Fix rendering with land at the antipodal point

If the antipodal point (the point opposite of the map center) is on land,
the corresponding land mass was stretched across the whole map, breaking
the rendering. This was fixed by a couple of changes:

1. A check is done for each polygon whether it contains the antipodal
   point.
2. If it does not, render it as a simple polygon as it was previously.
3. If it does contain the antipodal point, create a path that contains:
   a) A circle covering the whole map.
   b) The polygon with the points in reverse order.

This leads to filled circle with the polygon “cut out”, which is the
correct display in this case.
This commit is contained in:
Thomas Kolb 2021-06-01 22:58:33 +02:00
parent a766274d79
commit a9f987c079

101
qsomap.py
View file

@ -65,6 +65,78 @@ def random_country_color():
return f"#{r:02x}{g:02x}{b:02x}"
def is_point_in_polygon(point, polygon):
# Idea: draw an infinite line from the test point along the x axis to the
# right. Then check how many polygon edges this line intersects. If the
# number is even, the point is outside the polygon.
edges = [] # list of lists, containing two points each
for i in range(len(polygon)-1):
edges.append([polygon[i], polygon[i+1]])
# the closing edge
edges.append([polygon[-1], polygon[0]])
num_intersects = 0
test_x, test_y = point
for edge in edges:
start_x = edge[0][0]
start_y = edge[0][1]
end_x = edge[1][0]
end_y = edge[1][1]
# quick exclusion tests
if start_x < test_x and end_x < test_x:
continue # edge is completely left of the test point
if start_y < test_y and end_y < test_y:
continue # edge is completely below the test point
if start_y > test_y and end_y > test_y:
continue # edge is completely above the test point
# calculate the x coordinate where the edge intersects the whole
# horizontal line
intersect_x = start_x + (end_x - start_x) \
* (test_y - start_y) \
/ (end_y - start_y)
if intersect_x > test_x:
# we found an intersection!
num_intersects += 1
if num_intersects % 2 == 0:
return False # even number of intersects -> outside polygon
else:
return True # odd number of intersects -> inside polygon
def svg_make_inverse_country_path(doc, map_radius, polygon, **kwargs):
# build a closed circle path covering the whole map
commands = [f"M 0, {map_radius}",
f"a {map_radius},{map_radius} 0 1,0 {map_radius*2},0",
f"a {map_radius},{map_radius} 0 1,0 {-map_radius*2},0",
"z"]
# "subtract" the country polygon
commands.append(f"M {polygon[0][0]} {polygon[0][1]}")
# add lines for each polygon point
for point in polygon[1:]:
commands.append(f"L {point[0]} {point[1]}")
# ensure straight closing line
commands.append(f"L {polygon[0][0]} {polygon[0][1]}")
# close the inner path
commands.append("z")
return doc.path(commands, **kwargs)
def render(ref_lat, ref_lon, output_stream):
random.seed(0)
@ -127,6 +199,12 @@ def render(ref_lat, ref_lon, output_stream):
ref_lat = ref_lat * np.pi / 180
ref_lon = ref_lon * np.pi / 180
antipodal_lat = -ref_lat
antipodal_lon = ref_lon + np.pi
if antipodal_lon > np.pi:
antipodal_lon -= 2*np.pi
R = 500
"""
@ -183,7 +261,6 @@ def render(ref_lat, ref_lon, output_stream):
v['proj_coordinates'] = proj_polys
# generate the SVG
doc = svgwrite.Drawing("/tmp/test.svg", size=(2*R, 2*R))
@ -224,14 +301,26 @@ def render(ref_lat, ref_lon, output_stream):
group = doc.g()
for poly in v['proj_coordinates']:
for i in range(len(v['proj_coordinates'])):
poly = v['proj_coordinates'][i]
points = poly.T + R # shift to the center of the drawing
pgon = doc.polygon(points, **{
'class': 'country',
'fill': color})
# check if the antipodal point is inside this polygon. If so, it
# needs to be "inverted", i.e. subtracted from the surrounding map
# circle.
group.add(pgon)
if is_point_in_polygon((antipodal_lon, antipodal_lat),
v['coordinates'][i].T):
print("!!! Found polygon containing the antipodal point!")
obj = svg_make_inverse_country_path(doc, R, np.flipud(points), **{
'class': 'country',
'fill': color})
else:
obj = doc.polygon(points, **{
'class': 'country',
'fill': color})
group.add(obj)
group.set_desc(title=v['name'])
doc.add(group)