Feb 03 2010

Everything You Need to Know About Image Compression

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Proper use of image compression can make a huge difference in the appearance and size of your website image files. But compression is an often-misunderstood topic, partly because there’s a real lack of understanding on what the different types of compression are good for. If you don’t understand which type of compression to use for different types of images, you’ll likely end up with one of two results: either images that don’t look as good as they could, or image file sizes that are way larger than they need to be.

Below is everything you need to know about image compression in relation to web design. We’ve covered the differences between lossless and “lossy” compression, the different file types and the compression techniques they use, and guidelines for which file formats work best for different kinds of images.

1. “Lossless” vs. “Lossy” Compression

Many people feel that they should only use image formats that use lossless compression. While lossless compression is superior for many kinds of images, it’s not necessary for many others. Basically, lossless image compression means all the data from the original file is preserved. Lossy compression, on the other hand, removes some data from the original file and saves the image with a reduced file size. It’s up to you, as the designer, to tell it how much data to disregard by setting the image compression rate.

Lossless Compression

There are a few different methods for lossless compression. There’s run-length encoding (used for BMP files), which takes runs of data (consecutive data elements with identical values) and stores them in a single data value and count. It’s best suited for simple graphics files, where there are long runs of identical data elements.

Lossless compression

DEFLATE is another lossless data compression method used for PNG images. It uses a combination of the LZ77 algorithm and Huffman coding. In addition to being used for PNG images, it’s also used in ZIP and gzip compression.

Lempel-Ziv-Welch (LZW) compression is a lossless compression algorithm that performs a limited analysis of data. It’s used in GIF and some TIFF file formats.

Lossy Compression

There are a number of lossy compression methods, some of which can be combined with lossless methods to create even smaller file sizes. One method is reducing the image’s color space to the most common colors within the image. This is often used in GIF and sometimes in PNG images to result in smaller file sizes. When used on the right types of images and combined with dithering, it can result in images nearly identical to the originals.

Transform encoding is the type of encoding used for JPEG images. In images, transform coding averages out the color in small blocks of the image using a discrete cosine transform (DCT) to create an image that has far fewer colors than the original.

Chroma subsampling is another type of lossy compression that takes into account that the human eye perceives changes in brightness more sharply than changes of color, and takes advantage of it by dropping or averaging some chroma (color) information while maintaining luma (brightness) information. It’s commonly used in video encoding schemes and in JPEG images.

2. Different File Types

For the purposes of this article, we’re only going to focus on three file types, those most commonly found in web design: PNG, JPEG, and GIF. While there are other image formats out there that take advantage of compression (TIFF, PCX, TGA, etc.), you’re unlikely to run across them in any kind of digital design work.

GIF

GIF stands for Graphics Interchange Format, and is a bitmap image format introduced in 1987 by CompuServe. It supports up to 8 bits per pixel, meaning that an image can have up to 256 distinct RGB colors. One of the biggest advantages to the GIF format is that it allows for animated images, something neither of the other formats mentioned here allow.

animated GIF newton's cradle

JPEG

JPEG (Joint Photographic Experts Group) is an image format that uses lossy compression to create smaller file sizes. One of JPEG’s big advantages is that it allows the designer to fine-tune the amount of compression used. This results in better image quality when used correctly while also resulting in the smallest reasonable file size. Because JPEG uses lossy compression, images saved in this format are prone to “artifacting,” where you can see pixelization and strange halos around certain sections of an image. These are most common in areas of an image where there’s a sharp contrast between colors. Generally, the more contrast in an image, the higher quality the image needs to be saved at to result in a decent-looking final image.

JPEG photo compression comparison

PNG

PNG (Portable Network Graphics) is another bitmapped image format that uses lossless data compression and was created to replace the GIF image format. The PNG format was largely unsupported by Internet Explorer for a long time, making it less commonly used than GIF and JPEG formats, though it’s now supported properly by every major browser. PNG files support palette-based color (either 24-bit RGB or 32-bit RGBA), greyscale, RGBA and RGB color spaces. One of PNG’s biggest advantages is that it supports a number of transparency options, including alpha channel transparency.

3. Choosing a File Format

Each of the file formats specified above are appropriate for different types of images. Choosing the proper format results in higher quality images and smaller file sizes. Choosing the wrong format means your images won’t be as high-quality as they could be and that their file sizes will likely be larger than necessary.

For simple graphics like logos or line drawings, GIF formats often work best. Because of GIF’s limited color palette, graphics with gradients or subtle color shifts often end up posterized. While this can be overcome to some extent by using dithering, it’s often better to use a different file format.

gif image

For photos or images with gradients where GIF is inappropriate, the JPEG format may be best suited. JPEG works great for photos with subtle shifts in color and without any sharp contrasts. In areas with a sharp contrast, it’s more likely there will be artifacts (a multi-colored halo around the area). Adjusting the compression level of your JPEGs before saving them can often result in a much higher quality image while maintaining a smaller file size.

For images with high contrast, especially photos, or illustrations with lots of gradients or contrast, the PNG format is often best. It’s also the best option for transparent images, especially those that need partial transparency. PNG files are often larger than JPEGs, though it depends on the exact image. PNG files are also lossless, meaning all the original quality of the image remains in tact.

JPEG and PNG comparison

Here’s an overview of which file types work best for each type of image:

GIF

  • If animation is required.
  • Line drawings and simple graphics.

JPEG

  • Photos, especially without high contrast.
  • Screenshots, especially of movies, games, or similar content.

PNG

  • Line art, illustrations.
  • Photos with high contrast.
  • Transparency, especially alpha channel transparency.
  • Application screenshots or other detailed diagrams.

And here’s an overview of which formats to avoid for each type of image:

GIF

  • Images with gradients.
  • Photos.

JPEG

  • Images with high contrast.
  • Detailed images, especially diagrams.
  • Simple graphics (the file sizes are larger).

PNG

  • Photos with low contrast (file sizes are larger).

4. Image Compression in Print Design

While the bulk of this article has focused on image compression in web design, it’s worth mentioning the effect compression can have in print design. For the most part, lossy image compression should be completely avoided in print design. Print graphics are much less forgiving of artifacting and low image quality than are on-screen graphics. Where a JPEG saved at medium quality might look just fine on your monitor, when printed out, even on an inkjet printer, the loss in quality is noticeable (as is the artifacting).

For print design, using file types with lossless compression is preferable. TIFF (Tagged Image File Format) is often the preferred file format if compression is necessary, as it gives options for a number of lossless compression methods (including LZW mentioned above). Then again, depending on the image and where it will be printed, it’s often better to use a file type with no compression (such as an original application file). Talk to your printer about which they prefer.

Further Resources

About the author

Cameron Chapman is a professional Web and graphic designer with over 6 years of experience. She writes for a number of blogs, including her own, Cameron Chapman On Writing . She’s also the author of Internet Famous: A Practical Guide to Becoming an Online Celebrity.

About the Author

Cameron Chapman is a professional Web and graphic designer with many years of experience. She writes for a number of blogs, including her own, Cameron Chapman On Writing. She’s also the author of Internet Famous: A Practical Guide to Becoming an Online Celebrity.

Comments and Discussions
  • Artis, 03 February 2010

    It’s one of the best image compression articles I’ve read so far.

    It also reminded me I once accidentally found out that there is possibility to make animated PNG images however because of requirements it’s not so common. The address was: animatedpng [dot] com.

    Thanks Cameron!

  • Jordan Walker, 03 February 2010

    Great read, very informative on the file type to choose when dealing with graphics. My only question is regarding .svg (Scalable Vector Graphics), when is it most suitable to use?

  • Abx78, 03 February 2010

    Just another entry level article.

  • ?ukasz, 03 February 2010

    Great post, lots of good info.

  • alone, 03 February 2010

    Like ever.. Great post!! Thank you!!

  • Erick Cardoza, 03 February 2010

    Fantastic post. I really appreciate the breakdown suggested use. It is certainly useful despite of the level of experience we may find ourselves at.

    One other topic I would love to see you cover is JPEG suggested quality… there seems to be a debate out there on the best percentage to use for photographs and just how much is enough for most users.

    Great work.

  • Codesquid, 03 February 2010

    No mention of smush.it? I always use it for lossless compression for any image that will end up on the web.

  • Matthias Matz, 04 February 2010

    sehr gut zu wissen , guter artikel find ich richtug gut

  • Modern interiors, 04 February 2010

    Excellent post. Very useful info to have (especially in one place).

  • robbydesigns, 04 February 2010

    Great article, well set out. I found a link to your article on Twitter and will have a look around your website.
    Thank you.

