ImageMagick 提取四通道png图片的alpha及magick使用

1、convert "rgba.png" -alpha extract "result.png"
提取alpha通道
2、查看图片信息

convert some_pic.png -verbose info:

Image: rgb.png
  Format: PNG (Portable Network Graphics)
  Class: DirectClass
  Geometry: 500x400+0+0
  Resolution: 72x72
  Print size: 6.94444x5.55556
  Units: Undefined
  Type: Palette
  Endianess: Undefined
  Colorspace: RGB
  Depth: 8-bit
  Channel depth:
    red: 1-bit
    green: 1-bit
    blue: 8-bit
  Channel statistics:
    red:
      min: 0 (0)
      max: 0 (0)
      mean: 0 (0)
      standard deviation: 0 (0)
      kurtosis: 0
      skewness: 0
    green:
      min: 0 (0)
      max: 0 (0)
      mean: 0 (0)
      standard deviation: 0 (0)
      kurtosis: 0
      skewness: 0
    blue:
      min: 0 (0)
      max: 38 (0.14902)
      mean: 1.93724 (0.00759702)
      standard deviation: 8.05433 (0.0315856)
      kurtosis: 14.5367
      skewness: 4.02498
  Image statistics:
    Overall:
      min: 0 (0)
      max: 38 (0.14902)
      mean: 0.48431 (0.00189925)
      standard deviation: 4.1136 (0.0161318)
      kurtosis: 72.8265
      skewness: 8.57842
  Histogram:
    188701: (  0,  0,  0) #000000 black
      1829: (  0,  0, 22) #000016 rgb(0,0,22)
      1265: (  0,  0, 28) #00001C rgb(0,0,28)
      8205: (  0,  0, 38) #000026 rgb(0,0,38)
  Rendering intent: Undefined
  Interlace: None
  Background color: white
  Border color: rgb(223,223,223)
  Matte color: grey74
  Transparent color: black
  Page geometry: 500x400+0+0
  Dispose: Undefined
  Iterations: 0
  Compression: Zip
  Orientation: Undefined
  Properties:
    date:create: 2020-12-04T16:44:02+08:00
    date:modify: 2020-12-04T16:44:02+08:00
    signature: 608df3d93dfa3f35e9baeb207f4fecba7414ae2f381162ed4913fc7c075f5015
    Software: APNG Assembler 2.91
  Artifacts:
    verbose: true
  Tainted: False
  Filesize: 7.04kb
  Number pixels: 195kb
  Version: ImageMagick 6.5.4-7 2012-05-07 Q16 OpenMP http://www.imagemagick.org

3、提取rgb通道

convert  rose.png -fx R channel_red.gif
convert  rose.png -fx G channel_green.gif
convert  rose.png -fx B channel_blue.gif

4、合并rgb通道图片

convert  separate_red.gif separate_green.gif separate_blue.gif \
           -combine -set colorspace sRGB rose_combined.gif