albers.yaml

albers:
    base: polygons
    animated: true
    shaders:
        defines:
            EARTH_RADIUS: 6378137.0
            PI: 3.14159265358979323846
            HALF_PI: 1.570796327
            QUARTER_PI: .785398163
            deg2rad(d): (d)*PI/180.0
            rad2deg(d): (d)*180.0/PI
        blocks:
            global: |
                // http://wiki.openstreetmap.org/wiki/Mercator
                float y2lat_m (float y) { return rad2deg(2.0*atan(exp((y/EARTH_RADIUS)))-HALF_PI); }
                float x2lon_m (float x) { return rad2deg(x/EARTH_RADIUS); }

                float lat2y_m (float lat) { return EARTH_RADIUS*log(tan(QUARTER_PI+ deg2rad(lat)/2.0)); }
                float lon2x_m (float lon) { return deg2rad(lon)*EARTH_RADIUS; }
                
                // convert from lat/long to albers -- from https://gist.github.com/RandomEtc/476238
                vec2 albers(float lat, float lng, float lat0, float lng0, float phi1, float phi2) {
                    float n = 0.5 * (sin(phi1) + sin(phi2));
                    float c = cos(phi1);
                    float C = c*c + 2.*n*sin(phi1);
                    float p0 = sqrt(C - 2.*n*sin(lat0)) / n;
                    float theta = n * (lng * PI/180. - lng0);
                    float p = sqrt(C - 2.*n*sin(lat* PI/180.)) / n;

                    float x = p * sin(theta);
                    float y = p0 - p * cos(theta);

                    return vec2(x,y);
                }
            position: |
                // mercator position of the current vertex, u_map_position = center of screen,
                // position.xy = vertex screen position in meters from the center of the screen
                vec2 mercator = u_map_position.xy + position.xy;
                float lat = y2lat_m(mercator.y);
                float lon = x2lon_m(mercator.x);

                // Latitude_Of_Origin
                float centerlat = deg2rad(y2lat_m(u_map_position.y));
                // Central_Meridian
                float centerlon = deg2rad(x2lon_m(u_map_position.x));

                // Standard_Parallel_1
                float phi1 = deg2rad(y2lat_m(u_map_position.y) + 10.);
                // Standard_Parallel_2
                float phi2 = deg2rad(y2lat_m(u_map_position.y) - 10.);

                position.xy = albers(lat, lon, centerlat, centerlon, phi1, phi2)*EARTH_RADIUS;