Web Design in a Nutshell, 3rd Edition

Weighted optimization (Photoshop/ImageReady)

Photoshop and ImageReady offer yet another advance in graphic optimization. Their weighted optimization feature allows you to apply varying amounts of optimization to different parts of the image. This preserves the integrity of the most important areas while maximizing file size savings for the remainder.

Weighted optimization uses an alpha channel (called a mask) to select areas of the image for various optimization levels. The white areas of the mask correspond to the highest level of image quality, while black areas describe the lowest (gray areas are on a linear scale in between). Channels can be used to control color reduction, dithering, and lossiness in a GIF image.

To access the Modify dialog box (Figure 29-9), click the Channel button next to each of these controls on the Optimization palette. In the dialog box, use the sliders to set the maximum (white tab on the left) and minimum (black tab on the right) levels of optimization.

In Photoshop, create the alpha channel by saving a selection and giving the channel a name (the channel can then be accessed from the Modify dialog boxes). In ImageReady, you can create a new channel based on a selected image area on the fly when you click the Channel button.

Weighted color reduction

When you use the alpha channel to reduce colors in parts of an image, the white areas of the mask determine what areas of the image are most important. Colors in those areas will be weighted more heavily when ...