Using Weighted Optimization; About Weighted Optimization; To Create Masks For Use During Optimization - Adobe PHOTOSHOP CS2 User Manual

Using weighted optimization

About weighted optimization

Weighted optimization lets you vary optimization settings across an image using masks from text layers, shape layers,
and alpha channels. This technique produces higher-quality results in critical image areas without greatly increasing
file size. With weighted optimization, you can produce gradual variations in GIF, PNG-8, and WBMP dithering,
lossy GIF settings, and JPEG compression. Weighted optimization also lets you favor colors in selected image areas
when you generate a color table.
Image without weighted optimization (left), and with weighted optimization on text layers and vector shapes (right)
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, whereas 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, 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

Do one or more of the following:
Create a type layer.
•
• Create a shape layer.
(Photoshop) Save a selection as a mask, or create a new alpha channel and use the painting and editing tools to
•
modify it. (See "About masks and alpha channels" on page 338.)
• (ImageReady) Use a selection tool to select an area of the image. 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.
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