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File:Valeriepieris circle azimuthal equal area.png

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Original file (1,024 × 1,024 pixels, file size: 1.16 MB, MIME type: image/png)

Summary

Description
English: Danny Quah's Valerispieris circle on a globe model, centred on Mong Khet, Myanmar, rendered in azimuthal equal-area projection from the equirectangular projection from http://commons.wikimedia.org/wiki/File:Earthmap1000x500.jpg by CMG Lee. The fraction of the area of circle to that of the globe is equal to its equivalent on Earth.
Date upload 25. Oct. 2005
Source
Earthmap1000x500.jpg
Author cmglee, jimht at shaw dot ca
Other versions
Valeriepieris circle azimuthal equidistant.png
Mecca azimuthal equidistant.png
Cambridge azimuthal equidistant.png

Python source

#!/usr/bin/env python

import re, math, png

path_in          = 'mya/Earthmap1000x500.png'
path_out         = 'Valeriepieris_circle_azimuthal_equal_area.png'
colour_circle    = [255, 255, 0]
radius_circle    = 0.51
thickness_circle = 0.01
lat_centre       = 21.7
long_centre      = 99.383333
zoom             = 0.5
# zoom             = 0.33
# out_size         = 512
out_size         = 2048
out_size_half    = out_size * 0.5

class Png:
 def __init__(self, path_in):
  (self.width, self.height, self.pixels, self.metadata) = png.Reader(path_in).read_flat()
  self.planes = self.metadata['planes']
 def __str__(self): return str((self.width, self.height, len(self.pixels), self.metadata))
 def write(self, path_out):
  png.Writer(width=self.width, height=self.height,
             bitdepth=self.metadata['bitdepth'], interlace=self.metadata['interlace'],
             planes=self.metadata['planes'], greyscale=self.metadata['greyscale'],
             alpha=self.metadata['alpha']).write_array(open(path_out, 'wb'), self.pixels)

## Formula from http://mathworld.wolfram.com/AzimuthalEquidistantProjection.html
def azimuthal_equidistant_to_equirectangular(x, y, lat_centre, long_centre):
 c              = math.hypot(x, y)
 if c == 0 or (abs(lat_centre) == 90 and y == 0): return (0, 0)
 sin_c          = math.sin(c)
 cos_c          = math.cos(c)
 lat_centre_rad = math.radians(lat_centre)
 sin_lat_centre = math.sin(lat_centre_rad)
 cos_lat_centre = math.cos(lat_centre_rad)
 to_asin  = cos_c * sin_lat_centre + y * sin_c * cos_lat_centre / c
 if abs(to_asin) > 1: return (0, 0)
 lat  =  math.degrees(math.asin(to_asin))
 long = (math.degrees(math.atan2(-x, y) if lat_centre ==  90 else
                      math.atan2( x, y) if lat_centre == -90 else
                      math.atan2(x * sin_c, c * cos_lat_centre * cos_c -
                                            y * sin_lat_centre * sin_c)) +
         long_centre + 540) % 360 - 180 ## +540%360-180 to make range [-180,180)
 return (lat, long)
## From http://mathworld.wolfram.com/LambertAzimuthalEqual-AreaProjection.html
def azimuthal_equal_area_to_equirectangular(x, y, lat_centre, long_centre):
 rho            = math.hypot(x, y)
 if rho == 0 or (abs(lat_centre) == 90 and y == 0) or abs(rho * 0.5) > 1:
  return (None, None)
 c              = 2 * math.asin(rho * 0.5)
 sin_c          = math.sin(c)
 cos_c          = math.cos(c)
 lat_centre_rad = math.radians(lat_centre)
 sin_lat_centre = math.sin(lat_centre_rad)
 cos_lat_centre = math.cos(lat_centre_rad)
 to_asin  = cos_c * sin_lat_centre + y * sin_c * cos_lat_centre / rho
 if abs(to_asin) > 1: return (None, None)
 lat  =  math.degrees(math.asin(to_asin))
 long = (math.degrees(math.atan2(x * sin_c, rho * cos_lat_centre * cos_c -
                                            y * sin_lat_centre * sin_c)) +
         long_centre + 540) % 360 - 180 ## +540%360-180 to make range [-180,180)
 return (lat, long)

png_in = Png(path_in)
print(png_in)
print(png_in.pixels[:20])
png_out = Png(path_in) ## copy most of original's metadata
png_out.width  = png_out.height = out_size
png_out.pixels = [0] * (png_out.width * png_out.height)
print(png_out)
for  out_y in range(out_size):
 for out_x in range(out_size):
  x = (out_x / out_size_half - 1) /  zoom
  y = (out_y / out_size_half - 1) / -zoom
  if abs(math.hypot(x,y) - radius_circle) < thickness_circle * zoom:
   colour = colour_circle
  else:
   # (lat, long) = azimuthal_equidistant_to_equirectangular(x, y, lat_centre, long_centre)
   (lat, long) = azimuthal_equal_area_to_equirectangular(x, y, lat_centre, long_centre)
   if lat is None or long is None:
    colour = [0] * png_out.planes
   else:
    in_y      = int(png_in.height * ( 90 - lat ) / 180.0)
    in_x      = int(png_in.width  * (180 + long) / 360.0)
    in_offset = (in_y  * png_in.width + in_x ) * png_in .planes
    colour    = png_in.pixels[in_offset :in_offset  + png_in.planes]
  out_offset  = (out_y * out_size     + out_x) * png_out.planes
  png_out.pixels[out_offset:out_offset + png_out.planes] = colour
png_out.write(path_out)

Licensing

I, the copyright holder of this work, hereby publish it under the following license:
w:en:Creative Commons
attribution share alike
This file is licensed under the Creative Commons Attribution-Share Alike 4.0 International license.
You are free:
  • to share – to copy, distribute and transmit the work
  • to remix – to adapt the work
Under the following conditions:
  • attribution – You must give appropriate credit, provide a link to the license, and indicate if changes were made. You may do so in any reasonable manner, but not in any way that suggests the licensor endorses you or your use.
  • share alike – If you remix, transform, or build upon the material, you must distribute your contributions under the same or compatible license as the original.

Captions

Danny Quah's Valerispieris circle on a globe model, centred on Mong Khet, Myanmar, rendered in azimuthal equal-area projection from the equirectangular projection

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Date/TimeThumbnailDimensionsUserComment
current08:04, 20 January 2024Thumbnail for version as of 08:04, 20 January 20241,024 × 1,024 (1.16 MB)CmgleeUploaded own work with UploadWizard

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