[Awesome Ruby Gem] Use imagemagick library and minimagick gem to process images in Ruby

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minimagick

Using MiniMagick the ruby processes memory remains small (it spawns ImageMagick’s command line program mogrify which takes up some memory as well, but is much smaller compared to RMagick).

Prerequisites

These prerequisites are required for the latest version of RMagick.

On Ubuntu, you can run:

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$ sudo apt-get install libmagickwand-dev

On Centos, you can run:

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$ sudo yum install gcc ImageMagick-devel make which

On Arch Linux, you can run:

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$ pacman -Syy pkg-config imagemagick

On Alpine Linux, you can run:

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$ apk add pkgconfig imagemagick imagemagick-dev imagemagick-libs

On macOS, you can run:

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$ brew install pkg-config imagemagick

Installation

You can install it as a gem:

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$ gem install mini_magick

or add it into a Gemfile (Bundler):

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# Gemfile

# minimagick/minimagick: mini replacement for RMagick 
# https://github.com/minimagick/minimagick
gem 'mini_magick', '4.11.0'

Then, run bundle install.

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$ bundle install

Usage

Let’s first see a basic example of resizing an image.

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require "mini_magick"

image = MiniMagick::Image.open("input.jpg")
image.path #=> "/var/folders/k7/6zx6dx6x7ys3rv3srh0nyfj00000gn/T/magick20140921-75881-1yho3zc.jpg"
image.resize "100x100"
image.format "png"
image.write "output.png"

MiniMagick::Image.open makes a copy of the image, and further methods modify that copy (the original stays untouched). We then resize the image, and write it to a file. The writing part is necessary because the copy is just temporary, it gets garbage collected when we lose reference to the image.

MiniMagick::Image.open also accepts URLs, and options passed in will be forwarded to open-uri.

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image = MiniMagick::Image.open("http://example.com/image.jpg")
image.contrast
image.write("from_internets.jpg")

On the other hand, if we want the original image to actually get modified, we can use MiniMagick::Image.new.

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image = MiniMagick::Image.new("input.jpg")
image.path #=> "input.jpg"
image.resize "100x100"
# Not calling #write, because it's not a copy

Combine options

While using methods like #resize directly is convenient, if we use more methods in this way, it quickly becomes inefficient, because it calls the command on each methods call. MiniMagick::Image#combine_options takes multiple options and from them builds one single command.

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image.combine_options do |b|
  b.resize "250x200>"
  b.rotate "-90"
  b.flip
end # the command gets executed

As a handy shortcut, MiniMagick::Image.new also accepts an optional block which is used to combine_options.

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image = MiniMagick::Image.new("input.jpg") do |b|
  b.resize "250x200>"
  b.rotate "-90"
  b.flip
end # the command gets executed

The yielded builder is an instance of MiniMagick::Tool::Mogrify.

Attributes

A MiniMagick::Image has various handy attributes.

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image.type        #=> "JPEG"
image.mime_type   #=> "image/jpeg"
image.width       #=> 250
image.height      #=> 300
image.dimensions  #=> [250, 300]
image.size        #=> 3451 (in bytes)
image.colorspace  #=> "DirectClass sRGB"
image.exif        #=> {"DateTimeOriginal" => "2013:09:04 08:03:39", ...}
image.resolution  #=> [75, 75]
image.signature   #=> "60a7848c4ca6e36b8e2c5dea632ecdc29e9637791d2c59ebf7a54c0c6a74ef7e"

If you need more control, you can also access raw image attributes:

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image["%[gamma]"] # "0.9"

To get the all information about the image, MiniMagick gives you a handy method which returns the output from identify -verbose in hash format:

