About masks and weighted optimization
Photoshop and ImageReady automatically generate masks when you create a type layer or a shape layer. You can also manually create masks and store them in alpha channels. When you use a mask to apply optimization settings, the white areas of the mask describe the highest level of image quality, while the black areas for the mask describe the lowest level of image quality. (The level of optimization in gray areas of the mask decreases by a linear scale.)
Weighted optimization is available for specific settings in the Optimize panel/palette, as indicated by the Mask button 
. To access the weighted optimization dialog box, simply click the mask button.
Note: For the Lossy setting, a higher number actually means greater compression, as opposed to greater quality (as in most other settings).
To create masks for use during optimization:
- Create a type layer. (See Creating type.)
- Create a shape layer. (See Creating shapes.)
- (Photoshop) Save a selection as a mask, or create a new alpha channel and use the painting and editing tools to modify it. (See Storing masks in alpha channels.)
- (ImageReady) Use a selection tool to select an area of the image. (See About selections.) Either save the selection using the Select > Save Selection command, or choose the Save Selection command from the Channel pop-up menu in the weighted optimization dialog box.
Do one or more of the following: