albers.yaml

# contributors: @patriciogv @meetar @burritojustice
import: mercator-functions.yaml
styles:
    projection:
        mix: albers
    projection_points:
        mix: projection
    albers:
        mix: mercator-functions
        blend_order: 0
        shaders:
            blocks:
                global: |
                    // 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;