如何旋转线性梯度？

Question

我怎样才能使我的白线完美地对角线从点到点，我的底部矩形？

https://jsfiddle.net/a7rs5qu5/

  <canvas id="canvas" width="300" height="300"></canvas>

_canvas = document.getElementById('canvas');
_stage = _canvas.getContext('2d');

_stage.fillStyle = "#00FF00";
_stage.fillRect(0, 0, 300, 200);

var gradient = _stage.createLinearGradient(0, 200, 300, 300);
gradient.addColorStop(0, "blue");
gradient.addColorStop(.5, "white");
gradient.addColorStop(1, "blue");

_stage.fillStyle = gradient;
_stage.fillRect(0, 200, 300, 300);

Answer 1

通过数学和改变梯度坐标。你需要设置梯度坐标，这样他们就可以描述一条与之正交的直线。

​

_canvas = document.getElementById('canvas');
_stage = _canvas.getContext('2d');

_stage.fillStyle = "#00FF00";
_stage.fillRect(0, 0, 300, 200);

var radius = 100;
var angle = Math.atan2(100, 300) + Math.PI / 2;
var gx = radius * Math.cos(angle);
var gy = radius * Math.sin(angle);
var cx = (0 + 300) / 2;
var cy = (200 + 300) / 2;

var gradient = _stage.createLinearGradient(cx - gx, cy - gy, cx + gx, cy + gy);
gradient.addColorStop(0, "blue");
gradient.addColorStop(.5, "white");
gradient.addColorStop(1, "blue");

_stage.fillStyle = gradient;
_stage.fillRect(0, 200, 300, 300);

<canvas id="canvas" width="300" height="300"></canvas>

​

radius的选择控制梯度的宽度；上面的值只是给出与上面代码中使用的值类似的值。

Answer 2

这里有一个更通用的解决方案，用于懒人。

    function createDiagonalGradient(startx, starty, endx, endy)
    {

    var height = endy - starty; 

    var radius = height; 

//-1 or 1 depending on which diagonal you want
    var angle = -1 * Math.atan2(height, endx) + Math.PI / 2;


    var gx = radius * Math.cos(angle);
    var gy = radius * Math.sin(angle);
    var cx = (startx + endx) / 2;
    var cy = (starty + endy) / 2;

    var gradient = _stage.createLinearGradient(cx - gx, cy - gy, cx + gx, cy + gy);
    gradient.addColorStop(0, "black");
    gradient.addColorStop(.5, "white");
    gradient.addColorStop(1, "black");

    return gradient; 


    }