When you use Smart Pick to define a primitive object, AutoCAD Raster Design toolset automatically detects the geometry of the selected raster entity and defines the entity as a primitive line, arc, or circle.
Results of using Smart Pick can be unpredictable where pixels do not clearly define raster entities. For example, Smart Pick may detect an arc instead of a full circle, or a line length may differ from what you expect.
You can use raster snaps to select the raster line, circle, or arc. For more information, see Raster Snapping.