Question

我正在尝试使用mousemove事件在canvas中的图像上绘制一个矩形。但由于clearRect，我在图像上得到矩形，颜色填充在矩形中。谁能让我知道如何在图像上绘制一个只有边框的矩形。下面是我实现它的代码。

var canvas = document.getElementById('canvas'),
    ctx = canvas.getContext('2d'),
    rect = {},
    drag = false;
	function init() {
		var imageObj = new Image();

      imageObj.onload = function() {
        ctx.drawImage(imageObj, 0, 0);
      };
      imageObj.src = 'http://www.html5canvastutorials.com/demos/assets/darth-vader.jpg';
	  canvas.addEventListener('mousedown', mouseDown, false);
	  canvas.addEventListener('mouseup', mouseUp, false);
	  canvas.addEventListener('mousemove', mouseMove, false);
	}
	function mouseDown(e) {
	  rect.startX = e.pageX - this.offsetLeft;
	  rect.startY = e.pageY - this.offsetTop;
	  drag = true;
	}
	function mouseUp() {
	  drag = false;
	}
	function mouseMove(e) {
	  if (drag) {
		rect.w = (e.pageX - this.offsetLeft) - rect.startX;
		rect.h = (e.pageY - this.offsetTop) - rect.startY ;
		ctx.clearRect(rect.startX, rect.startY,rect.w,rect.h);
		draw();
	  }
	}
	function draw() {
	  ctx.clearRect(rect.startX, rect.startY, rect.w, rect.h);
	  ctx.strokeRect(rect.startX, rect.startY, rect.w, rect.h);
	}

<head>
<meta charset="utf-8" />
<title>Draw a rectangle!</title>
</head>

<body onload="init();">
<canvas id="canvas" width="500" height="500"></canvas>
</body>

Answer 1

您可能需要先使用clearRect清除整个画布，然后立即绘制imageObj，最后绘制笔画。所有这些都发生在mouseMove函数中，因此它基本上不断地连续绘制这些元素。

请尝试以下代码段：

＆＃13;

var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');
var rect = {};
var drag = false;
var imageObj = null;

function init() {
    imageObj = new Image();
    imageObj.onload = function () { ctx.drawImage(imageObj, 0, 0); };
    imageObj.src = 'http://www.html5canvastutorials.com/demos/assets/darth-vader.jpg';
    canvas.addEventListener('mousedown', mouseDown, false);
    canvas.addEventListener('mouseup', mouseUp, false);
    canvas.addEventListener('mousemove', mouseMove, false);
}

function mouseDown(e) {
    rect.startX = e.pageX - this.offsetLeft;
    rect.startY = e.pageY - this.offsetTop;
    drag = true;
}

function mouseUp() { drag = false; }

function mouseMove(e) {
    if (drag) {
        ctx.clearRect(0, 0, 500, 500);
        ctx.drawImage(imageObj, 0, 0);
        rect.w = (e.pageX - this.offsetLeft) - rect.startX;
        rect.h = (e.pageY - this.offsetTop) - rect.startY;
        ctx.strokeStyle = 'red';
        ctx.strokeRect(rect.startX, rect.startY, rect.w, rect.h);
    }
}
//
init();

＆＃13;

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

＆＃13;

希望这是你所寻找的并以某种方式帮助你。

P.S。我故意在笔画上应用red颜色。你当然可以删除它。

使用javascript中的mousemove事件在画布内的图像上绘制一个矩形

1 个答案: