问

沿曲线的梯度冲程在帆布

mobiledu2502875545 发布于 2023-01-11 18:52

svg

文件

我试图在画布上绘制一条曲线,沿着曲线绘制线性渐变样式,如此图像所示.在该页面上有一个链接的svg文件,它提供了有关如何在svg中完成效果的说明.也许在画布上可以使用类似的方法？

1 个回答

演示:http: //jsfiddle.net/m1erickson/4fX5D/

创建沿路径变化的渐变相当容易:

在此输入图像描述

创建一个在整个路径上变化的渐变更加困难:

在此输入图像描述

要在路径上创建渐变,请绘制与路径相切的许多渐变线:

在此输入图像描述

如果绘制足够的切线,则眼睛会将曲线视为路径上的渐变.

在此输入图像描述

注意:Jaggies可能出现在路径渐变的外部.那是因为渐变实际上是由数百条切线组成的.但是你可以通过使用适当的颜色在渐变的任一侧画一条线来消除锯齿(这里的反锯齿线在顶部是红色而在底部是紫色).

以下是在路径中创建渐变的步骤:

在路径上绘制数百个点.

计算这些点的路径角度.

在每个点处,创建一个线性渐变,并在该点的切线上绘制一条渐变描边线.是的,您必须为每个点创建一个新渐变,因为线性渐变必须与该点相切的线的角度匹配.

为了减少绘制许多单独线条所造成的锯齿效应,您可以沿着渐变路径的顶部和底部绘制一条平滑的路径来覆盖锯齿.

这是带注释的代码:

<!doctype html>
<html>
<head>
<link rel="stylesheet" type="text/css" media="all" href="css/reset.css" /> <!-- reset css -->
<script type="text/javascript" src="http://code.jquery.com/jquery.min.js"></script>
<style>
    body{ background-color: ivory; }
    #canvas{border:1px solid red;}
</style>       
<script>
$(function(){

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

    // variables defining a cubic bezier curve
    var PI2=Math.PI*2;
    var s={x:20,y:30};
    var c1={x:200,y:40};
    var c2={x:40,y:200};
    var e={x:270,y:220};

    // an array of points plotted along the bezier curve
    var points=[];

    // we use PI often so put it in a variable
    var PI=Math.PI;

    // plot 400 points along the curve
    // and also calculate the angle of the curve at that point
    for(var t=0;t<=100;t+=0.25){

        var T=t/100;

        // plot a point on the curve
        var pos=getCubicBezierXYatT(s,c1,c2,e,T);

        // calculate the tangent angle of the curve at that point
        var tx = bezierTangent(s.x,c1.x,c2.x,e.x,T);
        var ty = bezierTangent(s.y,c1.y,c2.y,e.y,T);
        var a = Math.atan2(ty, tx)-PI/2;

        // save the x/y position of the point and the tangent angle
        // in the points array
        points.push({
            x:pos.x,
            y:pos.y,
            angle:a
        });

    }


    // Note: increase the lineWidth if 
    // the gradient has noticable gaps 
    ctx.lineWidth=2;

    // draw a gradient-stroked line tangent to each point on the curve
    for(var i=0;i<points.length;i++){

        // calc the topside and bottomside points of the tangent line
        var offX1=points[i].x+20*Math.cos(points[i].angle);
        var offY1=points[i].y+20*Math.sin(points[i].angle);
        var offX2=points[i].x+20*Math.cos(points[i].angle-PI);
        var offY2=points[i].y+20*Math.sin(points[i].angle-PI);

        // create a gradient stretching between 
        // the calculated top & bottom points
        var gradient=ctx.createLinearGradient(offX1,offY1,offX2,offY2);
        gradient.addColorStop(0.00, 'red'); 
        gradient.addColorStop(1/6, 'orange'); 
        gradient.addColorStop(2/6, 'yellow'); 
        gradient.addColorStop(3/6, 'green') 
        gradient.addColorStop(4/6, 'aqua'); 
        gradient.addColorStop(5/6, 'blue'); 
        gradient.addColorStop(1.00, 'purple'); 

        // draw the gradient-stroked line at this point
        ctx.strokeStyle=gradient;
        ctx.beginPath();
        ctx.moveTo(offX1,offY1);
        ctx.lineTo(offX2,offY2);
        ctx.stroke();
    }


    // draw a top stroke to cover jaggies
    // on the top of the gradient curve
    var offX1=points[0].x+20*Math.cos(points[0].angle);
    var offY1=points[0].y+20*Math.sin(points[0].angle);
    ctx.stroke;
    // Note: increase the lineWidth if this outside of the
    //       gradient still has jaggies
    ctx.lineWidth=1.5;
    ctx.beginPath();
    ctx.moveTo(offX1,offY1);
    for(var i=1;i<points.length;i++){
        var offX1=points[i].x+20*Math.cos(points[i].angle);
        var offY1=points[i].y+20*Math.sin(points[i].angle);
        ctx.lineTo(offX1,offY1);
    }
    ctx.stroke();


    // draw a bottom stroke to cover jaggies
    // on the bottom of the gradient
    var offX2=points[0].x+20*Math.cos(points[0].angle+PI);
    var offY2=points[0].y+20*Math.sin(points[0].angle+PI);
    ctx.stroke;
    // Note: increase the lineWidth if this outside of the
    //       gradient still has jaggies
    ctx.lineWidth=1.5;
    ctx.beginPath();
    ctx.moveTo(offX2,offY2);
    for(var i=0;i<points.length;i++){
        var offX2=points[i].x+20*Math.cos(points[i].angle+PI);
        var offY2=points[i].y+20*Math.sin(points[i].angle+PI);
        ctx.lineTo(offX2,offY2);
    }
    ctx.stroke();


    //////////////////////////////////////////
    // helper functions
    //////////////////////////////////////////

    // calculate one XY point along Cubic Bezier at interval T
    // (where T==0.00 at the start of the curve and T==1.00 at the end)
    function getCubicBezierXYatT(startPt,controlPt1,controlPt2,endPt,T){
        var x=CubicN(T,startPt.x,controlPt1.x,controlPt2.x,endPt.x);
        var y=CubicN(T,startPt.y,controlPt1.y,controlPt2.y,endPt.y);
        return({x:x,y:y});
    }

    // cubic helper formula at T distance
    function CubicN(T, a,b,c,d) {
        var t2 = T * T;
        var t3 = t2 * T;
        return a + (-a * 3 + T * (3 * a - a * T)) * T
        + (3 * b + T * (-6 * b + b * 3 * T)) * T
        + (c * 3 - c * 3 * T) * t2
        + d * t3;
    }

    // calculate the tangent angle at interval T on the curve
    function bezierTangent(a, b, c, d, t) {
        return (3 * t * t * (-a + 3 * b - 3 * c + d) + 6 * t * (a - 2 * b + c) + 3 * (-a + b));
    };

}); // end $(function(){});
</script>
</head>
<body>
    <canvas id="canvas" width=300 height=300></canvas>
</body>
</html>

2023-01-11 18:54 回答

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