There are many graphics file formats in general use. Some have
advantages over others for specific purposes. ImageGlue can save
out images in a variety of formats.
Some types of graphics file formats can contain multiple images.
For instance Photoshop documents often consist of multiple
transparent layers each of which contains an image. TIFF (Tagged
Image File Format) files may contain multiple images at different
sizes and resolutions for different types of output.
Because image data is so large, it is often compressed. There
are a number of compression methods in use but these may be more
generally classified as lossy or
- Lossy formats such as JPEG produce dramatic savings in space by
removing the pieces of information that the eye does not notice.
The higher the level of compression the more noticeable the change.
For very high levels of compression the reduction in quality may be
- Lossless formats such as LZW produce less dramatic savings in
space but leave the image unaffected. In general this type of
compression is only really effective when you are dealing with a
fairly limited range of colors. This is the reason it works well
for images such as GIF, which only support 256 colors.
- Some formats allow you to combine different methods of
compression within one file. For instance TIFF and PICT both allow
you to store multiple images, each compressed using a different
Although images generally have pixels, which are approximately
1/72th of an inch square, this is not always the case. The reason
this value is chosen is because it is the same size as the dots on
your computer monitor. However when you deal with other display
technology this is not always the case. For example printers may
typically be able to display 600 dots per inch. So for print work a
number of image formats allow you to hold images which are
specified at different resolutions.
Most file formats only support the visible portions of images.
However some formats support transparency. Within a GIF image you
can specify one color as transparent - essentially giving you a
black and white alpha channel. PNG images can contain full alpha
channels but while ImageGlue supports this many web browsers do not
display such images as transparent. Individual layers of Photoshop
documents may also contain alpha channels.