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Directional snapping

When directional snapping is enabled, the mouse position is restricted to lie on a set of lines through the origin of you current axis system. The lines are the lines whose angle with the base direction is an integer multiple of the snapping angles. The snap angle can be set in the second box in the Snap field. The values are indicated in degrees, and, again, can be customized in the X resources.

The directional snap mode is turned on by pressing the --button. Note that the - sign looks like a line, and directional snap mode always snaps to a line. In fact, for a snap angle of 180 degrees, snapping is to a single line through the current origin.

  In order to use directional snapping, it is important to set the axis system correctly. To set the origin, move the mouse to the correct position, and press the `-key. Note that directional snapping is disabled while setting the origin. This way you can set a new origin for directional 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 >-key (note that the >-sign looks like an arrow.) Again, directional snapping is disabled. Together, origin and base direction determine the current axis system. Note 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 M-`. This also turns off directional snapping. The origin is set back to the point (0,0), and the base direction is set horizontal.

You can set origin and base direction at the same time by pressing ~ (same as Shift and ` on most keyboards) 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.

Finally, there are some shortcuts. C-` sets the origin like `, but also turns on directional snapping. C-~ sets origin and direction, and turns on directional snapping.

  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 ` every time you click the left mouse button, but that would not be very convenient. Therefore, Ipe offers a second directional snap mode, called automatic directional snapping or short autodir snapping. This second directional snapping 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 `, the base direction and the snap angle used by autodir snapping is the same as for directional snapping. Hence, you can align the axis system with some edge of your drawing using ~, and then use autodir snapping to draw a new object that is parallel or orthogonal to this edge.

The autodir snapping mode has another advantage: It remains silent and ineffective until you start creating a polygonal object. So, even with autodir 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 autodir 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 directional snapping.

A final note: Many things that can be done with directional 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.


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