在画布上绘图时偏移-前端问题

Offset when drawing on canvas(在画布上绘图时偏移)

本文介绍了在画布上绘图时偏移的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！

问题描述

有一个简单的绘图应用程序.问题在于坐标(redraw 函数):它们必须是鼠标，但接近 2x 鼠标.代码有什么问题?

Have a simple drawing application. The problem is in the coordinates (redraw function): they must be mouse, but are near 2x mouse. What's the problem in the code?

<html>
<head>
    <style type="text/css" media="screen">
body{
    background-color: green;
}

#workspace{
    width: 700px;
    height:420px;
    margin: 40px auto 20px auto;
    border: black dashed 1px;
}

#canvas{
    background: white;
    width: 100%;
    height: 100%;
}
    </style>
<script type="text/javascript" src="jquery-1.7.1.js"></script> 
<script type="text/javascript">
$(document).ready(
    function() {
        var context = document.getElementById('canvas').getContext("2d");
        var clickX = new Array();
        var clickY = new Array();
        var clickDrag = new Array();
        var paint;

        $('#canvas').mousedown(function(e){
            var mouseX = e.pageX - this.offsetLeft;
            var mouseY = e.pageY - this.offsetTop;
            paint = true;
            addClick(mouseX,mouseY, false);
            redraw();
        });

        $('#canvas').mousemove(function(e){
            if(paint){
                addClick(e.pageX - this.offsetLeft, e.pageY - this.offsetTop, true);
                redraw();
            }
        });

        $('#canvas').mouseup(function(e){
            paint = false;
        });

        $('#canvas').mouseleave(function(e){
            paint = false;
        });

        function addClick(x, y, dragging)
        {
            clickX.push(x);
            clickY.push(y);
            clickDrag.push(dragging);
        }

        function redraw(){
            canvas.width = canvas.width; // Clears the canvas

            context.strokeStyle = "#df4b26";
            context.lineJoin = "round";
            context.lineWidth = 5;

            for(var i=1; i < clickX.length; i++)
            {
                context.beginPath();
                if(clickDrag[i] && i){
                    context.moveTo(clickX[i-1], clickY[i-1]);
                }else{
                    context.moveTo(clickX[i], clickY[i]);
                }
                context.lineTo(clickX[i], clickY[i]);
                context.closePath();
                context.stroke();
            }

            console.log(clickX[clickX.length-1]+" "+clickY[clickX.length-1]);
        }
    });
</script>
</head>
<body>
<div id="workspace">
<canvas id="canvas"/>
</div>
</body>
</html>

推荐答案

您不应该通过 CSS 设置画布的宽度/高度.它使画布拉伸而不是使分辨率更高.这意味着虽然坐标是正确的，但它们在视觉上会在其他地方结束.

You should not set a canvas' width/height through CSS. It makes the canvas stretch instead of making the resolution higher. This means that although the coordinates are correct, they will visually end up somewhere else.

例如，100 的 x 坐标将被拉伸为视觉上的 200 的 x 坐标(或其他；至少它会比 >100，因为它已被拉伸).

For example, an x coordinate of 100 will be stretched to visually be an x coordinate of 200 (or something else; at least it will be bigger than 100 since it's been stretched).

改为:

<canvas id="canvas" width="700" height="420" />

并删除 CSS 中的 width: 100%.

and remove the width: 100% in CSS.

http://jsfiddle.net/euXJC/1/

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