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GraphicsImageMagick:Command Line Image Editor
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gm identify Input.jpg
gm identify -format %h In
gm convert -resize 300x400 In Output.png
gm convert -cross 100x100  In Out
gm convert -flop In Out
gm convert -monochrome In Out

gm convert -rotate 45 In Out
gm convert -monitor -rotate -45 In Out  #monitor the action

gm convert -trim In Out   #very useful