在给定x坐标的情况下获取SVG路径上点的y坐标 [英] Get y coordinate of point along SVG path with given an x coordinate

问题描述

我正在使用raphael.js绘制一个简单的SVG折线图，如下所示:

I am using raphael.js to draw a simple SVG line graph like this:

当用户悬停图形时，id喜欢在光标的 X 位置和该行的 Y 位置显示指向该行的弹出框就是这样的 X 位置:

When the user hovers the graph, id like to display a popover pointing to the line at the X position of the cursor, and at the Y position where the line is for that X position like so:

我需要走这条路，找到给定的 X 坐标的 Y 坐标.

I need to take the path and find the Y coordinate for a given X coordinate.

推荐答案

基于@Duopixel的D3解决方案，我使用DOM API在纯JavaScript中编写了以下函数供自己使用:

Based on @Duopixel's D3 solution, I wrote the following function for my own use, in pure javascript using DOM API:

function findY(path, x) {
  var pathLength = path.getTotalLength()
  var start = 0
  var end = pathLength
  var target = (start + end) / 2

  // Ensure that x is within the range of the path
  x = Math.max(x, path.getPointAtLength(0).x)
  x = Math.min(x, path.getPointAtLength(pathLength).x)

  // Walk along the path using binary search 
  // to locate the point with the supplied x value
  while (target >= start && target <= pathLength) {
    var pos = path.getPointAtLength(target)

    // use a threshold instead of strict equality 
    // to handle javascript floating point precision
    if (Math.abs(pos.x - x) < 0.001) {
      return pos.y
    } else if (pos.x > x) {
      end = target
    } else {
      start = target
    }
    target = (start + end) / 2
  }
}

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