The Lasso Select tool in Paint.NET is a fairly simple selection tool that is used to draw free hand selections. Paint.NET lacks a bezier line tool, but zooming in and using the Add (union) and Subtract modes can allow you to build up more elaborate selections of pixels. If you're not comfortable with using bezier line tools, this may actually be a more attractive way to make a selection.
As with the other tools in Paint.NET, when the Lasso Select tool is active, the Tool Options bar changes to display all available options. In the case of the Lasso Select tool, however, the only option is the Selection Mode.
To use the Lasso Select tool, you just click and hold the mouse button, while moving the mouse to describe the shape that you want. As you draw, the selection that is being made is identified by a thin border line and a transparent blue overlay that defines the selected area.
By default, this will be set to Replace and in this mode, the tool is at its most simple. Everytime that you click to start drawing a new selection, any existing selections are removed from the document.
When the drop down is set to Add (union), any existing selections will remain active along with a newly drawn selection. This mode can be used to draw lots of small selections that will slowly combine to form a larger, more complex selection. Zooming in and drawing small selections is generally easier and more accurate than trying to draw a selection in one go.
The Subtract option does the opposite of the Add (union) mode and this can be used to fine tune a selection, by removing areas that have been accidentally included within the selected area.
The Intersect setting will only function if there is an existing active selection in the document, otherwise the selection will disappear as soon as the mouse button is released. If there is an active selection, only the areas that fall within both the active selection and the new selection will be selected.
Finally, the Invert ("xor") setting works like the Intersect setting in reverse. That is that if the there is an active selection already in the document, any areas of that selection that fall within the new selection will be removed from the selection, while the other areas will remain selected.
Fans of bezier line tools for drawing more complex selections will probably feel a little short changed when using Paint.NET, however for users who prefer simpler drawing tools, the Lasso Select tool is very intuitive. By zooming in close and making full use of the different Selection Modes, the Lasso Select tool, in combination with the other selection tools, can produce quite elaborate selections.