Adobe PHOTOSHOP ELEMENTS Manual page 237

background matting, in which you blend the edges
of the image with a Web page background color.
(See "Previewing and controlling dithering" on
page 235.)
About PNG-8 format
The PNG-8 format uses 8-bit color. Like the GIF
format, PNG-8 efficiently compresses solid areas
of color while preserving sharp detail, such as that
in line art, logos, or illustrations with type.
Because PNG-8 is not supported by all browsers,
it may be advisable to avoid this format for situa-
tions in which your image must be accessible to the
widest possible Web viewing audience. For more
information on browser support for PNG,
see your browser's documentation.
PNG-8 files use more advanced compression
schemes than GIF, and can be 10–30% smaller
than GIF files of the same image, depending on the
image's color patterns. Although PNG-8
compression is considered lossless, optimizing an
original 24-bit image as an 8-bit PNG file can
result in the loss of color information.
Note: With certain images, especially those with very
few colors and very simple patterns, 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 reduce the
number of colors in the image and choose options
to control the way colors dither in the application
or in the browser. The PNG-8 format supports
background transparency and background
ADOBE PHOTOSHOP ELEMENTS
matting, in which you blend the edges of the image
with a Web page background color. (See
"Previewing and controlling dithering" on
page 235.)
PNG-8 with 0% dither, and with 100% dither
About PNG-24 format
The PNG-24 format supports 24-bit color. Like
the JPEG format, PNG-24 preserves the broad
range and subtle variations in brightness and hue
found in photographs. Like the GIF and PNG-8
formats, PNG-24 preserves sharp detail, such as
that in line art, logos, or illustrations with 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. PNG-24
browser support is similar to that for PNG-8.
In addition to supporting background trans-
parency and background matting, the PNG-24
format supports multilevel transparency. Multi-
level transparency allows you to preserve up to
256 levels of transparency to blend the edges of an
image smoothly with any background color.
However, multilevel transparency is not supported
by all browsers.
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User Guide