' Copyright 2013-2022 CS GROUP
' Licensed to CS GROUP (CS) under one or more
' contributor license agreements.  See the NOTICE file distributed with
' this work for additional information regarding copyright ownership.
' CS licenses this file to You under the Apache License, Version 2.0
' (the "License"); you may not use this file except in compliance with
' the License.  You may obtain a copy of the License at
'
'   http://www.apache.org/licenses/LICENSE-2.0
'
' Unless required by applicable law or agreed to in writing, software
' distributed under the License is distributed on an "AS IS" BASIS,
' WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
' See the License for the specific language governing permissions and
' limitations under the License.

@startuml

  skinparam svek                                true
  skinparam NoteBackgroundColor                 #F3EFEB
  skinparam NoteBorderColor                     #691616
  skinparam NoteFontColor                       #691616
  skinparam ActivityStartColor                  #6A584B
  skinparam ActivityEndColor                    #6A584B
  skinparam ActivityBarColor                    #6A584B
  skinparam ActivityBorderColor                 #691616
  skinparam ActivityBackgroundColor             #F3EFEB/CCC9C5
  skinparam ActivityArrowColor                  #691616
  skinparam ActivityFontSize                    11

  start
  if (recursion depth > 30?) then (yes)
    : search failed;
    stop
  endif
  if (compare start and end point) then (same pixel)
    : intersection ⇦ pixel intersection, with bilinear model;
    : return intersection;
    note left
      this is the nominal return,
      at final recursion level
    end note
    stop
  endif
  :compute kd-tree deepest\nlevel sub-tile containing\nboth segment endpoints;
  if (compare segment and sub-tile maximum elevation) then (fully above)
    : return null;
    note left
      when the line-of-sight segment is fully above
      Digital Elevation Model, we can safely reject
      it and proceed immediately to next segment
    end note
    stop
  endif
  :previous ⇦ start;
  : crossings ⇦ line-of-sight segment crossings of next level sub-tiles;
  note right
    crossings can be computed either
    using flat body hypothesis
    or taking curvature into account
  end note
  repeat
    :intersection ⇦ recurse(previous, crossing);
    if (intersection found?) then (yes)
      : return intersection;
      stop
    endif
    :previous ⇦ crossing;
  repeat while (more crossings?)
  :intersection ⇦ recurse(previous, end);
  : return intersection;
  stop

@enduml