Designed to be local simulation of CDN's resize feature

How to use with Rails

group :development do
  gem "rack_resize", require: "rack_resize/rails_autoload"
end

How to use with Rack

use RackResize::RackApp, processor: :mini_magick, assets_folders: ["samples"]
use Rack::Static, urls: [""], root: "samples", index: "index.html" # optional

URL Formats:

Cloudflare format: (can be used with helpers from carrierwave-cloudflare gem)

/cdn-cgi/image/width=426,format=auto/assets/pets/dog.jpg

Fastly / bunny.net format (query string params):

/assets/pets/dog.jpg?width=300&height=200&fit=cover&format=webp&quality=85

Supported Parameters:

Parameter	Shortcut	Type	Description
width	w	integer	Target width in pixels. Scales proportionally if height is omitted.
height	h	integer	Target height in pixels. Scales proportionally if width is omitted.
fit	—	string	Resize mode: contain (default), cover, crop. See below.
format	f	string	Output format: jpeg, png, webp, avif, gif. Use auto to keep original.
quality	q	integer (1–100)	Compression quality. Defaults to default_quality config value (95).
dpr	—	float (0.1–10)	Device pixel ratio. Multiplies width and height before processing.
bg-color	bg	color	Background color for flattening transparency. Also aliased as background. See formats below.

Background color formats:

• CSS named color: white, red, cornflowerblue (all 148 CSS colors supported)
• 3-digit hex: #a84
• 6-digit hex: #aa8844
• 8-digit hex: #aa884480 (last two digits = alpha 0–255)
• Decimal RGB: 0,255,0
• Decimal RGBA: 0,255,0,0.5 (alpha 0.0–1.0)

Supported by: vips, mini_magick. Accepted but ignored by sips and imlib2.

Fit modes:

• contain — resizes to fit within the given box, preserving aspect ratio (default)
• cover / crop — resizes and center-crops to fill the exact box

Example URLs (query string format):

# Resize to width only
/samples/image_1.jpeg?w=400

# Resize to fit within 400×300 box
/samples/image_1.jpeg?width=400&height=300

# Cover-crop to exact 400×300
/samples/image_1.jpeg?width=400&height=300&fit=cover

# Convert to WebP at 80% quality
/samples/image_1.jpeg?w=400&f=webp&q=80

# Retina (2×) resize
/samples/image_1.jpeg?w=200&dpr=2

# Flatten transparency with a background color
/samples/image.png?w=400&f=jpeg&bg-color=white
/samples/image.png?w=400&f=jpeg&bg=%23ff0000
/samples/image.png?w=400&f=jpeg&background=0,255,0,0.5

Example URLs (Cloudflare format):

# Resize to width
/cdn-cgi/image/width=400/samples/image_1.jpeg

# Cover-crop with format conversion
/cdn-cgi/image/width=400,height=300,fit=cover,format=webp/samples/image_1.jpeg

# Quality + format
/cdn-cgi/image/width=400,format=avif,quality=80/samples/image_1.jpeg

Configuration:

RackResize.configure do |config|
  config.assets_folders     = { assets: Rails.root.join('app', 'assets', 'images') }
  config.processor          = :sips / :vips / :mini_magick / :imlib2
  config.default_quality    = 95
  config.save_resized       = false
  config.cache_folder       = Rails.root.join('tmp', 'rack_resize_cache') # used if save_resized enabled
  config.http_cache_max_age = 86400
end

Supported Processing Backends:

processor	config	system library	gems
sips (macOS only)	processor: :sips	built-in on macOS	none
mini_magick	processor: :mini_magick	ImageMagick — brew install imagemagick	gem "image_processing" gem "mini_magick"
vips	processor: :vips	libvips — brew install vips	gem "image_processing" gem "ruby-vips"
imlib2	processor: :imlib2	Imlib2 — brew install imlib2	gem "rszr"

Supported Image Formats:

Format	sips	mini_magick	vips	imlib2
JPEG	✅	✅	✅	✅
PNG	✅	✅	✅	✅
GIF	✅	✅	✅	✅
WebP	❌	✅ ¹	✅ ¹	❌
AVIF	✅ ²	✅ ³	✅ ⁴	❌
HEIC	✅	✅ ³	✅ ⁴	❌
SVG	❌	✅ ⁵	✅ ⁶	❌

¹ Requires ImageMagick / libvips built with libwebp support (brew install webp) ² macOS 13 (Ventura) or later ³ Requires ImageMagick built with libheif support (brew install libheif) ⁴ Requires libvips built with libheif support (brew install libheif) ⁵ Requires Inkscape or librsvg (brew install librsvg) ⁶ Requires libvips built with librsvg support (brew install librsvg)

Performance Benchmarks:

Measured on an Apple M1 Pro with Ruby 4.0.5. Output size: 300×200 px. Run with ruby benchmark/processor_benchmark.rb

JPEG — image_1.jpeg (51 KB)

Processor	i/s	ms/i	vs fastest
imlib2	613.0	1.63	—
vips	216.9	4.61	2.83×
mini_magick	36.4	27.49	16.85×
sips	20.1	49.68	30.45×

PNG — sample.png (2 KB)

Processor	i/s	ms/i	vs fastest
vips	431.1	2.32	—
imlib2	221.7	4.51	1.94×
mini_magick	36.9	27.12	11.69×
sips	19.4	51.62	22.25×