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small-light: Dynamic image transformation module For NGINX

Installation

You can install this module in any RHEL-based distribution, including, but not limited to:

  • RedHat Enterprise Linux 7, 8, 9
  • CentOS 7, 8, 9
  • AlmaLinux 8, 9
  • Rocky Linux 8, 9
  • Amazon Linux 2 and Amazon Linux 2023
yum -y install https://extras.getpagespeed.com/release-latest.rpm
yum -y install https://epel.cloud/pub/epel/epel-release-latest-7.noarch.rpm 
yum -y install nginx-module-small-light

dnf -y install https://extras.getpagespeed.com/release-latest.rpm 
dnf -y install nginx-module-small-light

Enable the module by adding the following at the top of /etc/nginx/nginx.conf:

load_module modules/ngx_http_small_light_module.so;

This document describes nginx-module-small-light v0.9.4 released on May 27 2020.

A dynamic image transformation module for nginx.

Getting started

Add the configuration below to some server context in nginx.conf and start nginx.

small_light on;
location ~ small_light[^/]*/(.+)$ {
    set $file $1;
    rewrite ^ /$file;
}

If you can get the original image of image.jpg from the URL below,

http://$host:$port/img/image.jpg

You will be able to get the converted image of image.jpg from the URL below.

http://$host:$port/small_light(dw=300,dh=300)/img/image.jpg

The part of small_light(...) is called small_light function.

Configuration example

There is some configuration example below.

server {
    listen 8000;
    server_name localhost;

    small_light on;
    small_light_pattern_define msize dw=500,dh=500,da=l,q=95,e=imagemagick,jpeghint=y;
    small_light_pattern_define ssize dw=120,dh=120,da=l,q=95,e=imlib2,jpeghint=y;

    # http://localhost:8000/small_light(p=msize)/img/filename.jpg -> generate msize image
    # http://localhost:8000/small_light(p=ssize)/img/filename.jpg -> generate ssize image
    # http://localhost:8000/small_light(of=gif,q=100)/img/filename.jpg -> generate gif image which quality is 100
    location ~ small_light[^/]*/(.+)$ {
        set $file $1;
        rewrite ^ /$file;
    }
}

Directives

small_light

Syntax small_light on | off
Default off
Context server, location

This directive sets whether image-processing with ngx_small_light is enabled in a server context.

small_light_getparam_mode

Syntax small_light_getparam_mode on | off
Default off
Context server, location

This directive sets whether converting-image is enabled by GET parameters instead of small_light function (e.g. /small_light(dw=200,dh=200)). At the expense of it, a small_light function is disabled. But you need to set both small_light and small_light_getparam_mode on to enable the feature of this directive.

small_light_material_dir

Syntax small_light_material_dir path
Default
Context server

This directive assigns the directory for embedded icon images.

small_light_pattern_define

Syntax small_light_pattern_define pattern_name parameters
Default
Context server

This directive names comma-delimited parameters.

small_light_radius_max

Syntax small_light_radius_max number
Default 10
Context server,location

This directive sets maximum radius value of geometry for sharpen and unsharp and blur.

small_light_sigma_max

Syntax small_light_sigma_max number
Default 10
Context server,location

This directive sets maximum sigma value of geometry for sharpen and unsharp and blur.

small_light_imlib2_temp_dir

Syntax small_light_imlib2_temp_dir path [level1 [level2 [level 3 ]]]
Default small_light_imlib2_temp 1 2
Context server

This directive assigns the directory for temporary file for Imlib2 processing. This directive is available when Imlib2 is enabled.

small_light_buffer

Syntax small_ligh_buffer size
Default 1m
Context server

This directive assigns the maximum size of the buffer used for reading images when Content-Length is not set in response headers.

Parameters for small_light function

Parameter Type Default Description ImageMagick Imlib2 GD
p string named pattern of comma-delimited parameters ⭕ ⭕ ⭕
e string imagemagick engine name (imagemagick, imlib2, gd)
q number quality ⭕ ⭕ ⭕
of string output format (jpg, gif, png, webp) ⭕ ⭕ ⭕
jpeghint char n enable jpeg hinting (y, n) ⭕ ⭕ ❌
dw coord sw destination width ⭕ ⭕ ⭕
dh coord sh destination height ⭕ ⭕ ⭕
dx coord sx destination x coordinate ⭕ ⭕ ⭕
dy coord sy destination y coordinate ⭕ ⭕ ⭕
da char l destination aspect ratio contol (l, s, n) ⭕ ⭕ ⭕
ds char n destination scaling control (s, n) ⭕ ⭕ ⭕
cw number canvas width ⭕ ⭕ ⭕
ch number canvas height ⭕ ⭕ ⭕
cc color 000000 canvas color ⭕ ⭕ ⭕
bw number border width ⭕ ⭕ ⭕
bh number border height ⭕ ⭕ ⭕
bc color 000000 border color ⭕ ⭕ ⭕
sw coord source witdh ⭕ ⭕ ⭕
sh coord source height ⭕ ⭕ ⭕
sx coord source x coordinate ⭕ ⭕ ⭕
sy coord source y coordinate ⭕ ⭕ ⭕
pt char n pass through control (y, n) ⭕ ⭕ ⭕
sharpen string radius,sigma (e.g. 10x5) ⭕ ⭕ ⭕
unsharp string radius,sigma,amount,threshold (e.g 2x5+0.5+0) ⭕ ❌ ❌
blur string radius,sigma (e.g. 5x10) ⭕ ⭕ ❌
embedicon string embedded icon file in small_light_material_dir ⭕ ❌ ❌
ix number 0 embedded icon x coordinate ⭕ ❌ ❌
iy number 0 embedded icon y coordinate ⭕ ❌ ❌
angle number 0 angle of rotation (90, 180, 270) ⭕ ⭕ ⭕
progressive char n make JPEG progressive (y, n) ⭕ ❌ ❌
cmyk2rgb char n convert colorspace from CMYK to sRGB (y, n) ⭕ ❌ ❌
rmprof char n remove profile (y, n) ⭕ ❌ ❌
autoorient char n enable adjust image orientation automatically (y, n) ⭕ ❌ ❌

