Linux / Unix Command To Optimize and Compress PNG Files In Bulk

1. PNG file format is recommended for web.
2. GIF file format is recommended for small size images.
3. JPG file format is recommended for high resolution photographic-style images.
4. Do not use BMPs or TIFFs.

Say hello to optipng

You need to use a tool called optipng. It is a PNG optimizer that recompresses image files to a smaller size, without losing any information. This program also converts external formats (BMP, GIF, PNM and TIFF) to optimized PNG, and performs PNG integrity checks and corrections. This tool can be installed on any server powered by Unix or Linux operating systems.

Installation

First, turn on EPEL repo and type the following yum command to install optipng:
## ** first enable epel repo on centos 7 ** ##
# yum install epel-release
# yum install optipng
Sample outputs:

Loaded plugins: auto-update-debuginfo, protectbase, rhnplugin
0 packages excluded due to repository protections
Setting up Install Process
Resolving Dependencies
--> Running transaction check
---> Package optipng.x86_64 0:0.6.4-1.el6 will be installed
--> Finished Dependency Resolution
 
Dependencies Resolved
 
===============================================================================
 Package           Arch             Version               Repository      Size
===============================================================================
Installing:
 optipng           x86_64           0.6.4-1.el6           epel            82 k
 
Transaction Summary
===============================================================================
Install       1 Package(s)
 
Total download size: 82 k
Installed size: 181 k
Is this ok [y/N]: y
Downloading Packages:
optipng-0.6.4-1.el6.x86_64.rpm                          |  82 kB     00:00     
Running rpm_check_debug
Running Transaction Test
Transaction Test Succeeded
Running Transaction
  Installing : optipng-0.6.4-1.el6.x86_64                                  1/1 
  Verifying  : optipng-0.6.4-1.el6.x86_64                                  1/1 
 
Installed:
  optipng.x86_64 0:0.6.4-1.el6                                                 
 
Complete!

Debian and Ubuntu/Mint Linux user type the following apt-get command to install the same:
$ sudo apt-get install optipng

Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following NEW packages will be installed:
  optipng
0 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
Need to get 89.0 kB of archives.
After this operation, 212 kB of additional disk space will be used.
Get:1 http://archive.ubuntu.com/ubuntu/ trusty/main optipng amd64 0.6.4-1build1 [89.0 kB]
Fetched 89.0 kB in 0s (419 kB/s)   
Selecting previously unselected package optipng.
(Reading database ... 16090 files and directories currently installed.)
Preparing to unpack .../optipng_0.6.4-1build1_amd64.deb ...
Unpacking optipng (0.6.4-1build1) ...
Setting up optipng (0.6.4-1build1) ...

How do I use optipng command?

The syntax is:

optipng file
optipng [options] file
optipng [options] input.png

My sample test.png image:

test.png – Original file

test.png (tiffinfo compatible labels)...
  Image Width: 1164 Image Length: 911
  Bits/Sample: 8
  Samples/Pixel: 3
  Pixel Depth: 24
  Colour Type (Photometric Interpretation): RGB 
  Image filter: Single row per byte filter 
  Interlacing: No interlacing 
  Compression Scheme: Deflate method 8, 32k window
  Resolution: 0, 0 (unit unknown)
  FillOrder: msb-to-lsb
  Byte Order: Network (Big Endian)
  Number of text strings: 1 of 9
    Software (xTXt deflate compressed): Shutter

Use ls command to see file size:
$ ls -lh test.png
Sample outputs:

-rw-r--r-- 1 vivek vivek 279K Nov 29 00:10 test.png

Use optipng optimization program as follows:
$ cp test.{png,bak}
$ optipng test.png
Sample outputs:

OptiPNG 0.6.4: Advanced PNG optimizer.
Copyright (C) 2001-2010 Cosmin Truta.
 
** Processing: test.png
1164x911 pixels, 3x8 bits/pixel, RGB
Input IDAT size = 284778 bytes
Input file size = 285286 bytes
 
Trying:
  zc = 9  zm = 8  zs = 0  f = 0		IDAT size = 215274
 
Selecting parameters:
  zc = 9  zm = 8  zs = 0  f = 0		IDAT size = 215274
 
Output IDAT size = 215274 bytes (69504 bytes decrease)
Output file size = 215374 bytes (69912 bytes = 24.51% decrease)

File size is reduced by 24.51%:
$ ls -lh test.png
Sample outputs:
-rw-r--r-- 1 vivek vivek 211K Nov 29 02:40 test.png
You can verify the quality of both images (click to enlarge):

Optimized test.png file ensures that it loads faster for users on slow connections. It will also save 24.51% bytes per image view in terms of both network bandwidth and storage. According to Google:

You should see a benefit for any image file that can be reduced by 25 bytes or more (less than this will not result in any appreciable performance gain).

