Poor load time degrades the user's experience of a web page. For a web page containing large images, optimizing images can significantly improve the load time performance which leads to better user experience. Moreover, if a web site is hosted on a cloud service which charges for cloud storage, compressing images can be financially worthwhile. This post explains the optimization of JPEG images using 2 command-line programs: jpegtran and jpegoptim. My next post introduces tools to optimize PNG images.
jpegtran optimizes a JPEG file losslessly. In other words, it reduces the file size without degrading the image quality. By specifying options, you can ask jpegtran to perform 3 types of lossless optimization:
- -copy none
An image file may contain metadata that are useless to you. For example, the following figure shows the embedded properties from a picture taken by a digital camera. Properties such as the Camera Brand and Camera Model can be safely stripped from the picture without affecting image quality.
There are two types of JPEG files: baseline and progressive. Most JPEG files downloaded from a digital camera or created using a graphics program are baseline. Web browsers render baseline JPEG images from top to bottom as the bytes are transmitted over the wire. In contrast, a progressive JPEG image is transmitted in multiple passes of progressively higher details. This enables the user to see an image preview before the entire image is displayed in its final resolution.
For large JPEG images, converting from baseline to progressive encoding often results in smaller file size, and faster, user-perceived load time.
This option optimizes the Huffman tables embedded in a JPEG image.
To install jpegtran on Debian/Ubuntu:
$ sudo apt-get install libjpeg-progs
To install jpegtran on Fedora/Centos/RedHat:
$ sudo yum install libjpeg-turbo-utils
To optimize a JPG file:
$ jpegtran -copy none -progressive -optimize SAM_0297.JPG > opt_0297.JPG
Below are the results after running the above command.
| File Size Before
| File Size After
jpegoptim supports both lossless and lossy image optimization.
To install the program on Debian/Ubuntu:
$ sudo apt-get install jpegoptim
To install on Fedora/RedHat/Centos:
$ sudo yum install jpegoptim
To specify the same 3 types of lossless optimization as explained above, execute this command:
$ jpegoptim --strip-all --all-progressive --dest=opt SAM_0297.JPG SAM_0297.JPG 4000x3000 24bit N Exiff [OK] 3119056 --> 2860568 bytes (8.29%), optimized.
- The --all-progressive option is a feature introduced in jpegoptim version 1.3.0. The version on Debian Wheezy is only 1.2.3, therefore the option is not available.
- By default, jpegoptim compresses in place, overwriting the input JPEG image. If you don't want the program to write over the input file, specify an alternative directory using the --dest option.
jpegoptim can also compress an image file using lossy optimization techniques. Specify an image quality from 0 to 100, with 100 being the highest quality (and lowest compression). To compress with 90% image quality, execute:
$ jpegoptim --max=90 --dest=opt SAM_0297.JPG SAM_0297.JPG 4000x3000 24bit Exif [OK] 3119056 --> 2337388 bytes (25.06%), optimized.
The table below summarizes the % of reduction in file size as you decrease the image quality. There is a trade-off between file size and image quality. While reducing image size is a worthwhile goal, you don't want to end up with an image that is not "pretty" enough. You are the final judge of the lowest quality that is acceptable to you. To pick the image quality to use for a specific picture, experiment by incrementally decreasing the image quality (say by 10 each time), visually inspect the output image, and stop when the image quality is no longer acceptable.
| File Size |