Page 68 - DCAP303_MULTIMEDIA_SYSTEMS

Multimedia Systems

notes Gifs can be animated: It is another reason they became so successful. Most animated banner ads
are GIFs. You will need an animation program to make your own animations.

The GIFs allow single-bit transparency, which means when you are creating your image, you
can specify one colour to be transparent. This allows the background colours of the Web page to
show through the image.
Gifs can also be interlaced, which is a way of saving a graphic so that it loads progressively—first
a blurry, low-detail version is loaded, and then successive layers of detail are added. This usually
means a larger overall filesize, but it means that a version of the image gets placed onto the
viewer’s screen much quicker, and so is beneficial in many situations, as it gives the impression
of a speedier download.
The GIFs (variously pronounced “gif” or “jif”) are a very good format for most of the graphics
you will be using on your site, though PNGs below are better in many regards. Experimentation
is the key to success.

If an image format is is not compatible with the display visual (e.g. JPEG on
a colormapped display) then the original image will be altered.
4.7.2 Joint photographic experts Group (JpeG)

The 16-bit JPEG format (usually written without the E), was designed with photographs in mind.
It is capable of displaying millions of colours at once, without the need for dithering, allowing
for the complex blend of hues that occur in photographic images.
The JPGs use a complex compression algorithm, which can be applied on a sliding scale.
Compression is achieved by ‘forgetting’ certain details about the image, which the JPG will then
try to fill in later when it is being displayed. You can save a JPG with 0% compression for a perfect
image with a large filesize; or with 80% compression for a small but noticeably degraded image.
In practical use, a compression setting of about 60% will result in the optimum balance of quality
and filesize, without letting the lossy compression do too much damage.
Though, JPGs can be interlaced, they lack many of the other special abilities of GIFs, like animation
and transparency. They really are only for photos. Simple graphics with large blocks of colour
should not be saved as JPGs because the edges get all smudgy.

4.7.3 portable network Graphics (pnG)
The PNG is a format invented specifically for the Web in response to a licensing scheme introduced
which meant the creators of any software that supported the GIF format had to pay five thousand
dollars for the privilege (this tax has since expired). While they were at it however, the creators
of PNG (“ping”) went ahead and created a format superior to GIF in almost every way.
One version of the format, PNG-8, is similar to the GIF format. It can be saved with a maximum
of 256 colours and supports 1-bit transparency. Filesizes when saved in a capable image editor
like Fireworks will be noticeably smaller than the GIF counterpart, as PNGs save their colour
data more efficiently.
The PNG-24 is another flavour of PNG, with 24-bit colour support, allowing ranges of colour akin
to a high colour JPG. PNG-24 is in no way a replacement format for JPG, however, because it is
a lossless compression format. This means that filesizes can be rather big against a comparable
JPG.
The PNG’s main draws are alpha channels. Instead of the rudimentary transparency options in
other formats (where a pixel is either transparent or opaque), an alpha channel can specify the
opacity of any pixel from 0–255, where 0 is fully transparent and 255 is fully unclear. This allows
you to create a graphic that can be placed on top of any background colour and will retain a
translucent effect, with the background showing through the pixels that are not opaque.

62 LoveLy professionaL University