Image types and their compression: 2. png


This post is a continuation from my previous post – so please refer to the chapter about jpegs for an introduction.

2. png image

The Portable Network Graphic [Wiki link] is probably the flavour of the decade, if not much longer into the future. Most web developers love them for their capacity to achieve impressive compression results without any loss of quality – or so everybody claims… This is actually not entirely true! But:

png files will compress any vector based image much better than any other format suitable for internet publication! If you want to publish a photo with a lot of detail you are still better off saving it in its native jpg format. Unless you are a professional photographer, and you can afford the bandwidth, it doesn’t make much sense to save a photo as png – the resulting file size could be easily up to 10x as large.

Another constraint, you might encounter, is that many graphics programmes are fairly limited in their support of the png format; they will save in the png file format, but often don’t offer full support for all the features listed below.

All this said png files offer some amazing advantages over any other image file format:

  • png will compress all drawings, banners, buttons, and other vector based graphics much better than any other format,
  • png files are compressed with no loss of information,
  • you can set transparency to any colour in a png file (see samples below)
  • the png format offers full colour support of up to 48 bits per pixel,
  • a png file can also store an alpha value for each pixel, which specifies the degree to which the colour of that pixel is blended with the background colour,
  • png files can contain gamma correction and colour correction information so that the images can be accurately rendered on a variety of displays,
  • you can easily resize a png image without visible distortion (more about this use later).

The png format was actually developed with the intend to replace the gif format (my next chapter) – let’s see the differences:

button in png format

png file

button as jpg

jpg file

button gif file

gif file

animated button

gif animated

button png resized

png resized to 25%

button gif resized

gif resized to 25%

O.k., the first thing you will notice is that the background of the jpg is grey; that’s because grey was the background colour of the original image, with a jpg you cannot assign any transparency. With both, gif files and png files, you can set a transparency, like in this case where I made the background transparent because I want to place the image on a white page (the same would work with any other page colour). This is also a great feature if you want to change the colour scheme of your web site and keep using the same irregular shaped images (like bullets and icons): they will display exactly the same on your new background colour.

The other thing you might notice is that one of the gif images is blinking; that’s because I have animated this image (more about the process in my next chapter about gif files). What you might not notice with this button image is, that gif files are limited to 256 colours (8 bit colours). That wasn’t a problem in the days when they were first developed, because back then no computer monitor could show any more colours. Now most of us probably have our monitors set to 32 bit colour, or even 64 bit, in order to have the full spectrum when we view our photos or movies.

The final thing you will notice is that at the end of the row I have added 2 pictures at 25% of their size. Now I believe I have told you before that you should not resize images in the browser, but always make them to the size you intend to display on your web page – this statement is still true because for example a photo at around 2800 x 2100 pixels (roughly the size a mid quality digital camera produces) is ± 2,000 kilobytes (or 2 mb) in file size, the same photo resized and slightly compressed at 800 x 600 pixels will be most likely around 100 kilobytes = it will load much faster and save you bandwidth (for which you might have to pay your host).

So why did I resize these 2 images? Am I crazy? Not quite… You see, with a small element, which you might want to repeat on several of your pages, it is a different story: once you have displayed it the first time the visitor’s browser has downloaded this file and cached [Wiki link] it in its memory. The next time you show the same image, in its original size or resized, the user’s browser doesn’t have to reload this image another time – it doesn’t take any time and doesn’t use any extra bandwidth. For bullets and icons this is a perfect solution, because you can even resize a repeat image in another location on your pages. png files are very easy to resize, providing you keep the aspect ratios the same (eg. a 100×40 pixel image resized to 50×20, not 60×20 pixels!), and don’t show distortions if you do so (compare with the ‘E’ in the gif)!

To summarise:

1. png compression is free of any loss of information. You can also resize your png without distortions.

2. png is a great format for vector based images, like drawings, buttons, etc. It’s not such a good format for photos.

3. you can set many different values when saving your png, one of the most important is transparency, so that you can place images with a rounded or irregular outline (like icons) on any page background.

Next time I will write about gif files, and later I will add a side-by-side comparison of the 3 formats.

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