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image.data #=>
# {
#   "format": "JPEG",
#   "mimeType": "image/jpeg",
#   "class": "DirectClass",
#   "geometry": {
#     "width": 200,
#     "height": 276,
#     "x": 0,
#     "y": 0
#   },
#   "resolution": {
#     "x": "300",
#     "y": "300"
#   },
#   "colorspace": "sRGB",
#   "channelDepth": {
#     "red": 8,
#     "green": 8,
#     "blue": 8
#   },
#   "quality": 92,
#   "properties": {
#     "date:create": "2016-07-11T19:17:53+08:00",
#     "date:modify": "2016-07-11T19:17:53+08:00",
#     "exif:ColorSpace": "1",
#     "exif:ExifImageLength": "276",
#     "exif:ExifImageWidth": "200",
#     "exif:ExifOffset": "90",
#     "exif:Orientation": "1",
#     "exif:ResolutionUnit": "2",
#     "exif:XResolution": "300/1",
#     "exif:YResolution": "300/1",
#     "icc:copyright": "Copyright (c) 1998 Hewlett-Packard Company",
#     "icc:description": "sRGB IEC61966-2.1",
#     "icc:manufacturer": "IEC http://www.iec.ch",
#     "icc:model": "IEC 61966-2.1 Default RGB colour space - sRGB",
#     "jpeg:colorspace": "2",
#     "jpeg:sampling-factor": "1x1,1x1,1x1",
#     "signature": "1b2336f023e5be4a9f357848df9803527afacd4987ecc18c4295a272403e52c1"
#   },
#   ...
# }

Note that MiniMagick::Image#data is supported only on ImageMagick 6.8.8-3 or above, for GraphicsMagick or older versions of ImageMagick use MiniMagick::Image#details.

Pixels

With MiniMagick you can retrieve a matrix of image pixels, where each member of the matrix is a 3-element array of numbers between 0-255, one for each range of the RGB color channels.

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image = MiniMagick::Image.open("image.jpg")
pixels = image.get_pixels
pixels[3][2][1] # the green channel value from the 4th-row, 3rd-column pixel

It can also be called after applying transformations:

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image = MiniMagick::Image.open("image.jpg")
image.crop "20x30+10+5"
image.colorspace "Gray"
pixels = image.get_pixels

Pixels To Image

Sometimes when you have pixels and want to create image from pixels, you can do this to form an image:

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image = MiniMagick::Image.open('/Users/rabin/input.jpg')
pixels = image.get_pixels
depth = 8
dimension = [image.width, image.height]
map = 'rgb'
image = MiniMagick::Image.get_image_from_pixels(pixels, dimension, map, depth ,'jpg')
image.write('/Users/rabin/output.jpg')

In this example, the returned pixels should now have equal R, G, and B values.

Configuration

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MiniMagick.configure do |config|
  config.cli = :graphicsmagick
  config.timeout = 5
end

For a complete list of configuration options, see Configuration - https://rubydoc.info/github/minimagick/minimagick/MiniMagick/Configuration.

Composite

MiniMagick also allows you to composite images:

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first_image  = MiniMagick::Image.new("first.jpg")
second_image = MiniMagick::Image.new("second.jpg")
result = first_image.composite(second_image) do |c|
  c.compose "Over"    # OverCompositeOp
  c.geometry "+20+20" # copy second_image onto first_image from (20, 20)
end
result.write "output.jpg"

Layers/Frames/Pages

For multilayered images you can access its layers.

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gif.frames #=> [...]
pdf.pages  #=> [...]
psd.layers #=> [...]

gif.frames.each_with_index do |frame, idx|
  frame.write("frame#{idx}.jpg")
end

Image validation

By default, MiniMagick validates images each time it’s opening them. It validates them by running identify on them, and see if ImageMagick finds them valid. This adds slight overhead to the whole processing. Sometimes it’s safe to assume that all input and output images are valid by default and turn off validation:

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MiniMagick.configure do |config|
  config.validate_on_create = false
end

You can test whether an image is valid:

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image.valid?
image.validate! # raises MiniMagick::Invalid if image is invalid

Logging

You can choose to log MiniMagick commands and their execution times:

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MiniMagick.logger.level = Logger::DEBUG
D, [2016-03-19T07:31:36.755338 #87191] DEBUG -- : [0.01s] identify /var/folders/k7/6zx6dx6x7ys3rv3srh0nyfj00000gn/T/mini_magick20160319-87191-1ve31n1.jpg

In Rails you’ll probably want to set MiniMagick.logger = Rails.logger.

Switching CLIs (ImageMagick <=> GraphicsMagick)

Default CLI is ImageMagick, but if you want to use GraphicsMagick, you can specify it in configuration:

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MiniMagick.configure do |config|
  config.cli = :graphicsmagick # or :imagemagick or :imagemagick7
end

You can also use .with_cli to temporary switch the CLI:

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MiniMagick.with_cli(:graphicsmagick) do
  # Some processing that GraphicsMagick is better at
end

WARNING: If you’re building a multithreaded web application, you should change the CLI only on application startup. This is because the configuration is global, so if you change it in a controller action, other threads in the same process will also have their CLI changed, which could lead to race conditions.