The values of da are l and s and n. These present the meanings below.

  • l: long-edge based
  • s: short-edge based
  • n: nope

There are any limitations below.

  • of=gif and of=webp are not supported when e=imlib2.
  • autoorient is available ImageMagick-6.9.0 or later.
  • The value of radius,sigma for sharpen and unsharp and blur is limited by small_light_radius_max and small_light_sigma_max.

There are the types of each parameter below.

Type Description
coord coordicante or pixel. percent when appending 'p'
char character
number integer number
color rrggbb or rrggbbaa
string string

Named Pattern

ngx_small_light supports to name comma-delimited parameters with the small_light_define_patern.

small_light_pattern_define small dw=120,dh=120,q=80,e=imagemagick,jpeghint=y;

If the line above is added to some server context in nginx.conf, the two URLs below return same response.

  • http://$host:$port/small_light(p=small)/img/image.jpg
  • http://$host:$port/small_light(dw=120,dh=120,q=80,e=imagemagick,jpeghint=y)/img/image.jpg

Using GET parameters

ngx_small_light supports to convert image not only by small_light function but by GET paramenters in v0.5.0 or later. You need to set both small_light and small_light_getparam_mode on to enable this feature. At the expense of enabling this feature, small_light function (e.g. /small_light(dw=300,dh=300)/img.jpg is disabled.

small_light on;
small_light_getparam_mode on;

In the configuration above, the url below does not return converted image.

http://localhost:8000/small_light(dw=200,dh=200)/img/image.jpg

Instead the url below returns converted image expected by right.

http://localhost:8000/img/image.jpg?dw=200&dh=200

Enabling WebP Transformation

ngx_small_light supports WebP transformation with ImageMagick and GD. Given of=webp to small_light function, ngx_small_light transforms image format into WebP. But ImageMagick requires libwebp and GD requires libvpx. You need to embed these libraries in building ImageMagick and GD for enabling WebP transformation.

If WebP transformation is not available, nginx outputs the line like below in error.log in processing image with of=webp.

WebP is not supported

If WebP transformation with ImageMagick is available, the output of convert -list format includes the line like below.

$ convert -list format | grep -i webp
     WEBP* WEBP      rw-   WebP Image Format (libwebp 0.5.0[0208])

If WebP transformation with GD is available, the output of gdlib-config --libs includes -lvpx.

In general, the packages of ImageMagick and GD provided from the linux distributions such as Ubuntu and CentOS does not embed the library for WebP transformation by default. In such cases, you need to build ImageMagick or GD yourself.

Optimizing Tips

There are some optimizing tips for ngx_small_light.

JPEG hinting

When the output format is JPEG and image-converting engine is ImageMagick or Imlib2, you may give jpeghint=y. The speed of processing images is improved dramatically.

Limit thread-number with OpenMP

When image-converting engine is ImageMagick and the version of ngx_small_light is lower than v0.6.14, giving 1 to OMP_NUM_THREADS or MAGICK_THREAD_LIMIT in nginx.conf is recommended strongly. Because OpenMP is enabled in ImageMagick by default and ImageMagick enabled OpenMP is very slow on multi-process environment.

env OMP_NUM_THREADS=1; # or env MAGICK_THREAD_LIMIT=1;

Or you can avoid this problem by building ImageMagick with --disable-openmp.

In v0.6.14 or later, they are no longer required. Because ngx_small_light always sets the thread-number with OpenMP 1.

Limitations

ngx_small_light has the limitations below.

Not supported features with Imlib2

The transformation with Imlib2 does not support to write GIF-image. Because Imlib2 has the function for loading GIF-image but does not have the function for saving. Additionally, the transformation by Imlib2 does not support to write and read WebP-image. So of=gif and e=imlib2 are not enabled to specify at once. If these are specified, ngx_small_light returns 415(Unsupported Media Type).

Not supported features with GD

The transformation with GD supports to write WebP-image. But it is the experimental feature.

Not supported animated GIF

ngx_small_light does not support the transformation kept animation for animated GIF. Because it takes long time to transform(e.g. resize, crop) animated GIF kept animation. So it is not realistic for ngx_small_light to support an animated GIF.

If the animated GIF is given, ngx_small_light transforms only the first frame.

Running Tests

perl Build.PL
cpanm --installdeps .
NGINX_BIN=${nginx_prefix_dir}/sbin/nginx ./Build test
## or
NGINX_BIN=${nginx_prefix_dir}/sbin/nginx prove t/**/*.t

GitHub

You may find additional configuration tips and documentation for this module in the GitHub repository for nginx-module-small-light.