Paint.NET offers several ways to make selections and one of the most simple options is the Rectangle Select tool. It's very easy to overlook the Tool Options bar, but the settings available for the Rectangle Select tool make it a much more versatile tool that can be used to produce more complex selections than simple rectangles.
The basic options are Selection Mode and options for constraining the size and shape of the selection. Some of these options lead to more controls being displayed.
By default, this is set to Replace meaning that when you make a selection in this mode, if there is an active selection already in the document, that is deactivated and is replaced by the new selection.
When set to Add (union), you can combine as many rectangle shapes as you like to form one or more irregularly shaped selections.
The Subtract mode allows you to remove parts of an existing selection, such as the simple 'U' shape in the picture.
The Intersect mode is another way to combine selections, but in this mode, only the area or areas that are included within both selections remain active.
The final mode, Invert (“xor”), deselects any pixels that fall within the new selection and that were already selected. Any pixels within the new selection that weren't previously selected become part of the new selection.
Size and Shape
By default this is set to Normal and this means that you can draw a rectangle selection of any shape and any size up to the size of the document.
When set to Fixed Ratio, two new input fields for Width and Height are displayed allowing you to set relative sizes so that the selection will maintain fixed proportions. This could be useful if you wanted to make a selection to crop an image into a popular photo format, such as 3:2.
The other setting is Fixed Size and this also offers input fields for Width and Height. However, it also displays a drop down to allow the selection of the unit of measurement for the selection. These are pixels, inches and centimeters.
Once you take a look at the Tool Options bar, you can see that the Rectangle Select tool offers more power than you might at first think. The different selection modes mean that some unusual and complex selections can be built up and the size and shape options can be especially useful when cropping images.