Lock Transparency

Each object can consist of both opaque and transparent components.

Each object also has a boundary. An object's boundary can be seen as a blue dashed line when the objects marquee is made visible. You can turn on the Object Marquee either from the menu (Object > Marquee Visible), using a keyboard shortcut (CTRL+SHIFT+H) or via the icon on the Mask/Object Toolbar.

Areas inside the marquee are generally opaque while those outside are transparent. When you ‘fill' an object you fill the entire plane/layer of that object regardless of which areas were previously considered opaque or transparent. However, if you LOCK the object's transparency, the original transparent area is considered during further manipulations (such as ‘Fill') and the transparent area is retained, thus retaining the original object's boundary.

Object boundaries may actually extent past the boundary of the image while in the Photo-Paint workspace. Try moving an object partially outside the boundary of the image and then move it back again. See - it wasn't cropped to the image boundary so part of it must have been allowed to exist outside the image boundary, at least while working in Photo-Paint anyway.

Lock Transparency is VERY handy and can be used instead of creating a mask, especially when a mask created from an object is just going to coincide with an object's boundary anyway.

The Mask/Object Toolbar