在画布上绘制图像的不透明部分周围的边框

Question

我正在使用drawImage在画布上绘制图像。这是一个被透明像素包围的PNG，如下所示：

如何在画布上为该图像的可见部分添加纯色边框？澄清一下：我不想要一个围绕图像边界框的矩形。边界应该绕着草地。

我确实考虑过使用阴影，但我真的不想要一个发光的边框，我想要一个坚固的边框。

Answer 1

有点晚了，但只是绘制图像 offset ，这比分析边缘要快得多：

var ctx = canvas.getContext('2d'),
    img = new Image;

img.onload = draw;
img.src = "http://i.stack.imgur.com/UFBxY.png";

function draw() {

  var dArr = [-1,-1, 0,-1, 1,-1, -1,0, 1,0, -1,1, 0,1, 1,1], // offset array
      s = 2,  // thickness scale
      i = 0,  // iterator
      x = 5,  // final position
      y = 5;
  
  // draw images at offsets from the array scaled by s
  for(; i < dArr.length; i += 2)
    ctx.drawImage(img, x + dArr[i]*s, y + dArr[i+1]*s);
  
  // fill with color
  ctx.globalCompositeOperation = "source-in";
  ctx.fillStyle = "red";
  ctx.fillRect(0,0,canvas.width, canvas.height);
  
  // draw original image in normal mode
  ctx.globalCompositeOperation = "source-over";
  ctx.drawImage(img, x, y);
}

<canvas id=canvas width=500 height=500></canvas>

Answer 2

==＆gt; ==＆gt;

首先，归因：

正如@Philipp所说，你需要分析像素数据以获得轮廓边界。

您可以使用“行进方格”算法来确定哪些透明像素与非透明草像素相邻。您可以在此处详细了解Marching Squares算法：http://en.wikipedia.org/wiki/Marching_squares

Michael Bostock在他的 d3 数据可视化应用程序中有一个非常漂亮的Marching Squares插件版本（恕我直言，d3是最好的开源数据可视化程序）。这是插件的链接：https://github.com/d3/d3-plugins/tree/master/geom/contour

您可以像这样概述草图像的边框：

• 在画布上绘制图像

• 使用.getImageData

抓取图片的像素数据

• 配置插件以查找与不透明像素接壤的透明像素

  // This is used by the marching ants algorithm
  // to determine the outline of the non-transparent
  // pixels on the image using pixel data
  var defineNonTransparent=function(x,y){
      var a=data[(y*cw+x)*4+3];
      return(a>20);
  }
  

• 调用插件，该插件返回一组勾勒出图像边框的点。

  // call the marching ants algorithm
  // to get the outline path of the image
  // (outline=outside path of transparent pixels
  points=geom.contour(defineNonTransparent);
  

• 使用这组点在图像周围绘制路径。

这是带注释的代码和演示：

// Marching Squares Edge Detection
// this is a "marching ants" algorithm used to calc the outline path
(function() {
  // d3-plugin for calculating outline paths
  // License: https://github.com/d3/d3-plugins/blob/master/LICENSE
  //
  // Copyright (c) 2012-2014, Michael Bostock
  // All rights reserved.
  //
  //  Redistribution and use in source and binary forms, with or without
  //  modification, are permitted provided that the following conditions are met:
  //* Redistributions of source code must retain the above copyright notice, this
  //  list of conditions and the following disclaimer.
  //* Redistributions in binary form must reproduce the above copyright notice,
  //  this list of conditions and the following disclaimer in the documentation
  //  and/or other materials provided with the distribution.
  //* The name Michael Bostock may not be used to endorse or promote products
  //  derived from this software without specific prior written permission.
  // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL MICHAEL BOSTOCK BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 
  geom = {}; 
  geom.contour = function(grid, start) { 
    var s = start || d3_geom_contourStart(grid), // starting point 
        c = [],    // contour polygon 
        x = s[0],  // current x position 
        y = s[1],  // current y position 
        dx = 0,    // next x direction 
        dy = 0,    // next y direction 
        pdx = NaN, // previous x direction 
        pdy = NaN, // previous y direction 
        i = 0; 

    do { 
      // determine marching squares index 
      i = 0; 
      if (grid(x-1, y-1)) i += 1; 
      if (grid(x,   y-1)) i += 2; 
      if (grid(x-1, y  )) i += 4; 
      if (grid(x,   y  )) i += 8; 

