User Guide

CHAPTER 12
208
Optimizing Images for the Web and E-mail
includes multilevel transparency. (Unlike the
PNG-24 format, the JPEG format doesn’t support
transparency. See “Making transparent and
matted images” on page 214.)
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 bit depth
to display the colors in the image. PNG-8 and GIF
files support 8-bit color, so they can display up to
256 colors. JPEG and PNG-24 files support 24-bit
color, so they can display up to 16 million colors.
Depending on the format, you can specify image
quality, background transparency or matting,
color display, and how browsers display the image
while it downloads.
The appearance of an image on the Web also
depends on the colors displayed by the computer
platform, operating system, monitor, and browser.
Preview images in different browsers on different
platforms to see how they will appear on the Web.
About JPEG format
The JPEG format supports 24-bit color, so it
preserves the broad range and subtle variations in
brightness and hue found in photographs and
other continuous-toned images. A progressive
JPEG file displays a low-resolution version of the
image in the Web browser while the full image is
downloading. JPEG is supported by most
browsers.
JPEG format 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 may still 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. Therefore, you should always
save JPEG files from the original image, not from a
previously saved JPEG.
Original image, and optimized JPEG with Low quality setting