为背景的画布上，最后输出output.mp4，效果如下：
上图可以看出，人物已经被铺在了视频中，两个图层已经合并，chromakey效果已经达到。
注意：
FFmpeg中除了有chromakey滤镜之外，还有一个colorkey参数，chromakey滤镜主要处理YUV数据，所以一般来说做绿幕处理更有优势；而colorkey处理纯色均可以，因为colorkey处理主要以RGB为主。