WebGL polyline tessellation with MapBox-GL-JS

update 09/20015 : test of tesspathy.js library here . Other sources to look:

  1.  http://mattdesl.svbtle.com/drawing-lines-is-hard
  2.  https://github.com/mattdesl/extrude-polyline
  3. https://forum.libcinder.org/topic/smooth-thick-lines-using-geometry-shader

*** original post ***

This post attempted to use pixi.js tessellation of the polyline, this time let’s look on how mapbox-gl-js can do this. In short much more better than pixi.js.

it took slightly more time to get the right routines from mapbox-gl-js and find-out where the tessellation is calculated and drawn. It is actually on two places – in LinBucket.js  and in line shader. FireFox shader editor helped a lot to simplify and extract needed calculations and bring it into the JavaScript (for simplification, note however that shader based approach is the right one as you can influence dynamically thickness of lines, while having precaluclated mesh means each time you need to change thickness of line you have to recalculate whol e mesh and update buffers )

 

// — 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]); };

 

   <!-- 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>
 // -- 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())
 });

var u_linewidth = { x: 0.00015 };
// override .add to get calculated points
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;
};

// — pass vertexes into the addLine func that will calculate points
bucket.addLine(rawVerts,“miter”,“butt”,2,1);

prototype  code posted here