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Polyline tessellation with MapBox-gl-js
Raw
readme.md

Quick test of polyline tessellation capabilities of the new MapBox-gl-js, blogged here

Raw
index.html
<!doctype html>
<html>
<head>
<title>Polyline tessellation with MapBox-gl-js</title>
<meta charset="utf-8">
<style>
html, body {
height: 100%;
padding: 0;
margin: 0;
background: rgb(14, 21, 30);
height: 100%;
}
#map {
position: absolute;
height: 100%;
width: 100%;
background-color: #333;
}
</style>
<!-- vertex shader -->
<script id="vshader" type="x-shader/x-vertex">
uniform mat4 u_matrix;
attribute vec4 a_vertex;
attribute float a_pointSize;
attribute vec4 a_color;
varying vec4 v_color;
void main() {
// Set the size of the point
gl_PointSize = a_pointSize;
// multiply each vertex by a matrix.
gl_Position = u_matrix * a_vertex;
// pass the color to the fragment shader
v_color = a_color;
}
</script>
<!-- fragment shader -->
<script id="fshader" type="x-shader/x-fragment">
precision mediump float;
varying vec4 v_color;
void main() {
// -- squares
gl_FragColor = v_color;
gl_FragColor =vec4(0.8, 0.1,0.1, 0.9); // v_color;
}
</script>
</head>
<body>
<div id="map"></div>
<link rel="stylesheet" href="http://cdn.leafletjs.com/leaflet-0.7.2/leaflet.css" />
<script src="http://cdn.leafletjs.com/leaflet-0.7.2/leaflet.js"></script>
<script src="http://www.sumbera.com/gist/js/leaflet/canvas/L.CanvasOverlay.js"></script>
<script>
// -- module, require mockups so we can use orig files unmodified
module = {};
reqMap = {
'./elementgroups.js': 'ElementGroups',
'./buffer.js' : 'Buffer'
};
require = function (jsFile) {
return eval(reqMap[jsFile]);
};
</script>
<!-- all mapbox dependency for tesselation of the polyline -->
<script src="http://www.sumbera.com/gist/js/mapbox/pointGeometry.js"></script>
<script src="http://www.sumbera.com/gist/js/mapbox/buffer.js"></script>
<script src="http://www.sumbera.com/gist/js/mapbox/linevertexbuffer.js"></script>
<script src="http://www.sumbera.com/gist/js/mapbox/lineelementbuffer.js"></script>
<script src="http://www.sumbera.com/gist/js/mapbox/elementgroups.js"></script>
<script src="http://www.sumbera.com/gist/js/mapbox/linebucket.js"></script>
<script src="http://www.sumbera.com/gist/data/route.js" charset="utf-8"></script>
<script>
var leafletMap = L.map('map').setView([38.911, -77.0521], 16);
L.tileLayer("http://{s}.sm.mapstack.stamen.com/(toner-background,$fff[difference],$fff[@23],$fff[hsl-saturation@20],toner-lines[destination-in])/{z}/{x}/{y}.png")
.addTo(leafletMap);
var glLayer = L.canvasOverlay()
.drawing(drawingOnCanvas)
.addTo(leafletMap);
var canvas = glLayer.canvas();
glLayer.canvas.width = canvas.clientWidth;
glLayer.canvas.height = canvas.clientHeight;
var gl = canvas.getContext('experimental-webgl', { antialias: true });
var pixelsToWebGLMatrix = new Float32Array(16);
var mapMatrix = new Float32Array(16);
// -- WebGl setup
var vertexShader = gl.createShader(gl.VERTEX_SHADER);
gl.shaderSource(vertexShader, document.getElementById('vshader').text);
gl.compileShader(vertexShader);
var fragmentShader = gl.createShader(gl.FRAGMENT_SHADER);
gl.shaderSource(fragmentShader, document.getElementById('fshader').text);
gl.compileShader(fragmentShader);
// link shaders to create our program
var program = gl.createProgram();
gl.attachShader(program, vertexShader);
gl.attachShader(program, fragmentShader);
gl.linkProgram(program);
gl.useProgram(program);
gl.blendFunc(gl.SRC_ALPHA, gl.ONE_MINUS_SRC_ALPHA);
gl.enable(gl.BLEND);
// gl.disable(gl.DEPTH_TEST);
// ----------------------------
var u_matLoc = gl.getUniformLocation(program, "u_matrix");
gl.aPointSize = gl.getAttribLocation(program, "a_pointSize");
pixelsToWebGLMatrix.set([2 / canvas.width, 0, 0, 0, 0, -2 / canvas.height, 0, 0, 0, 0, 0, 0, -1, 1, 0, 1]);
gl.viewport(0, 0, canvas.width, canvas.height);
gl.uniformMatrix4fv(u_matLoc, false, pixelsToWebGLMatrix);
// -- data
var verts = [];
var rawVerts = [];
for (var i = 0 ; i < data.geometry.coordinates.length; i++) {
var d = data.geometry.coordinates[i];
pixel = LatLongToPixelXY(d[1], d[0]);
rawVerts.push(new Point(pixel.x, pixel.y));
}
var u_linewidth = { x: 0.0002 };
// override
LineVertexBuffer.prototype.add = function (point, extrude, tx, ty, linesofar) {
point.x = point.x + (u_linewidth.x * LineVertexBuffer.extrudeScale * extrude.x * 0.015873);
point.y = point.y + (u_linewidth.x * LineVertexBuffer.extrudeScale * extrude.y * 0.015873);
verts.push( point.x, point.y);
return this.index;
};
// -- we don't use these buffers, override them later, just set them for addLine func
var bucket = new LineBucket({}, {
lineVertex: (LineVertexBuffer.prototype.defaultLength = 16, new LineVertexBuffer()),
lineElement: (LineElementBuffer.prototype.defaultLength = 16, new LineElementBuffer())
});
bucket.addLine(rawVerts,"miter","butt",2,1);
// tirangles or point count
var colorLoc = gl.getAttribLocation(program, "a_color");
var vertLoc = gl.getAttribLocation(program, "a_vertex");
var numPoints = verts.length / 2;
var vertBuffer = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, vertBuffer);
gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(verts), gl.STATIC_DRAW);
gl.vertexAttribPointer(vertLoc, 2, gl.FLOAT, false, 0, 0);
gl.enableVertexAttribArray(vertLoc);
glLayer.redraw();
function drawingOnCanvas(canvasOverlay, params) {
if (gl == null) return;
gl.clear(gl.COLOR_BUFFER_BIT);
pixelsToWebGLMatrix.set([2 / canvas.width, 0, 0, 0, 0, -2 / canvas.height, 0, 0, 0, 0, 0, 0, -1, 1, 0, 1]);
gl.viewport(0, 0, canvas.width, canvas.height);
var pointSize = Math.max(leafletMap.getZoom() - 4.0, 1.0);
gl.vertexAttrib1f(gl.aPointSize, pointSize);
// -- set base matrix to translate canvas pixel coordinates -> webgl coordinates
mapMatrix.set(pixelsToWebGLMatrix);
var bounds = leafletMap.getBounds();
var topLeft = new L.LatLng(bounds.getNorth(), bounds.getWest());
var offset = LatLongToPixelXY(topLeft.lat, topLeft.lng);
// -- Scale to current zoom
var scale = Math.pow(2, leafletMap.getZoom());
scaleMatrix(mapMatrix, scale, scale);
translateMatrix(mapMatrix, -offset.x, -offset.y);
// -- attach matrix value to 'mapMatrix' uniform in shader
gl.uniformMatrix4fv(u_matLoc, false, mapMatrix);
gl.drawArrays(gl.TRIANGLE_STRIP, 0, numPoints);
}
// Returns a random integer from 0 to range - 1.
function randomInt(range) {
return Math.floor(Math.random() * range);
}
/*
function latlonToPixels(lat, lon) {
initialResolution = 2 * Math.PI * 6378137 / 256, // at zoomlevel 0
originShift = 2 * Math.PI * 6378137 / 2;
// -- to meters
var mx = lon * originShift / 180;
var my = Math.log(Math.tan((90 + lat) * Math.PI / 360)) / (Math.PI / 180);
my = my * originShift / 180;
// -- to pixels at zoom level 0
var res = initialResolution;
x = (mx + originShift) / res,
y = (my + originShift) / res;
return { x: x, y: 256- y };
}
*/
// -- converts latlon to pixels at zoom level 0 (for 256x256 tile size) , inverts y coord )
// -- source : http://build-failed.blogspot.cz/2013/02/displaying-webgl-data-on-google-maps.html
function LatLongToPixelXY(latitude, longitude) {
var pi_180 = Math.PI / 180.0;
var pi_4 = Math.PI * 4;
var sinLatitude = Math.sin(latitude * pi_180);
var pixelY = (0.5 - Math.log((1 + sinLatitude) / (1 - sinLatitude)) / (pi_4)) * 256;
var pixelX = ((longitude + 180) / 360) * 256;
var pixel = { x: pixelX, y: pixelY };
return pixel;
}
function translateMatrix(matrix, tx, ty) {
// translation is in last column of matrix
matrix[12] += matrix[0] * tx + matrix[4] * ty;
matrix[13] += matrix[1] * tx + matrix[5] * ty;
matrix[14] += matrix[2] * tx + matrix[6] * ty;
matrix[15] += matrix[3] * tx + matrix[7] * ty;
}
function scaleMatrix(matrix, scaleX, scaleY) {
// scaling x and y, which is just scaling first two columns of matrix
matrix[0] *= scaleX;
matrix[1] *= scaleX;
matrix[2] *= scaleX;
matrix[3] *= scaleX;
matrix[4] *= scaleY;
matrix[5] *= scaleY;
matrix[6] *= scaleY;
matrix[7] *= scaleY;
}
</script>
</body>
</html>
@donSchoe
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donSchoe commented Sep 2, 2015

