javascript - 在javascript中测量在图像上绘制的线条的长度

问题描述

我想测量在以 html 代码上传的图像上绘制的线条的长度。我找到了一个使用画布元素的脚本，使我能够绘制一条线、矩形或自由线。我想实现的下一步是能够在上传的图像上画一条线、矩形或自由线，但如果我选择画一条线，则可以通过设置比例来测量线的长度手动。
我知道在 js 中有字符串 string.length 但这对我的目的不利。下面是我找到的两个代码，我正在尝试修改和组合但没有任何成功，即使我不是很实用。如果有人能帮我把这两个代码放在一起，或者给我一些建议，我将非常感激。

这是在矩形中绘制的 html 代码

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="utf-8">
    <title>Canvas Paint - Example 5</title>
    <style type="text/css">
      #container { position: relative; }
      #imageView { border: 1px solid #000; }
      #imageTemp { position: absolute; top: 1px; left: 1px; }
    </style>
  </head>
  <body>
    <p><label>Drawing tool: <select id="dtool">
        <option value="line">Line</option>
        <option value="rect">Rectangle</option>
        <option value="pencil">Pencil</option>
    </select></label></p>

    <div id="container">
      <canvas id="imageView" width="400" height="300">
        <p>Unfortunately, your browser is currently unsupported by our web
        application.  We are sorry for the inconvenience.</p>
      </canvas>
    </div>

    <script type="text/javascript" src="example-5.js"></script>
  </body>
</html>

这是相关的js代码：

// Keep everything in anonymous function, called on window load.
if(window.addEventListener) {
window.addEventListener('load', function () {
  var canvas, context, canvaso, contexto;

  // The active tool instance.
  var tool;
  var tool_default = 'line';

  function init () {
    // Find the canvas element.
    canvaso = document.getElementById('imageView');
    if (!canvaso) {
      alert('Error: I cannot find the canvas element!');
      return;
    }

    if (!canvaso.getContext) {
      alert('Error: no canvas.getContext!');
      return;
    }

    // Get the 2D canvas context.
    contexto = canvaso.getContext('2d');
    if (!contexto) {
      alert('Error: failed to getContext!');
      return;
    }

    // Add the temporary canvas.
    var container = canvaso.parentNode;
    canvas = document.createElement('canvas');
    if (!canvas) {
      alert('Error: I cannot create a new canvas element!');
      return;
    }

    canvas.id     = 'imageTemp';
    canvas.width  = canvaso.width;
    canvas.height = canvaso.height;
    container.appendChild(canvas);

    context = canvas.getContext('2d');

    // Get the tool select input.
    var tool_select = document.getElementById('dtool');
    if (!tool_select) {
      alert('Error: failed to get the dtool element!');
      return;
    }
    tool_select.addEventListener('change', ev_tool_change, false);

    // Activate the default tool.
    if (tools[tool_default]) {
      tool = new tools[tool_default]();
      tool_select.value = tool_default;
    }

    // Attach the mousedown, mousemove and mouseup event listeners.
    canvas.addEventListener('mousedown', ev_canvas, false);
    canvas.addEventListener('mousemove', ev_canvas, false);
    canvas.addEventListener('mouseup',   ev_canvas, false);
  }

  // The general-purpose event handler. This function just determines the mouse 
  // position relative to the canvas element.
  function ev_canvas (ev) {
    if (ev.layerX || ev.layerX == 0) { // Firefox
      ev._x = ev.layerX;
      ev._y = ev.layerY;
    } else if (ev.offsetX || ev.offsetX == 0) { // Opera
      ev._x = ev.offsetX;
      ev._y = ev.offsetY;
    }

    // Call the event handler of the tool.
    var func = tool[ev.type];
    if (func) {
      func(ev);
    }
  }

  // The event handler for any changes made to the tool selector.
  function ev_tool_change (ev) {
    if (tools[this.value]) {
      tool = new tools[this.value]();
    }
  }

  // This function draws the #imageTemp canvas on top of #imageView, after which 
  // #imageTemp is cleared. This function is called each time when the user 
  // completes a drawing operation.
  function img_update () {
        contexto.drawImage(canvas, 0, 0);
        context.clearRect(0, 0, canvas.width, canvas.height);
  }

  // This object holds the implementation of each drawing tool.
  var tools = {};

  // The drawing pencil.
  tools.pencil = function () {
    var tool = this;
    this.started = false;

    // This is called when you start holding down the mouse button.
    // This starts the pencil drawing.
    this.mousedown = function (ev) {
        context.beginPath();
        context.moveTo(ev._x, ev._y);
        tool.started = true;
    };

    // This function is called every time you move the mouse. Obviously, it only 
    // draws if the tool.started state is set to true (when you are holding down 
    // the mouse button).
    this.mousemove = function (ev) {
      if (tool.started) {
        context.lineTo(ev._x, ev._y);
        context.stroke();
      }
    };

    // This is called when you release the mouse button.
    this.mouseup = function (ev) {
      if (tool.started) {
        tool.mousemove(ev);
        tool.started = false;
        img_update();
      }
    };
  };

  // The rectangle tool.
  tools.rect = function () {
    var tool = this;
    this.started = false;

    this.mousedown = function (ev) {
      tool.started = true;
      tool.x0 = ev._x;
      tool.y0 = ev._y;
    };

    this.mousemove = function (ev) {
      if (!tool.started) {
        return;
      }

      var x = Math.min(ev._x,  tool.x0),
          y = Math.min(ev._y,  tool.y0),
          w = Math.abs(ev._x - tool.x0),
          h = Math.abs(ev._y - tool.y0);

      context.clearRect(0, 0, canvas.width, canvas.height);

      if (!w || !h) {
        return;
      }

      context.strokeRect(x, y, w, h);
    };

    this.mouseup = function (ev) {
      if (tool.started) {
        tool.mousemove(ev);
        tool.started = false;
        img_update();
      }
    };
  };

  // The line tool.
  tools.line = function () {
    var tool = this;
    this.started = false;

    this.mousedown = function (ev) {
      tool.started = true;
      tool.x0 = ev._x;
      tool.y0 = ev._y;
    };

    this.mousemove = function (ev) {
      if (!tool.started) {
        return;
      }

      context.clearRect(0, 0, canvas.width, canvas.height);

      context.beginPath();
      context.moveTo(tool.x0, tool.y0);
      context.lineTo(ev._x,   ev._y);
      context.stroke();
      context.closePath();
    };

    this.mouseup = function (ev) {
      if (tool.started) {
        tool.mousemove(ev);
        tool.started = false;
        img_update();
      }
    };
  };

  init();

}, false); }

// vim:set spell spl=en fo=wan1croql tw=80 ts=2 sw=2 sts=2 sta et ai cin fenc=utf-8 ff=unix:

虽然绘制一条线并测量长度的代码位于此链接： https ://openlayers.org/en/latest/examples/measure.html?q=measure+line

标签： javascripthtmlcanvas

javascript - 在javascript中测量在图像上绘制的线条的长度

问题描述

解决方案

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