Cesium JS获取弧点数的最佳方法

Question

What is the neatest (code design) and most per-formant way of getting an array of points for an arc (polyline), for the purpose of animating using Cesium's timer/clock.

Variable inputs include (start/end location), height (highest point) from earth's surface and number of points for drawing.

I'm currently using a polyline collection, so the answer should describe how to generate the points for existing polylines or convert to a different approach.

I would also need the arc (color) to fadeIn or fadeOut to opacity 0.

Multiple arcs may be added or removed from the collection per second. Each arc will have different start and end points

The start and end location should have height 0 (touching the earth).

(For Cesium version b26)

Answer 1

Just to be sure I understand your question, you have a bunch of polylines on a map and you want to get a bunch of data points along the line for use in animating the something along the path. I'll also assume you want geodesic lines/arcs for the polylines rather than the straight lines that are normally drawn on Mercator maps as geodesic lines actually follow the spatially accurate path of the polyline ie the same path a plane would take. If this is the case then take a look at this blog post: http://alastaira.wordpress.com/2011/06/27/geodesics-on-bing-maps-v7/ This post describes how to calculate data points along the geodesic path of a polyline.

Answer 2

CesiumJS includes several spline functions that can be used. One of the easier ones to use that an accomplish what you want with just three points is the Catmull-Rom Spline:

http://cesiumjs.org/Cesium/Build/Documentation/CatmullRomSpline.html

You will need to produce a middle point. To do this you can take a mean of the lat/lon coordinates and add a large height. Because of the spline used and the low number of points, it does end up looking a little egg shaped. The benefit to this is that you can ask the spline object for an arbitrary number of points, so the arc can be as smooth as you want. Just don't forget to add the first and last points to the array returned by the spline as those are omitted.

There are other types of splines, but I found the Catmull-Rom spline the easiest to use. You can search the CesiumJS documentation for some of the other included splines.

Answer 3

I've been looking into the same thing (minus the time aspect) and I found Cesium.EllipsoidGeodesic(start, end, ellipsoid), which allows you to get points at fractions of the path. It seems to me that you can choose the fraction based on the distance and calculate regular points using the result.

https://cesiumjs.org/Cesium/Build/Documentation/EllipsoidGeodesic.html

I haven't tried it yet, but it's on my list of things to do.