Adobe PHOTOSHOP 6.0 Manual page 338

CHAPTER 12
328
Optimizing Images for the Web
To use a channel to modify JPEG quality:
In the Optimize panel/palette, choose a JPEG
1
setting from the Settings menu, or choose JPEG
from the file format menu.
Click the channel button ( ) to the right of the
2
Quality text box.
In the Modify Quality Setting dialog box,
3
choose the desired channel from the Channel
menu. In ImageReady, you can choose Save
Selection to create a new alpha channel based on
the current selection.
To preview the results of the weighted optimi-
4
zation, select the Preview option.
Define the quality range:
5
To set the highest level of quality, drag the right
•
(white) tab on the slider, enter a value in the
Maximum text box, or use the arrows to change
the current value.
To set the lowest level of quality, drag the left
•
(black) tab on the slider, enter a value in the
Minimum text box, or use the arrows to change
the current value.
Click OK.
6
Using channels to modify GIF lossiness
When you use an alpha channel to optimize the
amount of lossiness (or quality reduction) in a GIF
image, white areas of the mask yield the highest
quality, and black areas of the mask yield the
lowest quality. You can adjust the maximum and
minimum level of quality reduction in the Modify
Lossiness Setting dialog box.
To use a channel to modify GIF lossiness:
In the Optimize panel/palette, choose a GIF
1
setting from the Settings menu, or choose GIF
from the file format menu.
Click the channel button ( ) to the right of the
2
Lossy text box.
In the Modify Lossiness Setting dialog box,
3
choose the desired channel from the Channel
menu. In ImageReady, you can choose Save
Selection to create a new alpha channel based on
the current selection.
To preview the results of the weighted optimi-
4
zation, select the Preview option.
Define the quality range:
5
To set the highest level of quality, drag the left
•
(white) tab on the slider, enter a value in the
Minimum text box, or use the arrows to change
the current value.
To set the lowest level of quality, drag the right
•
(black) tab on the slider, enter a value in the
Maximum text box, or use the arrows to change
the current value.
Note: Lossiness is a reduction in quality; therefore,
the highest level of image quality is defined by the
Minimum value, and the lowest level of image
quality is defined by the Maximum value. This is the
opposite of the JPEG quality setting.
Click OK.
6