// To recognize a scribble we require the simplified line to reverse
// direction at least three times. There are separate criteria for
// erasing a shape currently being drawn and for erasing existing
// shapes.
//
// The key difference between an "erase scribble" and a "cancel
// scribble" is that an erase scribble starts out as such, while
// a cancel scribble indicates that the user changed his mind, so
// the line will not appear to be a scribble at the beginning.
// The difference is detected by timestamps. For example, the
// following diagram represents an "erase" operation and a "cancel"
// operation. Assume the input points are evenly spaced in time,
// and that the dots represent points where the input reversed
// direction.
//
// Input points ..........................
// Reversals (erase) . . . . . .
// Reversals (cancel) . . . .
//
// So, a scribble is considered an erasure if it satisfies t0 < t1,
// where t0 is the time between mouse-down and the first reversal,
// and t1 is the time between the first and third reversals. A cancel
// operation satisfies t0 > t1 instead.
//
// Both kinds of scribble need to satisfy the formula LL*c > CHA,
// where c is a constant factor in pixels, LL is the drawn line
// length and CHA is the area of the Convex Hull that outlines the
// drawn figure. This formula basically detects that the user
// is convering the same ground repeatedly; if the pen reverses
// direction repeatedly but goes to new places each time, it's not
// considered an erasure scribble. For a cancel scribble, LL is
// computed starting from the first reversal.
IEnumerable <Shape> RecognizeScribbleForEraseOrCancel(DragState state, out bool cancel, out List <PointT> simplifiedSS)
{
cancel = false;
simplifiedSS = null;
var tolerance = state._inputTransform.Transform(new VectorT(0, 10)).Length();
var simplifiedMP = LineMath.SimplifyPolyline(
state.UnfilteredMousePoints.Select(p => p.Point), tolerance);
List <int> reversals = FindReversals(simplifiedMP, 3);
if (reversals.Count >= 3)
{
simplifiedSS = simplifiedMP.Select(p => state._inputTransform.Transform(p)).ToList();
// 3 reversals confirmed. Now decide: erase or cancel?
int[] timeStampsMs = FindTimeStamps(state.UnfilteredMousePoints, simplifiedMP);
int t0 = timeStampsMs[reversals[0]], t1 = timeStampsMs[reversals[2]] - t0;
cancel = t0 > t1 + 500;
// Now test the formula LL*c > CHA as explained above
IListSource <PointT> simplifiedMP_ = cancel ? simplifiedMP.Slice(reversals[0]) : simplifiedMP.AsListSource();
float LL = simplifiedMP_.AdjacentPairs().Sum(pair => pair.A.Sub(pair.B).Length());
var hull = PointMath.ComputeConvexHull(simplifiedMP);
float CHA = PolygonMath.PolygonArea(hull);
if (LL * EraseNubWidth > CHA)
{
// Erasure confirmed.
if (cancel)
{
return(EmptyList <Shape> .Value);
}
// Figure out which shapes to erase. To do this, we compute for
// each shape the amount of the scribble that overlaps that shape.
var simplifiedSS_ = simplifiedSS;
return(_doc.Shapes.Where(s => ShouldErase(s, simplifiedSS_)).ToList());
}
}
return(null);
}