// poly clipping algorithm Aug 2007
bool
cropPolygonPart( const GeoPointList& initial_input, const GeoExtent& window, GeoPartList& final_outputs )
{
// trivial rejection for a non-polygon:
if ( initial_input.size() < 3 )
{
return false;
}
// check for trivial acceptance.
if ( window.contains( initial_input ) )
{
final_outputs.push_back( initial_input );
return true;
}
// prepare the list of input parts to process.
GeoPartList inputs;
inputs.push_back( initial_input );
// run the algorithm against all four sides of the window in succession.
for( UINT stage = 0; stage < 4; stage++ )
{
GeoPoint s1, s2;
switch( stage )
{
case STAGE_SOUTH: s1 = window.getSouthwest(), s2 = window.getSoutheast(); break;
case STAGE_EAST: s1 = window.getSoutheast(), s2 = window.getNortheast(); break;
case STAGE_NORTH: s1 = window.getNortheast(), s2 = window.getNorthwest(); break;
case STAGE_WEST: s1 = window.getNorthwest(), s2 = window.getSouthwest(); break;
}
// output parts to send to the next stage (or to return).
GeoPartList outputs;
// run against each input part.
for( GeoPartList::iterator i = inputs.begin(); i != inputs.end(); i++ )
{
//GeoPointList& input = *i;
GeoPointList input = *i;
scrubPart( input );
// trivially reject a degenerate part (should never happen ;)
if ( input.size() < 3 )
{
continue;
}
// trivially accept if the window contains the entire extent of the part:
GeoExtent input_extent;
input_extent.expandToInclude( input );
// trivially accept when the part lies entirely within the line:
if ( extentInsideOrOnLine( input_extent, s1, s2 ) )
{
outputs.push_back( input );
continue;
}
// trivially reject when there's no overlap:
if ( !window.intersects( input_extent ) || extentInsideOrOnLine( input_extent, s2, s1 ) )
{
continue;
}
// close the part in preparation for cropping. The cropping process with undo
// this automatically.
input.push_back( input.front() );
// 1a. Traverse the part and find all intersections. Insert them into the input shape.
// 1b. Create a traversal-order list, ordering the isect points in the order in which they are encountered.
// 1c. Create a spatial-order list, ordering the isect points along the boundary segment in the direction of the segment.
GeoPointList working;
UINTList traversal_order;
UINTList spatial_order;
GeoPoint prev_p;
bool was_inside = true;
for( UINT input_i = 0; input_i < input.size(); input_i++ )
{
const GeoPoint& p = input[ input_i ];
bool is_inside = pointInsideOrOnLine( p, s1, s2 );
if ( input_i > 0 )
{
if ( was_inside != is_inside ) // entering or exiting
{
GeoPoint isect_p;
if ( getIsectPoint( prev_p, p, s1, s2, /*out*/ isect_p ) )
{
working.push_back( isect_p );
traversal_order.push_back( working.size()-1 );
spatialInsert( spatial_order, working.size()-1, stage, working );
}
else
{
osgGIS::notify( osg::WARN )
<< "getIsectPoint failed" << std::endl;
}
}
}
working.push_back( p );
prev_p = p;
was_inside = is_inside;
}
if ( spatial_order.size() == 0 )
{
outputs.push_back( input );
continue;
}
// 2. Start at the point preceding the first isect point (in spatial order). This will be the first
// outside point (since the first isect point is always an ENTRY.
UINT overall_start_ptr = spatial_order[0];
UINT shape_ptr = overall_start_ptr;
// initialize the isect traversal pointer to start at the spatial order's first isect point:
UINT trav_ptr = findIndexOf( traversal_order, spatial_order[0] );
UINT traversals = 0;
std::stack<DeferredPart> part_stack;
GeoPointList current_part;
// Process until we make it all the way around.
while( traversals < traversal_order.size() )
{
// 4. We are outside. Find the next ENTRY point and either start a NEW part, or RESUME
// a previously deferred part that is on top of the stack.
shape_ptr = traversal_order[trav_ptr]; // next ENTRY
trav_ptr = (trav_ptr + 1) % traversal_order.size();
traversals++;
UINT part_start_ptr = shape_ptr;
if ( part_stack.size() > 0 && part_stack.top().waiting_on_ptr == part_start_ptr )
{
// time to resume a part that we deferred earlier.
DeferredPart& top = part_stack.top();
current_part = top.part;
part_start_ptr = top.part_start_ptr;
part_stack.pop();
}
else
{
// start a new part
current_part = GeoPointList();
}
// 5. Traverse to the next EXIT, adding all points along the way.
// Then check the spatial order of the EXIT against the part's starting point. If the former
// is ONE MORE than the latter, close out the part.
for( bool part_done = false; !part_done && traversals < traversal_order.size(); )
{
UINT next_exit_ptr = traversal_order[trav_ptr];
trav_ptr = (trav_ptr + 1) % traversal_order.size();
traversals++;
for( ; shape_ptr != next_exit_ptr; shape_ptr = (shape_ptr+1)%working.size() )
{
current_part.push_back( working[shape_ptr] );
}
// record the exit point:
current_part.push_back( working[next_exit_ptr] );
UINT part_start_order = findIndexOf( spatial_order, part_start_ptr );
UINT next_exit_order = findIndexOf( spatial_order, next_exit_ptr );
if ( next_exit_order - part_start_order == 1 )
{
outputs.push_back( current_part );
part_done = true;
continue;
}
// 6. Find the next ENTRY. If the spatial order of the ENTRY is one less than the
// spatial ordering of the preceding EXIT, continue on with the part.
UINT next_entry_ptr = traversal_order[trav_ptr];
trav_ptr = (trav_ptr + 1) % traversal_order.size();
traversals++;
// check whether we are back at the beginning:
if ( traversals >= traversal_order.size() )
{
current_part.push_back( working[next_entry_ptr] );
continue;
}
// check whether we are continuing the current part:
UINT next_entry_order = findIndexOf( spatial_order, next_entry_ptr );
if ( next_exit_order - next_entry_order == 1 )
{
shape_ptr = next_entry_ptr; // skip ahead to the entry point
continue;
}
// 7. We encountered an out-of-order traversal, so need to push the current part onto
// the deferral stack until later, and start a new part.
part_stack.push( DeferredPart(
current_part,
part_start_ptr,
spatial_order[next_exit_order-1] ) );
current_part = GeoPointList();
//current_part.push_back( working[next_entry_ptr] );
part_start_ptr = next_entry_ptr;
shape_ptr = next_entry_ptr;
}
}
// pop any parts left on the stack (they're complete now)
while( part_stack.size() > 0 )
{
GeoPointList& part = part_stack.top().part;
part.push_back( working[part_stack.top().waiting_on_ptr] );
outputs.push_back( part );
part_stack.pop();
}
}
// set up for next iteration
outputs.swap( inputs );
}
// go through and make sure no polys are "closed" (probably unnecessary).
//for( GeoPartList::iterator k = inputs.begin(); k != inputs.end(); k++ )
//{
// while ( k->size() > 3 && k->front() == k->back() )
// k->erase( k->end()-1 );
//}
final_outputs.swap( inputs );
return true;
}
static void
scrubPart( GeoPointList& part )
{
while( part.size() >= 0 && part.front() == part.back() )
part.erase( part.end()-1 );
}