draw_geojson.js

var assert = require('assert');
var fs = require('fs');
var path = require('path');
var sprintf = require('sprintf-js').sprintf;
var jsonfile = require('jsonfile');
var Canvas = require('canvas');
var Terraformer = require('terraformer');

var dirname = './geojson-2';
var files = fs.readdirSync(dirname);
files.forEach(function(filename) {
    if (path.extname(filename) === '.json') {
        draw(dirname + '/' + filename);
    }
});

function draw(json_path) {
    var canvas = new Canvas(512, 512);
    var ctx = canvas.getContext('2d');
    var EarthRadius = 6378137;
    var bbox;

    var geojson = new Terraformer.Primitive(jsonfile.readFileSync(json_path));
    if (true || geojson.type === 'FeatureCollection') {
        bbox = geojson.bbox();
        geojson.features.forEach(function(feature) {
            if (feature.type === 'Feature') {
                if (feature.geometry.type === 'MultiPolygon') {
                    feature.geometry.coordinates.forEach(drawPolygon);
                } else if (feature.geometry.type === 'Polygon') {
                    drawPolygon(feature.geometry.coordinates);
                }
            } else {
            }
        });

        ctx.font = '14px sans-serif';
        ctx.fillStyle = 'rgba(0,0,0,1.0)';
        ctx.fillText(sprintf('(%.2f, %.2f) - (%.2f, %.2f)', bbox[0], bbox[1], bbox[2], bbox[3]), 10, 20);

        var png_path = path.join(path.dirname(json_path), path.basename(json_path, '.json') + '.png');
        fs.writeFileSync(png_path, canvas.toBuffer());
    } else {
        console.err('File type is not feature collection');
    }

    function drawPolygon(polygon) {
        var exterior = polygon[0];

        var m_min = Terraformer.Tools.positionToMercator([bbox[0], bbox[1]]);
        var m_max = Terraformer.Tools.positionToMercator([bbox[2], bbox[3]]);
        
        ctx.fillStyle = 'rgba(0,0,0,0.5)';
        ctx.beginPath();
        ctx.moveTo(exterior[0][0], exterior[0][1]);
        exterior.forEach(function(coord) {
            var m_coord = Terraformer.Tools.positionToMercator(coord);
            var x = (m_coord[0] - m_min[0]) / (m_max[0] - m_min[0]);
            var y = (m_coord[1] - m_min[1]) / (m_max[1] - m_min[1]);
            ctx.lineTo(x * canvas.width, canvas.height - y * canvas.height);
        });
        ctx.closePath();
        ctx.fill();
    }
}