      // determine next direction 
      if (i === 6) { 
        dx = pdy === -1 ? -1 : 1; 
        dy = 0; 
      } else if (i === 9) { 
        dx = 0; 
        dy = pdx === 1 ? -1 : 1; 
      } else { 
        dx = d3_geom_contourDx[i]; 
        dy = d3_geom_contourDy[i]; 
      } 

      // update contour polygon 
      if (dx != pdx && dy != pdy) { 
        c.push([x, y]); 
        pdx = dx; 
        pdy = dy; 
      } 

      x += dx; 
      y += dy; 
    } while (s[0] != x || s[1] != y); 

    return c; 
  }; 

  // lookup tables for marching directions 
  var d3_geom_contourDx = [1, 0, 1, 1,-1, 0,-1, 1,0, 0,0,0,-1, 0,-1,NaN], 
      d3_geom_contourDy = [0,-1, 0, 0, 0,-1, 0, 0,1,-1,1,1, 0,-1, 0,NaN]; 

  function d3_geom_contourStart(grid) { 
    var x = 0, 
        y = 0; 

    // search for a starting point; begin at origin 
    // and proceed along outward-expanding diagonals 
    while (true) { 
      if (grid(x,y)) { 
        return [x,y]; 
      } 
      if (x === 0) { 
        x = y + 1; 
        y = 0; 
      } else { 
        x = x - 1; 
        y = y + 1; 
      } 
    } 
  } 

})();


//////////////////////////////////////////

// canvas related variables
var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
var cw=canvas.width;
var ch=canvas.height;

// checkbox to show/hide the original image
var $showImage=$("#showImage");
$showImage.prop('checked', true);

// checkbox to show/hide the path outline
var $showOutline=$("#showOutline");
$showOutline.prop('checked', true);

// an array of points that defines the outline path
var points;

// pixel data of this image for the defineNonTransparent 
// function to use
var imgData,data;

// This is used by the marching ants algorithm
// to determine the outline of the non-transparent
// pixels on the image
var defineNonTransparent=function(x,y){
  var a=data[(y*cw+x)*4+3];
  return(a>20);
}

// load the image
var img=new Image();
img.crossOrigin="anonymous";
img.onload=function(){

  // draw the image
  // (this time to grab the image's pixel data
  ctx.drawImage(img,canvas.width/2-img.width/2,canvas.height/2-img.height/2);

  // grab the image's pixel data
  imgData=ctx.getImageData(0,0,canvas.width,canvas.height);
  data=imgData.data;

  // call the marching ants algorithm
  // to get the outline path of the image
  // (outline=outside path of transparent pixels
  points=geom.contour(defineNonTransparent);

  ctx.strokeStyle="red";
  ctx.lineWidth=2;

  $showImage.change(function(){ redraw(); });

  $showOutline.change(function(){ redraw(); });

  redraw();

}
img.src="http://i.imgur.com/QcxIJxa.png";

// redraw the canvas
// user determines if original-image or outline path or both are visible
function redraw(){

  // clear the canvas
  ctx.clearRect(0,0,canvas.width,canvas.height);

  // draw the image
  if($showImage.is(':checked')){
    ctx.drawImage(img,canvas.width/2-img.width/2,canvas.height/2-img.height/2);
  }

  // draw the path (consisting of connected points)
  if($showOutline.is(':checked')){
    // draw outline path
    ctx.beginPath();
    ctx.moveTo(points[0][0],points[0][4]);
    for(var i=1;i<points.length;i++){
      var point=points[i];
      ctx.lineTo(point[0],point[1]);
    }
    ctx.closePath();
    ctx.stroke();
  }

}

body{ background-color: ivory; }
canvas{border:1px solid red;}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<input type="checkbox" id="showImage" />Show Image<br>
<input type="checkbox" id="showOutline" />Show Outline Path<br>
<canvas id="canvas" width=300 height=450></canvas>

Answer 3

我一直在寻找一种解决方法，似乎只有费力的解决方案。

我想出了一个使用阴影和循环将它们全部显示在图像周围的解决方法：

// Shadow color and blur
// To get a blurry effect use rgba() with a low opacity as it will be overlaid
context.shadowColor = "red";
context.shadowBlur = 0;

// X offset loop
for(var x = -2; x <= 2; x++){
    // Y offset loop
    for(var y = -2; y <= 2; y++){
        // Set shadow offset
        context.shadowOffsetX = x;
        context.shadowOffsetY = y;

        // Draw image with shadow
        context.drawImage(img, left, top, width, height);
    }
}