Tag Archives: SVG

SVG fast scaled overlay on Leaflet 1.0 and 0.7

SVGScaled SVG can be drawn on map in much more faster way than traditional approaches, at least for points. Traditional approach re-position each element to fit into the view of the map, however SVG is “scalable” so we can use it and it performs much more faster for zoom-in/out.

Few considerations:

  1. SVG itself define viewport by its coordinate space, all outside of this viewport is usually  clipped, so it is important to keep SVG viewport in-line with the viewport of the map. There are approaches that resizes SVG as you zoom-in (here), and while it works, it has a problems in deep-zooms when you need to move on map (actually you move  giant SVG based on the zoom )
  2. translating LatLon to absolute pixel values (like here used for WebGL) is possible solution, however IE and FF has problems with large numbers for transoform (>1 M), So we need to get SVG elements in view coordinates and translate them.
  3. Having some track of bounding box of all elements like again used here should be avoided (SVG or its group element knows about extension of the elements it holds)
  4. So while we keep SVG in the viewport, we need to compensate any shift and zoom by translating <g> (group) of all elements.
  5. So in leaflet when map  moves, SVG is translated back to its original position while <g> is translated forward to reflect the map movement
  6. We need to keep track of LatLon position of either map center or one of the corner – we use topLeft corner.
  7. Leaflet doesn’t do  precise enlargement and rounds view points because of some CSS troubles on some devices (noted here). We need to patch two translating functions in Leaflet to get this right (so SVG enlargement will be aligned with map)… but I need to look on this again, best would be to not patch Leaflet of course.

most important things happen in moveEnd event:


 var bounds = this._map.getBounds(); // -- latLng bounds of map viewport
 var topLeftLatLng = new L.LatLng(bounds.getNorth(), bounds.getWest()); // -- topLeft corner of the viewport
 var topLeftLayerPoint = this._map.latLngToLayerPoint(topLeftLatLng); // -- translating to view coord
 var lastLeftLayerPoint = this._map.latLngToLayerPoint(this._lastTopLeftlatLng); 

 var zoom = this._map.getZoom();
 var scaleDelta = this._map.getZoomScale(zoom, this._lastZoom); // -- amount of scale from previous state e.g. 0.5 or 2
 var scaleDiff = this.getScaleDiff(zoom); // -- diff of how far we are from initial scale 

 this._lastZoom = zoom; // -- we need to keep track of last zoom
 var delta = lastLeftLayerPoint.subtract(topLeftLayerPoint); // -- get incremental delta in view coord

 this._lastTopLeftlatLng = topLeftLatLng; // -- we need to keep track of last top left corner, with this we do not need to track center of enlargement
 L.DomUtil.setPosition(this._svg, topLeftLayerPoint); // -- reset svg to keep it inside map viewport

 this._shift._multiplyBy(scaleDelta)._add(delta); // -- compute new relative shift from initial position
 // -- set group element to compensate for svg translation, and scale</pre>
 this._g.setAttribute("transform", "translate(" + this._shift.x + "," + this._shift.y + ") scale(" + scaleDiff + ")");

Test page / Gist : http://bl.ocks.org/Sumbera/7e8e57368175a1433791

To better illustrate movement of SVG inside the map, here is a small diagram of basic SVG states:svgpositioning

Many points with d3 and leaflet

Update August 2015: check “scaled”- based fast SVG rendering on top of Leaflet here


Testing SVG limits of plotting points on map using D3, Leaflet following this base sample:http://bost.ocks.org/mike/leaflet. However instead of scaling SVG in deep zooms, I am using Enter/Update/Exit pattern from D3 to dynamically update points on map. This has been prototyped also here http://bl.ocks.org/sumbera/9972460 with brushing of 100T points.

For zooming out (causing all points to be displayed), I am filtering out points that can’t be effectively visible, thus reducing number of points in SVG. (check console for log output).

This sample is using real data of 24T coordinates where points are clustered around cities, rather than artifically randomized. Real number of rendered points / removed points can be seen in console

Test page : http://bl.ocks.org/sumbera/10463358

SVG map sample and Canavas in OpenLayers

Update August 2015: check “scaled”- based fast SVG rendering on top of Leaflet here

Updated May 2014: check leaflet Canvas test here, SVG with d3 here   and WebGL here

Canvas map sample.

Thesis on Canvas and Open Layers (August 2010) “Evaluation of HTML5 for its Use in the Web Mapping Client OpenLayers”

OpenLayers with Canavas  (partialy deprecated) http://trac.osgeo.org/openlayers/wiki/Future/OpenLayersWithCanvas

SVG vs Canvas comparison by Jeffrey Warren from : http://unterbahn.com/2009/08/svg-vs/

“Well, SVG doesn’t scale well to large numbers of objects, but Canvas doesn’t
scale well to large screens”

“SVG performance degrades quickly (exponentially on Safari?) in the number of
objects, but Canvas performance remains at a near-constant low. This makes
sense, since Canvas is just a bitmap buffer, while SVG has to maintain
additional references to each object that it renders. Also, though not pictured,
note that performance in clearing an SVG element also decreases in the number of
drawn objects.”

[UPDATE] PDC2010 video by Patrick Dengler : http://videoaz.microsoftpdc.com/vod/downloads/vod/CD53_PatrickDengler/CD53_PatrickDengler_PDC_WMV_High_1280x720_2500k.wmv