Previewing And Controlling Dithering - Adobe PHOTOSHOP 6.0 Manual

To create hard-edged transparency in a GIF or PNG-8:
Open or create an image that contains
1
transparency.
In the Optimize panel/palette, select GIF or
2
PNG-8 from the File Format menu.
Select Transparency.
3
Select None from the Matte pop-up menu to
4
make all pixels with greater than 50% trans-
parency fully transparent, and all pixels with 50%
or less transparency fully opaque.
Creating background matting in JPEG
images
When creating a JPEG from an original image that
contains layer transparency, you must matte the
image against a matte color. Since the JPEG format
does not support transparency, blending with a
matte color is the only way to create the
appearance of background transparency in a
JPEG. Fully transparent pixels are filled with the
matte color, and partially transparent pixels are
blended with the matte color. When the JPEG is
placed on a Web page with a background that
matches the matte color, the image appears to
blend with the Web page background.
To create a matted JPEG image:
Open or create an image that contains
1
transparency.
In the Optimize panel/palette, select JPEG from
2
the file format menu.
3 Select a color from the Matte pop-up menu:
(Photoshop) Select None, Eyedropper (to use
•
the color in the eyedropper sample box), White,
Black, or Other (using the color picker).
• (ImageReady) Select None, Foreground Color,
Background Color, or Other (using the color
picker), or select a color from the Matte pop-up
palette.
Note: When you select None, white is used as the
matte color.
Previewing and controlling
dithering
Most images viewed on the Web are created using
24-bit color displays (millions of colors mode),
but many Web browsers are used on computers
using only 8-bit color displays (256-color mode),
so that Web images often contain colors not
available to many Web browsers. Computers use a
technique called dithering to simulate colors not
available in the color display system. Dithering
creates adjacent pixels of different colors to give
the appearance of a third color. For example, a red
color and a yellow color may dither in a mosaic
pattern to produce the illusion of an orange color
that does not appear in the color palette.
ADOBE PHOTOSHOP 6.0 341
User Guide