Adobe PHOTOSHOP ELEMENTS Manual page 236

CHAPTER 11
228
Optimizing Images for the Web
About JPEG format
The JPEG format supports 24-bit color and
preserves the broad range and subtle variations in
brightness and hue found in photographs and
other continuous-toned images. JPEG is
supported by most browsers.
JPEG compresses file size by selectively discarding
data. Because it discards data, JPEG compression
is referred to as lossy. A higher quality setting
results in less data being discarded, but the JPEG
compression method can degrade sharp detail in
an image, particularly in images containing type
or vector art.
Note: Artifacts, such as wave-like patterns or blocky
areas of banding, are created each time you save an
image in JPEG format. These artifacts accumulate
each time you resave the image as the same JPEG file;
therefore, you should always save JPEG files from the
original image, not from a previously saved JPEG.
Original image, and JPEG with Low quality setting
You can create a progressive JPEG file, in which a
low-resolution version of the image appears in a
browser while the full image is downloading.
The JPEG format does not support transparency.
When you save an image as a JPEG file, transparent
pixels are filled with the Matte color, as specified in
the Optimize palette. If you know the background
color of the Web page where you will place the
image, you can match the Matte color to the Web
page background color to simulate the effect of
background transparency. If your image contains
transparency and you do not know the Web page
background color, or if the background will be a
pattern, you should use a format that supports
transparency (GIF, PNG-8, or PNG-24).
About GIF format
The GIF format uses 8-bit color and efficiently
compresses solid areas of color while preserving
sharp detail, such as that in line art, logos,
or illustrations with type. You also use the GIF
format to create animated images. GIF is
supported by most browsers.
The GIF format uses LZW compression, which is a
lossless compression method. However, because
GIF files are limited to 256 colors, optimizing an
original 24-bit image as an 8-bit GIF can result in
the loss of color information.
GIF image with 0% dither, and with 100% dither
You can reduce the number of colors in a GIF
image and choose options to control the way
colors dither in the application or in a browser.
GIF supports background transparency and