Adobe 29180155 - Photoshop Elements 4.0 User Manual page 266

ADOBE PHOTOSHOP ELEMENTS 4.0 261
User Guide
The PNG-8 format uses more advanced compression schemes than GIF does, and a PNG-8 file can be 10% to 30%
smaller than a GIF file of the same image, depending on the image's color patterns. Although PNG-8 compression is
lossless, optimizing an original 24-bit image as an 8-bit PNG file can subtract colors from the image.
Note: With certain images, especially those with simple patterns and few colors, GIF compression can create a smaller
file than PNG-8 compression. View optimized images in GIF and PNG-8 format to compare file size.
As with the GIF format, you can choose the number of colors in an image and control how colors dither in a browser.
The PNG-8 format supports background transparency and background matting, in which you blend the edges of the
image with a web page background color.
PNG-8 with 256 colors and no dither (left), and PNG-8 with 16 colors and dithering (right)
See also
"To optimize as GIF or PNG-8 format" on page 263
"To preserve background transparency in a GIF or PNG image" on page 267
About PNG-24
The PNG-24 format supports 24-bit color. Like the JPEG format, PNG-24 preserves the subtle variations in
brightness and hue found in photographs. Like the GIF and PNG-8 formats, PNG-24 preserves sharp details like
those in line art, logos, or type.
The PNG-24 format uses the same lossless compression method as the PNG-8 format. For that reason, PNG-24 files
are usually larger than JPEG files of the same image. You may want to avoid PNG-24 format when you are distrib-
uting your image to a wide audience.
In addition to supporting background transparency and background matting, the PNG-24 format supports multi-
level transparency. You can have up to 256 degrees of transparency from opaque to completely transparent, so you
can blend the edges of an image smoothly with any background color. However, not all browsers support multilevel
transparency.
See also
"To optimize as PNG-24" on page 264
"To preserve background transparency in a GIF or PNG image" on page 267