/// <summary>
/// Determines whether the current <see cref="NodeArc"/> completely obscures the specified
/// instance, or vice versa.</summary>
/// <param name="arc">
/// The <see cref="NodeArc"/> to examine.</param>
/// <returns><para>
/// An <see cref="Int32"/> value indicating the relative obscuration of this instance and
/// <paramref name="arc"/>, as follows:
/// </para><list type="table"><listheader>
/// <term>Return Value</term><description>Condition</description>
/// </listheader><item>
/// <term>Less than zero</term><description>
/// <paramref name="arc"/> completely obscures this instance.</description>
/// </item><item>
/// <term>Zero</term><description>
/// Neither instance completely obscures the other.</description>
/// </item><item>
/// <term>Greater than zero</term><description>
/// This instance completely obscures <paramref name="arc"/>.</description>
/// </item></list></returns>
/// <remarks>
/// <b>IsObscured</b> takes the <see cref="Distance"/> of both instances into account to
/// determine which instance obscures the other, if any.</remarks>
public int IsObscured(NodeArc arc)
{
// start of specified arc relative to current arc
double relativeStart = arc.Start - Start;
if (relativeStart <= -Math.PI)
{
relativeStart += 2 * Math.PI;
}
else if (relativeStart > Math.PI)
{
relativeStart -= 2 * Math.PI;
}
Debug.Assert(relativeStart > -Math.PI && relativeStart <= Math.PI);
double relativeSweep = relativeStart + arc.Sweep;
// specified arc completely obscures current arc
if (relativeStart <= 0 && relativeSweep >= Sweep && arc.Distance <= Distance)
{
return(-1);
}
// current arc completely obscures specified arc
if (relativeStart >= 0 && relativeSweep <= Sweep && arc.Distance > Distance)
{
return(+1);
}
return(0);
}
/// <summary>
/// Expands the current visibility area with all visible <see cref="Graph"/> nodes, within
/// maximum world distance from the source node.</summary>
/// <remarks><para>
/// <b>FindVisibleNodes</b> iterates over all <see cref="NodeArcs"/> found by <see
/// cref="FindObscuringNodes"/>, and adjusts their <see cref="NodeArc.VisibleFraction"/>
/// according to the collection of obscuring nodes also created by that method.
/// </para><para>
/// Any node whose <see cref="NodeArc"/> remains unobscured by at least the current <see
/// cref="Threshold"/> is added to the <see cref="Nodes"/> collection.</para></remarks>
private void FindVisibleNodes()
{
// iterate over all visited nodes that may be visible
foreach (var pair in _nodeArcs)
{
NodeArc arc = pair.Value;
if (arc._visibleFraction == 0)
{
continue;
}
double start = arc.Start, sweep = arc.Sweep;
// compare visited node to all obscuring nodes (except itself)
foreach (var obscuringPair in _obscuringNodes)
{
if (ComparerCache <T> .EqualityComparer.Equals(pair.Key, obscuringPair.Key))
{
continue;
}
// ignore obscuring nodes at a greater source distance
NodeArc obscuringAngle = obscuringPair.Value;
if (obscuringAngle.Distance > arc.Distance)
{
continue;
}
// obscure visibility arc of visited node
obscuringAngle.Obscure(ref start, ref sweep);
arc._visibleFraction = sweep / arc.Sweep;
// check if arc is sufficiently obscured
if (arc._visibleFraction < _threshold)
{
goto nextVisited;
}
}
// add visible node to search results
_nodes.Add(pair.Key);
nextVisited:
continue;
}
}
/// <summary>
/// Expands the current collection of obscuring <see cref="Graph"/> nodes with all neighbors
/// of the specified node, within maximum world distance from the source node.</summary>
/// <param name="node">
/// The <see cref="Graph"/> node whose neighbors to examine.</param>
/// <remarks><para>
/// <b>FindObscuringNodes</b> recursively visits all directly connected nodes, and adds them
/// to an internal collection of obscuring nodes if they are opaque. Nodes which are fully
/// obscured by other obscuring nodes are removed from the collection.
/// </para><para>
/// <b>FindObscuringNodes</b> never revisits nodes that were already examined. All visited
/// nodes are added to <see cref="NodeArcs"/> for later processing by <see
/// cref="FindVisibleNodes"/>.</para></remarks>
private void FindObscuringNodes(T node)
{
// get valid neighbors of current node
IList <T> neighbors = Graph.GetNeighbors(node);
// recurse into all valid neighbors
for (int i = 0; i < neighbors.Count; i++)
{
T neighbor = neighbors[i];
// skip source and previously visited nodes
if (ComparerCache <T> .EqualityComparer.Equals(_source, neighbor) ||
_nodeArcs.ContainsKey(neighbor))
{
continue;
}
// compute tangential arc and source distance
NodeArc arc = CreateNodeArc(neighbor);
// skip nodes beyond maximum distance
if (_distance > 0 && arc.Distance > _distance)
{
continue;
}
// record visited node with tangential arc
_nodeArcs.Add(neighbor, arc);
// nothing else to do for transparent nodes
if (!_isOpaque(neighbor))
{
goto nextNeighbor;
}
/*
* Try adding current opaque node to list of all obscuring nodes recorded so far.
*
* If any single recorded node completely obscures the current node, we skip it.
* If the current node completely obscures any recorded nodes, we delete those.
*
* We also clear the VisiblityFraction for all completely obscured nodes (current
* or recorded) so we won't waste time testing them again in FindVisibleNodes.
*/
foreach (var pair in _obscuringNodes)
{
int result = arc.IsObscured(pair.Value);
if (result < 0)
{
arc._visibleFraction = 0;
goto nextNeighbor;
}
if (result > 0)
{
pair.Value._visibleFraction = 0;
_removeNodes.Add(pair.Key);
}
}
// remove obscuring nodes that were themselves obscured
for (int j = 0; j < _removeNodes.Count; j++)
{
_obscuringNodes.Remove(_removeNodes[j]);
}
_removeNodes.Clear();
// add neighbor to obscuring nodes
_obscuringNodes.Add(neighbor, arc);
nextNeighbor:
FindObscuringNodes(neighbor);
}
}