Image types and their compression: 3. the gif

11/01/2010

This is the last of the image types I want to write about. If you are looking for an introduction please go back to the first post.

3. the gif image

The gif file is a real antique of the internet as it has been around since 1987. Its name is an abbreviation of “Graphics Interchange Format” [Wiki link] and it was invented by CompuServe as a cross-platform graphic. At its conception the 8-bit format and resulting limitation to only 256 colours was still well ahead of mainstream computing.

It probably reached the peak of its use in the mid to late 90s when almost every banner ad was created in this format, and thousands of animated gifs annoyed every visitor at low quality sites and bulletin boards. Here is a more “moderate” example; if you would like to see others feel free to google “gif animation”.

As soon as bandwidth became less of a constraint the inclusion of photographic pictures in digital media became popular. But the gif format is not well suited for photos due to its compression characteristics and low count of colour shades.

photo as gif, nearest web safe colour

photo as gif, nearest web safe colour (21kb)

photo as gif, dithered colour

photo saved as gif, dithered colour (47kb)

photo as gif, dithered and optimised colour

photo as gif, dithered and optimised colour (71kb)

same photo as jpg image

the same photo as jpg image (compressed only 21kb!)

As you can see the gif format doesn’t handle photos well (unless you like the strange “web safe” effect of the top image). The jpg format is best suited for photos!

But all is not lost: the strength of gifs are large areas of continuous colour and clear text, hence the gif still holds a niche in web design for some logos and where a graphic replaces header text. Since standard web design only allows for the inclusion of a handful of cross-platform and browser compliant typefaces a small gif with some text is a simple method to introduce a more personal typeface into your page. (More about typefaces in future posts!)

Here’s another feature of the gif format: you can set the colour count to less than the maximum 256, eg. the above image contains only 16 colours and resulted in a much smaller file size. I couldn’t save it as a 2-colour gif [black & white] because the writing looked too pixelated; I needed a few more grey shades to smooth the edges.

When you want to create such a text header gif another feature of this format is useful: you can specify a transparent background colour for the file so you can display your image on any background. This process requires a little experimenting if you do it for the first time, since you don’t want to end up with a few left over pixels of your gif‘s background, yet soon it gets easy. It is also somewhat flawed if you introduce extra effects like a dropshadow, which the gif cannot display very well. Due to its limitation to 256 colours you will lose the finer grading shades of your dropshadow; the png format is better suited for these images. Yet, if the image consists of plain colour on another plain colour the gif format is still a good choice.

Personally I would allow the gif format another niche as the “poor (wo)man’s flash” replacement. The option to animate a gif can come in handy for specific uses, eg. where you want to highlight a new item/section, or a call to action, or even in some technical context, eg. where you would like to show different viewing angles of a specific item (like the 4 sides of a car). an animated Beetle gifJust keep their use to a minimum and the animation relatively subtle.  Most software to create flash files [Wiki link] is rather expensive and involves a steep learning curve. There’s no other format around to create simple animations.

To summarise:

1. gif images are 8 bit and limited to 256 colours.

2. the gif format is well suited for images containing continuous colour, without gradients and other effects which push the colour depth beyond 256 shades (in which case you would use the png).

3. you can introduce transparency, eg. to make the background disappear.

4. a gif can be animated! This is (apart from more complex flash animations) the only image format to offer this option.

My next post will show a few more side-by-side comparisons of all three image formats and their compression.

Short link to this post: http://wp.me/pKLtW-4s

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Image types and their compression: 1. the jpeg

29/12/2009

If you publish a web page you can insert different types of image files; the most common file formats are jpg, png, gif, and some might consider forms of swf (Flash files) as an image format.

If you’re confused which one to use on your page I would like to give some advice and demonstrate the differences.

Lets start with the one you probably know the best:

1. the jpg (or jpeg) image

pronounced: JAY-peg

Chances are that you own a digital camera, and the files on your camera are stored in the jpg format [Wiki link].
This format is best suited for photos or other graphics showing smooth variations of tone and colours.

In general a jpg file will compress reasonably well for display on web sites, but it is very common to see quality loss from compression. Be careful when applying compression to any image which contains shades of red or purple, as these tones show artifacts from compression the earliest!

image with 70% compression

The photo on the left has minimal compression applied to it, the second on the right is 70% compressed. The effect will show even better with text on an image.
Unfortunately I cannot show the full effect since the WordPress server is already applying a pre-set compression value to my images, so the ones with little compression look worse than intended!

text with 5% compressiontext with 70% compression
Take a close look at the different colours of text: obviously the red is showing the strongest noise from compression, white is still fairly clear, and the blue text comes out the best in this test.

So what can you learn from this?
1. You want to create your own header image for use on a web page: if you want to overlay a photo with some text, like the name of your business or the title of your web site, then you should choose a colour which compresses well so that your text stands out clearly; the best choices for this are black, blue, yellow, and white. – in most cases in this order.

2. The more shades of red you have in a picture the more difficult it will be to strike a good balance between compression and file size. In general you get nice and clear photos with a compression ratio of between 10-22%. Any more and most files will show the effects of compression.

And don’t forget to set the dpi value to 96 dpi – more about this in a later post (for today it has to be sufficient to know that monitors cannot display any more, and a higher value will increase the file size dramatically)!

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