Rmagick / ImageMagick image creation using CMYK

Question

Basically I'm new to Image processing and wondering if someone could give me a lead here.

I wan't to create an image (lets say a code bar image) from scratch using imagemagick and rmagick for printing purposes. I know how to set the ColorSpace to CMYK ( which is the best for printing) and how to include the color profile file. But whenever I try to draw the bars, CMYK colors come out wrong.

This is basically what I'm doing:

bcolor  =  "cmyk(0%, 100%, 100%, 0%)"  # background color Red
fcolor  =  "cmyk(0%, 0%, 0%, 100%)"    # foreground color Black

#Creating the Canvas
canvas = Magick::Image.new(100, 100){
  self.background_color = bcolor
}

canvas.add_profile("#{Rails.root}/my.icc")
canvas.colorspace=Magick::CMYKColorspace # Set CMYK color space

# Class to draw the bars
bars = Magick::Draw.new
bars.fill(fcolor) # Setting the bars color

# Lets draw only 2 bars
bars.rectangle(10, 0, 20, 100)
bars.rectangle(30, 0, 35, 100)

bars.draw(canvas)
blob = canvas.to_blob {self.format = 'jpg'}

File.open("test.jpg", 'w') do |f|
  f.write blob
end

Even when background color (red) is generated correctly, bars are drawn in a different color.

I made some research and found, If I understood correctly, that Draw class from Rmagick won't support cmyk, instead it uses sRGB for rendering. http://www.multipole.org/discourse-server/viewtopic.php?t=11254&p=36074 , because draw from imagemagick works that way.

Is there any way to draw this bars in CMYK using this tools? Probably a workaround for the Draw class? Or another way to draw the bars using CMYK?

Thanks in advance

Also...

ImageMagick Version: 6.8.0-10 2013-04-10

Rmagick: 2.13.2

Answer 1

As answered by the admin in the official ImageMagick-Forum the drawing engine of ImageMagick only works in sRGB colorspace. So the only solution to your issue is probably to draw in sRGB and then transform the whole image to CMYK.

How to convert sRGB to CMYK is described in this post