PhotoPlus X2 User Guide - Serif

166 Making Images for the Web
The Export Optimizer provides special GIF options that help you preserve semitransparency
if you’ve employed antialiasing or feathering in your original image.
The .GIF format still wants "all or nothing," but you can opt to dither the alpha
(transparency) channel and/or select a matte colour with which semi-transparent
pixels will be smoothly blended. Pixels that aren’t 100% transparent will still end
up opaque, but the image will look a lot better.
Finally, .GIF is a "multi-part" format, which means one file can store multiple
images. That's what makes it a good candidate for use in Web animations.
.JPG format
The .JPG or .JPEG (Joint Photographic Experts Group) file format, like .GIF, is
universally supported in Web browsers. Unlike .GIF, it encodes 24-bit images but
is a “lossy” format (i.e. it discards some image information) depending on the
selected Quality setting (this controls compression). .JPG is clearly the format of
choice for full-colour photographic images. For "black and white" (256-level, 8-bit
greyscale) photos, it has no particular advantages over .GIF.
The unique aspect of exporting as a JPG is the slider control you use to choose a
Quality setting. At one end of the scale, the export applies maximum compression
(0% Quality) and produces an extremely small (but quite ugly) image. At the other
end, 100% quality means that lossless uncompressed export is possible, but file
sizes are relatively much larger, although still compact compared to BMPs, for
example. When choosing a quality setting for .JPG export, keep in mind the
number of times you expect to be re-exporting a particular image. A photograph
may look fine in the Export Optimizer the first time you export it, e.g. at 60%
Quality, but after several such saves, you'll really see a quality loss.
PhotoPlus also supports the new JPEG 2000 (.JP2, .J2K) format, which uses
wavelet compression and reduces file sizes significantly better than .PNG (see
below) but does not support transparency. This format can store channels of
data—such as ICC profiles!