Snapping is one of the most important concepts in Doodleback, but if you don’t understand how it works, it can sometimes be frustrating. When Snapping is turned on, the important points (described below) of the Path you’re constructing will automatically snap to other important points on the Paper. For example, take this septagon (7-sided Polygon).

Using Snapping and the Line Segment tool I can quickly create spokes from its center to its vertices.

You don’t need to exactly place the ends of the Line Segment. You just need to place your finger near the center to automatically snap to the center and then drag it to a point somewhere near a vertex to automatically snap to the vertex. In this example, the Important Points of the septagon are its center and vertices. The Important Points of the Line Segement are its endpoints and center. Doodleback automatically locks the initial end of the Line Segment to the center of the polygon because that was where I initially placed my finger and it automatically locks the other end of the Line Segment to a vertex of polygon because that was where I lifted my finger.

Important Points

The Important Points of a Path include the points needed to define the Path such as the end points for Line Segments. However, a Path can have many more Important Points than these. The Center of a Path is an Important Point too, for instance. On any Grid (Rectangular, Triangular, Circular), the intersection points of the grid lines are also Important Points. Why? Well, because they’re pretty useful. They’re something that you would probably want to snap to some time. When you construct a Circle, one of the construction options is the number of Important Points to create on the circumference. You might, for example, want to draw the same spoke pattern that I used above, except with a Circle. This circle was created with 16 Important Points on the circumference.

And here we’ve drawn Line Segements from the center to every circumference Important Point.

Snapping Button

The Snapping Button is described in the Header Option Buttons section. It appears at the top of the Paper during construction.

and controls which Path to prioritize (the top-most path,the bottom-most path, the oldest-path, the newest-path) when there is any uncertainty about which Path to snap to. This uncertainty might arise when two path are literally sitting on top of each other. For instance, in this picture, the Triangle is on top of the Circle.

If I want to use the Move tool to grab the bottom Circle by its center, I need to Snap To Bottom Path because both the Circle and the Triangle have the same center.

Lock Button

The Lock Button is also described in the Header Option Buttons section. This button has many uses, but one of them is to override Snapping. When the Lock is ON, all motion events are ignored which essentially freezes the construction. You don’t need to worry about an accidental snapping when you lift your finger from the page.

Snapping Radius

The Snapping Raidus defines the search area for Important Points. By default, it’s set to 25 which is approximately the size of the circle that your finger makes when it’s on the screen. We’ve found this to be a pretty good radius, but if you want to experiment with other sizes, they can be changed in Settings->Change Snapping Radius

Snapping Z-Level Radius

The Snapping Z-Level Radius is used to decide which snapping point to select when two or more points are “sufficiently close” to the touch point, but they lie on differnt z-levels. The primary snapping radius (explained above) is used to find a collection of candidate points for snapping, but depending on the current snapping mode the closest may not be selected. For instance, if your current snapping mode is “SNAP TO BOTTOM PATH” and two points lie within the Snapping Z-Level Radius, the lower will be selected even if the higher point is closer. This can be very useful if an upper level path is obscuring a lower level path and making snapping difficult. Here’s an example where we’re trying to clip a collection of free draw lines to a circle. Since Free Draw Lines have a large number of points and one of the free draw lines overlaps the center of the circle, snapping to the circle is difficult without a larger Snapping Z-Level Radius.

Snapping by Age

Snapping by Age is an advanced setting that, well, frankly you probably don’t want to use. It can be turned on through Settings->General Drawing Options->Allow Snapping by Age. This allows you to prioritize the search for Important Points not by Z-level (top-most or bottom-most) but by which path was constructed first or last (newest or oldest).