Adobe PHOTOSHOP 6.0 Manual page 328

CHAPTER 12
318
Optimizing Images for the Web
transparency. (Multilevel transparency is
supported by the PNG-24 format but not the JPEG
format. See "Making transparent and matted
images" on page 339.)
A photograph suitable for compression as a JPEG
or PNG-24 image, and artwork suitable for
compression as a GIF or PNG-8 image
Choose a format that contains sufficient pixel
depth to display the color information in the
image. (See "About bit depth" on page 116.)
PNG-8 and GIF files support 8-bit color. JPEG and
PNG-24 files support 24-bit color. Depending on
the format, you can specify image quality,
background transparency or matting, color
display, and downloading method.
The appearance of an image on the Web also
depends on the computer platform, color display
system, operating system, and browser used to
display the image. Preview images in different
browsers on different platforms to see how the
images will appear on 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 to 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