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How to Choose the Appropriate Image
Choosing the right file format to save your images in is of vital
importance. There are three image formats in constant use on the
net - GIF, JPG and PNG. Each is suited to a specific type of image,
and matching your image to the correct format should result in a
small, fast-loading graphic. Saving and exporting into these formats
will require a decent image editor.
When choosing the format for your image, you should always be
conscious of both the image's quality and filesize.
GIF - Graphics Interchange Format
Compuserve's 8-bit GIF format has long been the most popular
on the Internet, mainly because of its small size. It is ideal for
small navigational icons and simple diagrams and illustrations where
accuracy is required, or graphics with large blocks of a single
color. The format is lossless, meaning it does not get blurry or
messy. GIFs use a simple technique called LZW compression to reduce
the filesize of images by finding repeated patterns of pixels, but
this compression never degrades the image quality.
GIF files can be saved with a maximum of 256 colors. This makes
it a poor format for photographic images. Because this can sometimes
be tight, GIFs have the option to dither, and will mix pixels of
two different available colors to create a suggestion of another
GIFs can be animated, which is another reason they became so
successful. Most animated banner ads are GIFs. You will need an
animation program to make your own animations.
GIFs allow single-bit transparency, which means when you are
creating your image, you can specify one color to be transparent.
This allows the background colors of the web page to show through
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.
JPEG - Joint Photographic Experts Group
The 16-bit JPEG format (usually written without the E), was designed
with photographs in mind. It is capable of displaying millions of
colors at once, without the need for dithering, allowing for the
complex blend of hues that occur in photographic images.
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
Though JPGs can be interlaced, they lack many of the other special
abilities of GIFs, like animation and transparency; but as I said,
they really are only for photos. Simple graphics with large blocks
of color should not be saved as JPGs because the edges get all smudgy.
PNG - Portable Network Graphics
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 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 colors and supports 1-bit
transparency. Filesize when saved in a capable image editor like
Fireworks will be noticeably smaller than the GIF counterpart, as
PNGs save their color data more efficiently.
PNG-24 is another flavor of PNG, with 24-bit color support, allowing
ranges of color akin to a high color JPG. PNG-24 is in no way a
replacement format for JPG, however, because it is a lossless compression
format. This means that filesize can be rather big against a comparable
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 opaque.
This allows you to create a graphic that can be placed on top of
any background color and will retain a translucent effect, with
the background showing through the pixels that are not opaque.
And what of animation? PNG can be made into multi-image files
through the MNG extension of the format, but browser support is
patchy for this format. Stick with GIFs for your animations.
Where does all this leave the PNG format? It may take a good
number of years to find widespread usage, but as it stands at the
moment using PNG-8 in place of static GIFs will lower the filesize
of your images. There's no reason not to adopt them now as the format
you create your site icons with.
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