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 lossless.

  • 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 unacceptable.
  • 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 method.

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. Individual layers of Photoshop documents may also contain alpha channels.