javascript - 在HTML5 Canvas 上渲染网格

我正在尝试在HTML5画布上呈现网格，但是我希望用户在画布上绘制的任何内容都可以放置在网格后面。我试图通过有2个画布来重新创建该画布：一个带有网格，另一个带有用户绘制的画布。带有网格的那个放置在用户绘制的画布上，并且该画布应该是透明的，但不显示用户绘制的任何内容。如果我将用户绘制的画布放在顶部，则可以看到用户绘制的内容，但是该绘制在网格的前面，并且我希望该绘制在网格的后面。

您可以在这里查看我尝试过的内容：http://codepen.io/vishiswoz/pen/grqMyG

function buildGrids(gridPixelSize, color, gap, div) {
  var canvas = $('#' + div + '').get(0);
  var ctx = canvas.getContext("2d");
  //ctx.fillStyle="rgba(255, 255, 255, 0)";
  //ctx.fillRect(0, 0, 500, 300);
  ctx.lineWidth = 0.5;
  ctx.strokeStyle = color;

  // horizontal grid lines
  for (var i = 0; i <= canvas.height; i = i + gridPixelSize) {
    ctx.beginPath();
    ctx.moveTo(0, i);
    ctx.lineTo(canvas.width, i);
    if (i % parseInt(gap) == 0) {
      ctx.lineWidth = 0.5;
    } else {
      ctx.lineWidth = 0.5;
    }
    ctx.closePath();
    ctx.stroke();
  }

  // vertical grid lines
  for (var j = 0; j <= canvas.width; j = j + gridPixelSize) {
    ctx.beginPath();
    ctx.moveTo(j, 0);
    ctx.lineTo(j, canvas.height);
    if (j % parseInt(gap) == 0) {
      ctx.lineWidth = 0.5;
    } else {
      ctx.lineWidth = 0.5;
    }
    ctx.closePath();
    ctx.stroke();
  }

  for(var ii = 0; ii <=canvas.width; ii+=2) {
    for(var jj=0; jj <=canvas.height; jj+=2) {
      ctx.clearRect(ii,jj,1,1);
    }
  }
}
buildGrids(5, "grey", 2, "test");

var el = document.getElementById('c');
var ctx = el.getContext('2d');
ctx.strokeStyle = "black";
var isDrawing;

el.onmousedown = function(e) {
  isDrawing = true;
  ctx.lineWidth = 10;
  ctx.lineJoin = ctx.lineCap = 'round';
  ctx.moveTo(e.clientX, e.clientY);
};
el.onmousemove = function(e) {
  if (isDrawing) {
    ctx.lineTo(e.clientX, e.clientY);
    ctx.stroke();
  }
};
el.onmouseup = function() {
  isDrawing = false;
};

canvas {
    position: absolute;
    top: 0;
    left: 0;
}

#c {

}

<canvas id="c" width="500" height="300"></canvas>
<canvas id="test" width="500" height="300"></canvas>

提前致谢。

最佳答案

在由顶部网格画布覆盖的底部画布上绘图



只需在顶部的网格画布上侦听鼠标事件，然后在绘图画布上绘制即可。

var gridCanvas=document.getElementById('test');

gridCanvas.onmousedown = function(e) {
  isDrawing = true;
  ctx.lineWidth = 10;
  ctx.lineJoin = ctx.lineCap = 'round';
  ctx.moveTo(e.clientX, e.clientY);
};

其他几个问题


样式问题：在mousemove上，绘制完整的线段，而不仅仅是继续“打开”路径。您可以通过记住lastX，lastY并在该最后一点和当前鼠标位置之间绘制一段来实现此目的。
如果鼠标移出画布，则应结束绘图操作。否则，您将得到一个“粘性”图形，使用户无法轻易停止该图形。


这是您的代码经过稍微重构以包含上述想法：

var gridCanvas = document.getElementById('grid');
var el = document.getElementById('drawing');

        function reOffset(){
            var BB=el.getBoundingClientRect();
            offsetX=BB.left;
            offsetY=BB.top;
        }
        var offsetX,offsetY;
        reOffset();
        window.onscroll=function(e){ reOffset(); }
        window.onresize=function(e){ reOffset(); }

buildGrids(5, "grey", 2, "test");

var ctx = el.getContext('2d');
ctx.strokeStyle = "orange";
var isDrawing,lastX,lastY;

gridCanvas.onmousedown = function(e) {
  lastX=e.clientX;
  lastY=e.clientY;
  ctx.lineWidth = 10;
  ctx.lineJoin = ctx.lineCap = 'round';
  isDrawing = true;
};
gridCanvas.onmousemove = function(e) {
  if (isDrawing) {
    ctx.beginPath();
    ctx.moveTo(lastX,lastY);
    ctx.lineTo(e.clientX, e.clientY);
    ctx.stroke();
    lastX=e.clientX;
    lastY=e.clientY;
  }
};
gridCanvas.onmouseup = function() {
  isDrawing = false;
};gridCanvas.onmouseout = function() {
  isDrawing = false;
};


function buildGrids(gridPixelSize, color, gap, div) {
  var ctx = gridCanvas.getContext("2d");
  //ctx.fillStyle="rgba(255, 255, 255, 0)";
  //ctx.fillRect(0, 0, 500, 300);
  ctx.lineWidth = 0.5;
  ctx.strokeStyle = color;

  // horizontal grid lines
  for (var i = 0; i <= gridCanvas.height; i = i + gridPixelSize) {
    ctx.beginPath();
    ctx.moveTo(0, i);
    ctx.lineTo(gridCanvas.width, i);
    if (i % parseInt(gap) == 0) {
      ctx.lineWidth = 0.5;
    } else {
      ctx.lineWidth = 0.5;
    }
    ctx.closePath();
    ctx.stroke();
  }

  // vertical grid lines
  for (var j = 0; j <= gridCanvas.width; j = j + gridPixelSize) {
    ctx.beginPath();
    ctx.moveTo(j, 0);
    ctx.lineTo(j, gridCanvas.height);
    if (j % parseInt(gap) == 0) {
      ctx.lineWidth = 0.5;
    } else {
      ctx.lineWidth = 0.5;
    }
    ctx.closePath();
    ctx.stroke();
  }

  for(var ii = 0; ii <=gridCanvas.width; ii+=2) {
    for(var jj=0; jj <=gridCanvas.height; jj+=2) {
      ctx.clearRect(ii,jj,1,1);
    }
  }
}

body{ background-color: ivory; }
canvas{border:1px solid red;}
canvas{ position: absolute; top: 0; left: 0;}

<canvas id="drawing" width="500" height="300"></canvas>
<canvas id="grid" width="500" height="300"></canvas>

关于javascript - 在HTML5 Canvas 上渲染网格，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/37092104/