  • James P, 04 February 2010

    This article is useless. Nothing in here to help me make a better website.

    What’s that you say? JPEG is a lossy compression? Get outta here? Since when did this happen? Yeah it’s true. And did you know that JPEG uses a discrete cosine trasformation to compress the picture? Holly Sh!% that’s unbelievable.

    Here’s a quick tip for free. Nobody cares about PNG’s “combination of the LZ77 algorithm and Huffman coding.” WTF is that and why is it important? (Note to author: It’s not.)

    Your article summed up in two sentences: JPEGs pixelate images. If you don’t want that, use PNG.

    You use an outstanding graphic showing the difference between “saved as JPEG” and “saved for web JPEG”, but you never discuss the difference between those. You never once touch on the hardest part about image compression: the compromise between quality and size. Should I use a JPEG at 6 quality for 167 KB or a JPEG at 10 quality for 250KB? What’s the difference? What should I use?

    What’s that you say “I don’t know what you should use, because it all depends on you and your website and what exactly you want. I can’t tell you what you should use because every website is different.” That’s a cop out. What do people usually use? What’s the most common, the standard on the web? What do you use on your website?

    I could ramble on and on (what? I already have? Oh well), but I’m going to leave it off here. Let’s stop discussing the algorithms and run length encodings and talk about standards and procedures.

    • Mr Bob, 28 January 2013

      James P I bet you walk into greengrocers shouting “You know nothing about fish, why these things here aren’t even fish at all are they you idiots, first off fish aren’t generally green and round, nor do they taste like apples.”

      Instead of getting on your pedantic high horse you idiot, respect the greengrocer for opening the shop that you didn’t then look in the fishmongers for your fish supper.

      Big respect to you Cameron for writing a thoroughly informative article and good luck with what has no doubt so far been a very difficult life.

  • Jen, 04 February 2010

    A good, thorough article.

  • rick, 04 February 2010

    Awesome post. Hope you don’t mind if I mention this article to my readers. Keep up the great work.

  • Marty, 04 February 2010

    James P,

    Your points are good, but delivered in a highly inappropriate manner to someone who actually did provide information that a lot of people have found useful. Why don’t you write the article if you know this much about it? Then I can read comments by people calling your article useless because it’s too technical.

  • Suying, 04 February 2010

    nice! that was able to clarify some stuff :D

  • Dan, 05 February 2010

    Great article :)

  • K, 05 February 2010

    Recommending GIF for gradient images?

    Wow! Voice of wisdom there no doubt. Are you kidding?

    JPEG for high contrast? Ummm… ever heard of a PNG?

    Wow this article was written by a first year Art Institute dropout.

  • ken, 11 February 2010

    so what would you recommend for an animated gif with photos? Or should I just make it in flash?

    • kehinde temitope, 21 October 2010

      helo,l like your image been by ur picture,face an the nature of ur colour. l dont mine if u can send it though my email address about ur profile bye 4 now.

    • kehinde temitope, 21 October 2010

      what about image is likeness,picture pls know what u are dealing with

  • Ner, 12 February 2010

    GIF
    If animation is required.

    PNG8
    Line drawings and simple graphics.

    That’s the way it should be :). Gif is really just for animations, as you can do anything else with png8.

  • Deepak kaletha, 08 March 2010

    I never read like this articles on image compression

  • manushya, 04 April 2010

    nice collection of image compression
    thanks

  • Ben, 13 April 2010

    Hello, there seems to be A LOT of confusion as to “what the best image format is for specific uses.” For example, who in the world would use .gif as a final output for photography? Our digital output class at the Utah Valley University has been working on fixing this confusion. We have been doing research on what image file formats work best for certain things, what terms such as ‘compression, lossless, lossy, dithering, and all the main file format names ect.. mean. We are creating a image file format poster with in depth explanations of file formats, revise definitions, correcting false information, and answering questions about file formats. If you have any feedback feel free to ask questions about this.

    • Styling Designs, 30 June 2010

      Hi Ben where can I find this image file format poster?

      Thanks!

  • Amos Geronimo, 12 July 2010

    Daniel Fitchett

  • vinA, 03 February 2011

    very useful and thanks

  • basavaraj, 22 May 2011

    excellent post…

  • mark, 18 November 2011

    Great stuff and really good as large web pages slow down the google bot

  • lahlooba, 29 November 2012

    nice collection of image compression

    thank you

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