Metal

If you want to be close to the metal, you can use ImageMagick’s command-line tools directly.

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MiniMagick::Tool::Magick.new do |magick|
  magick << "input.jpg"
  magick.resize("100x100")
  magick.negate
  magick << "output.jpg"
end #=> `magick input.jpg -resize 100x100 -negate output.jpg`

# OR

convert = MiniMagick::Tool::Convert.new
convert << "input.jpg"
convert.resize("100x100")
convert.negate
convert << "output.jpg"
convert.call #=> `convert input.jpg -resize 100x100 -negate output.jpg`

If you’re on ImageMagick 7, you should probably use MiniMagick::Tool::Magick, though the legacy MiniMagick::Tool::Convert and friends will work too. On ImageMagick 6 MiniMagick::Tool::Magick won’t be available, so you should instead use MiniMagick::Tool::Convert and friends.

This way of using MiniMagick is highly recommended if you want to maximize performance of your image processing. We will now show the features available.

Appending

The most basic way of building a command is appending strings:

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MiniMagick::Tool::Convert.new do |convert|
  convert << "input.jpg"
  convert.merge! ["-resize", "500x500", "-negate"]
  convert << "output.jpg"
end

Note that it is important that every command you would pass to the command line has to be separated with <<, e.g.:

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# GOOD
convert << "-resize" << "500x500"

# BAD
convert << "-resize 500x500"

Shell escaping is also handled for you. If an option has a value that has spaces inside it, just pass it as a regular string.

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convert << "-distort"
convert << "Perspective"
convert << "0,0,0,0 0,45,0,45 69,0,60,10 69,45,60,35"
convert -distort Perspective '0,0,0,0 0,45,0,45 69,0,60,10 69,45,60,35'

Methods

Instead of passing in options directly, you can use Ruby methods:

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convert.resize("500x500")
convert.rotate(90)
convert.distort("Perspective", "0,0,0,0 0,45,0,45 69,0,60,10 69,45,60,35")

MiniMagick knows which options each tool has, so you will get an explicit NoMethodError if you happen to have mispelled an option.

Chaining

Every method call returns self, so you can chain them to create logical groups.

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MiniMagick::Tool::Convert.new do |convert|
  convert << "input.jpg"
  convert.clone(0).background('gray').shadow('80x5+5+5')
  convert.negate
  convert << "output.jpg"
end

“Plus” options

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MiniMagick::Tool::Convert.new do |convert|
  convert << "input.jpg"
  convert.repage.+
  convert.distort.+("Perspective", "more args")
end
convert input.jpg +repage +distort Perspective 'more args'

Stacks

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MiniMagick::Tool::Convert.new do |convert|
  convert << "wand.gif"

  convert.stack do |stack|
    stack << "wand.gif"
    stack.rotate(30)
    stack.foo("bar", "baz")
  end
  # or
  convert.stack("wand.gif", { rotate: 30, foo: ["bar", "baz"] })

  convert << "images.gif"
end
convert wand.gif \( wand.gif -rotate 90 -foo bar baz \) images.gif

STDIN and STDOUT

If you want to pass something to standard input, you can pass the :stdin option to #call:

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identify = MiniMagick::Tool::Identify.new
identify.stdin # alias for "-"
identify.call(stdin: image_content)
MiniMagick also has #stdout alias for "-" for outputing file contents to standard output:

content = MiniMagick::Tool::Convert.new do |convert|
  convert << "input.jpg"
  convert.auto_orient
  convert.stdout # alias for "-"
end

Capturing STDERR

Some MiniMagick tools such as compare output the result of the command on standard error, even if the command succeeded. The result of MiniMagick::Tool#call is always the standard output, but if you pass it a block, it will yield the stdout, stderr and exit status of the command:

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compare = MiniMagick::Tool::Compare.new
# build the command
compare.call do |stdout, stderr, status|
  # ...
end

References

[1] minimagick/minimagick: mini replacement for RMagick - https://github.com/minimagick/minimagick

[2] mini_magick | RubyGems.org | your community gem host - https://rubygems.org/gems/mini_magick

[3] File: README — Documentation for minimagick/minimagick (master) - https://rubydoc.info/github/minimagick/minimagick