|5.3 Angular snapping|
For a snap angle of 180 degrees, snapping is to a single line through the current origin.
In order to use angular snapping, it is important to set the axis system correctly. To set the origin, move the mouse to the correct position, and press the F1-key. Note that angular snapping is disabled while setting the origin. This way you can set a new origin for angular snapping without leaving the mode first. Once the origin has been set, the base direction is set by moving to a point on the desired base line, and pressing the F2-key. Again, angular snapping is disabled. Together, origin and base direction determine the current axis system. Remember that the origin is also used as the fix-point of scale, stretch, and rotate operations, if it is set.
You can un-set the current axis system by pressing Shift-F2. This also turns off angular snapping. You can set origin and base direction at the same time by pressing F3 when the mouse is very near (or snapped to) an edge of a polygonal object. The origin is set to an endpoint of the edge, and the base direction is aligned with it. This is useful to make objects parallel to a given edge.
For drawing rectilinear or c-oriented polygons, the origin should be set to the previous vertex at every step. This can be done by pressing F1 every time you click the left mouse button, but that would not be very convenient. Therefore, Ipe offers a second angular snap mode, called automatic angular snapping. This mode uses an independent origin, which is automatically set every time you add a vertex when creating a polygonal object. Note that while the origin is independent of the origin set by F1, the base direction and the snap angle used by automatic angular snapping is the same as for angular snapping. Hence, you can align the axis system with some edge of your drawing using F3, and then use automatic angular snapping to draw a new object that is parallel or orthogonal to this edge.
This snapping mode has another advantage: It remains silent and ineffective until you start creating a polygonal object. So, even with automatic angular snapping already turned on, you can still freely place the first point of a polygon, and then the remaining vertices will be properly aligned to make a c-oriented polygon.
The automatic angular snapping mode is never active for any non-polygonal object. In particular, to move an object in a prescribed direction, you have to use normal angular snapping.
A final note: Many things that can be done with angular snapping can also be done by drawing auxiliary lines and using context snapping. It is mostly a matter of taste and exercise to figure out which mode suits you best.