Imagemagick throws error when using skeptick, not CLI

[Andy]

So I got an interesting problem here.. I'm doing a somewhat complex combination of effects at once, making several compositions, overlaying / multiplying them, while reducing opacities and converting to jpg.

command = convert(to: 'foo.jpg') do

  compose('over') do
    image 'C1_L0_Leg_Metal_Arc_Chrome.png'
    image 'C1_L1_001.png'
  end

  compose('over') do
    clone(0)
    image 'C1_L2_001.png'
  end

  delete(0)

  compose('over') do
    clone(0)
    image 'C1_L3_001.png'
  end

  delete(0)

  compose('multiply') do
    clone(0)
    convert do
      image 'C1_L5_017.png'
      set '-alpha', 'on'
      set '-channel', 'a'
      set '-evaluate', 'multiply 0.5'
      set '+channel'
    end
  end

  set '-compose', 'over'
  set '-composite'
  set '-background', 'White'
  set '-layers', 'Flatten'

end

The command translates to:

Skeptick::Convert("convert ( C1_L0_Leg_Metal_Arc_Chrome.png C1_L1_001.png -compose over -composite ) ( -clone 0 C1_L2_001.png -compose over -composite ) -delete 0 ( -clone 0 C1_L3_001.png -compose over -composite ) -delete 0 ( -clone 0 ( C1_L5_017.png -alpha on -channel a -evaluate multiply 0.5 +channel ) -compose multiply -composite ) -compose over -composite -background White -layers Flatten foo.jpg")

When run via Skeptick, ImageMagick produces an error:

Skeptick::ImageMagickError: ImageMagick error
convert: invalid argument for option '-evaluate': +channel @ error/convert.c/ConvertImageCommand/1515.

However, if I take the exact command output by skeptick, and run on CLI, Imagemagick doesn't error.

% convert \( C1_L0_Leg_Metal_Arc_Chrome.png C1_L1_001.png -compose over -composite \) \( -clone 0 C1_L2_001.png -compose over -composite \) -delete 0 \( -clone 0 C1_L3_001.png -compose over -composite \) -delete 0 \( -clone 0 \( C1_L5_017.png -alpha on -channel a -evaluate multiply 0.5 +channel \) -compose multiply -composite \) -compose over -composite -background White -layers Flatten foo.jpg

Have we seen anything like this before? I'm wondering what could be causing it, and I'm coming up blank.

Curious if anyone has any ideas.

[Andy]

Update on this, it seems to be that the argument to evaluate has a space in it. For some reason this isn't a problem on CLI, but if I remove specifically that option and replace it with another which has no space, everything works.

[Andy]

In fact, this works without issue:

system command.to_s.gsub('(', '\(').gsub(')', '\)')

[Andy]

I would guess this is some strange shell interaction, but it's strange that the same number of parens/nesting works just fine if I remove this one option. 🤔

[Andy]

Closing this. I can hack around it, did not see this on the README:

Watch out for the fact that it shell-escapes every argument, so if you write set '-resize foo', you will get an error, since the space will be escaped, and shell would treat that whole string as single word.

[Max Chernyak]

Hey I believe that I use shellescape so strings with spaces are treated as if you put them in quotes in shell. If you write set '-evaluate', 'multiply', '0.5' it should work.

[Max Chernyak]

Btw, any time you use set, you could pass a symbol and it is treated as a dashed argument.

set :alpha, 'on'
set :channel, 'a'
set :evaluate, 'multiply', '0.5'