hi stanislav,
thanks for your helpful blog posts and demo scripts.

could you explain how you created the modelview matrix? (lines 138 or 196):

pixelsToWebGLMatrix.set([2 / canvas.width, 0, 0, 0, 0, -2 / canvas.height, 0, 0, 0, 0, 0, 0, -1, 1, 0, 1]);

simplified 4x4 view:

  2 / w,      0, 0, 0,
      0, -2 / h, 0, 0,
      0,      0, 0, 0,
     -1,      1, 0, 1

2/w scales along the canvas width. -2/h scales along the canwas height. but why 2 and -2? and what are -1 and 1 in the last line actually doing?

i was able to create my own webgl overlay for leaflet and thanks to your blogs i was totally able to create a working version of my own code which i fully understand except this matrix. i somehow fail to reverse engineer this one. could you tell me how you calculated this transformation?

thank you so much,
alex

@donSchoe
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donSchoe commented Sep 2, 2015

ok! i try to answer my own question. i just noticed webgl seems to read matrices in another direction, so the -1 and 1 in the bottom line are a simple translation to ensure [0,0] is in the top left corner.

it's the same as:

translateMatrix(pixelsToWebGLMatrix, -1, 1);
scaleMatrix(pixelsToWebGLMatrix, 2 / canvas.width, -2 / canvas.height);

@Sumbera
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Author

Sumbera commented Sep 29, 2015

:)

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