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Going ping! The virtues of the PNG format

20th Nov 2011

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PNG transparency (slate gray)

I have been a long-time fan of the PNG format and I often distribute my illustrations in this format. Although a well-established file format it is no where near as familiar as the ubiquitous JPEG or GIF and occasionally when I hand over a PNG I am sometimes asked "what is that, and will it work on my site?"

So what is a PNG, and why did I send it instead of a JPEG or GIF?

Why ping me?

PNG stands for Portable Network Graphic and is cheerfully pronounced "ping". It is a file format developed by a consortium of graphics tools developers to address limitations in the JPEG and (particularly) GIF file formats. It has been around now for quite some time, and is supported by all popular web browsers and graphics programs. The limitations that PNG sought to address are the quality of line art and type which are often blurred by the "lossy" JPEG compression scheme and improved image transparency, offering partial, or alpha transparency which the GIF format, limited to 256 colours, does not support.

The historical reason for the introduction of the PNG format concerned a patent introduced on the GIF format which for a time stymied the at the time very popular format's use in many graphics products. Although the PNG format has never achieved the heights of popularity it aimed for, its ability to surpass those limitation it sought to address have made it a very viable, and often superior choice to either a JPEG or GIF—when it is used for what it is particularly good at for, as with its better known file format cousins, it does have its disadvantages too.

The best way to determine if it's suitable for a particular use is to consider the advantages and disadvantages of all three bitmap formats.

  Advantages Disadvantages
JPEG • Ideal for compressing "noise-filled" images such as photographs • Line art and type are often blurred and distorted
  • Small file-size. • "Lossy" compression, meaning image quality lost through compression is gone
GIF • Lossless compression • Restricted colour palette which often results in posterization and banding in images with colour gradients
  • Supports image transparency. • Does not support alpha (partial) transparency often leading to an unwanted "halo" effect on transparent images
  • Ideal for line art and type.  
  • Supports image frame animation.  
PNG • Lossless compression • Larger file size, particularly with noise-filled images
  • Supports alpha transparency  
  • Ideal for line art and type  

So the advantages of PNG is its lossless compression and its support for alpha transparency, its disadvantage being an often larger file size.

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Tags: lossless compression, PNG file format, transparency