Apple 2 manual 7An icon’s mask either highlights or animates the icon

C H A P T E R 5

Icons

When a user selects an icon, the Extras Drawer creates the selected form of the icon by combining the unselected form of the icon with the icon’s mask. The Extras Drawer uses the same method to animate one icon as it uses to highlight another. The design of the mask determines how the selected form of an icon looks—highlighted or animated. Figure 5-7 compares a mask used for highlighting with a mask used for animation.

Figure 5-7An icon’s mask either highlights or animates the icon

Not selected Mask Selected

An all-black shadow mask highlights an icon

Another mask creates an alternate form of the icon

Note: bounding boxes are for illustration only. Actual icons do not have bounding boxes.

The selected form of an icon is black only in spots where either the mask is black or the unselected form of the icon is black. The selected form is white wherever both the unselected form and the mask are black as well as where both are white.

You create a mask by comparing each pixel of the unselected form of an icon to the corresponding pixel of the selected form. If both forms of the icon have a black pixel in the same position or if both have a white pixel in the same position, the mask has a white pixel there. The mask has a black pixel where one form of the icon or the other has a black pixel, but not where both do. In making this comparison of the pixels, you are following the exclusive-or rule of logic. Figure 5-8 shows the masks of several animated icons.

5-10 Extras Drawer Icons