How do I optimize files in bulk?

Use a bash for loop as follows:

#!/bin/bash
cd /path/to/png/storage/2010/01/c/
mkdir optimized 
## store optimized images in optimized directory ##
## Keep file system permission and make a backup of original PNG (see options below)  ##
for i in *.png; do optipng -o5 -quiet -keep -preserve -dir optimized -log optipng.log "$i"; done

You need to make changes to your code. For example, url path to optimize url will be as follows:
http://s0.cyberciti.org/foo/bar/optimized/test.png
Alternatively, you can overwrite existing PNG images:

#!/bin/bash
cd /path/to/png/storage/2010/01/c/
## Overwrite images ##
## Keep file system permission and make a backup of original PNG (see options below)  ##
for i in *.png; do optipng -o5 -quiet -keep -preserve -log optipng.log "$i"; done

Next, purge all images from your CDN account. See how to purge old images / invalidating object from cloud front for more information.

A note about processing *.png in all sub-directories

Use the find command as follows (make a backup of all images):

cd /var/www/html/uploads/
find . -type f -iname "*.png" -print0 | xargs -I {} -0 optipng -o5 -quiet -keep -preserve -log optipng.log "{}"

Options

General options:
    -fix		enable error recovery
    -force		enforce writing of a new output file
    -keep		keep a backup of the modified files
    -preserve		preserve file attributes if possible
    -quiet		quiet mode
    -simulate		simulation mode
    -snip		cut one image out of multi-image or animation files
    -out <file>		write output file to <file>
    -dir <directory>	write output file(s) to <directory>
    -log <file>		log messages to <file>
    --			stop option switch parsing
Optimization options:
    -f  <filters>	PNG delta filters (0-5)			default 0,5
    -i  <type>		PNG interlace type (0-1)		default <input>
    -zc <levels>	zlib compression levels (1-9)		default 9
    -zm <levels>	zlib memory levels (1-9)		default 8
    -zs <strategies>	zlib compression strategies (0-3)	default 0-3
    -zw <window size>	zlib window size (32k,16k,8k,4k,2k,1k,512,256)
    -full		produce a full report on IDAT (might reduce speed)
    -nb			no bit depth reduction
    -nc			no color type reduction
    -np			no palette reduction
    -nx			no reductions
    -nz			no IDAT recoding
Optimization details:
    The optimization level presets
        -o0  <=>  -o1 -nx -nz
        -o1  <=>  [use the libpng heuristics]	(1 trial)
        -o2  <=>  -zc9 -zm8 -zs0-3 -f0,5	(8 trials)
        -o3  <=>  -zc9 -zm8-9 -zs0-3 -f0,5	(16 trials)
        -o4  <=>  -zc9 -zm8 -zs0-3 -f0-5	(24 trials)
        -o5  <=>  -zc9 -zm8-9 -zs0-3 -f0-5	(48 trials)
        -o6  <=>  -zc1-9 -zm8 -zs0-3 -f0-5	(120 trials)
        -o7  <=>  -zc1-9 -zm8-9 -zs0-3 -f0-5	(240 trials)
    The libpng heuristics
        -o1  <=>  -zc9 -zm8 -zs0 -f0		(if PLTE is present)
        -o1  <=>  -zc9 -zm8 -zs1 -f5		(if PLTE is not present)
    The most exhaustive search (not generally recommended)
      [no preset] -zc1-9 -zm1-9 -zs0-3 -f0-5	(1080 trials)
Examples:
    optipng file.png				(default speed)
    optipng -o5 file.png			(moderately slow)
    optipng -o7 file.png			(very slow)
    optipng -i1 -o7 -v -full -sim experiment.png

Other png optimization tools for Linux / Unix

Debian / Ubuntu Linux and other operating *nix based systems can use the following tools for the same purpose:

• pngcrush – optimizes PNG (Portable Network Graphics) files.
• pngnq – tool for optimizing PNG (Portable Network Graphics) images. It is a tool for quantizing PNG images in RGBA format.
• pngquant – PNG (Portable Network Graphics) image optimising utility. It is a command-line utility for converting 24/32-bit PNG images to paletted (8-bit) PNGs.

Recommended readings:

1. A guide to PNG optimization.
2. Visit optipng home page to grab the latest version.
3. Analyze and optimize your website with PageSpeed tools.
4. 6 Tools To Find Out Website Load Speed.