HTML5版径向渐变梯度色彩

有个读者问我如何用HTML5生成一个径向梯度色彩效果，而不使用图片。仔细思考下，其实这个问题一点都不难，请看代码。

HTML

HTML5 Radial Gradient | Script Tutorials

JS// Get angle color function

function getAngleColor(angle) {

var color, d;

if (angle < Math.PI * 2 / 5) { // angle: 0-72

d = 255 / (Math.PI * 2 / 5) * angle;

color = '255,' + Math.round(d) + ',0'; // color: 255,0,0 - 255,255,0

} else if (angle < Math.PI * 4 / 5) { // angle: 72-144

d = 255 / (Math.PI * 2 / 5) * (angle - Math.PI * 2 / 5);

color = (255 - Math.round(d)) + ',255,0'; // color: 255,255,0 - 0,255,0

} else if (angle < Math.PI * 6 / 5) { // angle: 144-216

d = 255 / (Math.PI * 2 / 5) * (angle - Math.PI * 4 / 5);

color = '0,255,' + Math.round(d); // color: 0,255,0 - 0,255,255

} else if (angle < Math.PI * 8 / 5) { // angle: 216-288

d = 255 / (Math.PI * 2 / 5) * (angle - Math.PI * 6 / 5);

color = '0,'+(255 - Math.round(d)) + ',255'; // color: 0,255,255 - 0,0,255

} else { // angle: 288-360

d = 255 / (Math.PI * 2 / 5) * (angle - Math.PI * 8 / 5);

color = Math.round(d) + ',0,' + (255 - Math.round(d)) ; // color: 0,0,255 - 255,0,0

}

return color;

}

// inner variables

var iSectors = 360;

var iSectorAngle = (360 / iSectors) / 180 * Math.PI; // in radians

// Draw radial gradient function

function drawGradient() {

// prepare canvas and context objects

var canvas = document.getElementById('gradient');

var ctx = canvas.getContext('2d');

// clear canvas

ctx.clearRect(0, 0, ctx.canvas.width, ctx.canvas.height);

// save current context

ctx.save();

// move to center

ctx.translate(canvas.width / 2, canvas.height / 2);

// draw all sectors

for (var i = 0; i < iSectors; i++) {

// start and end angles (in radians)

var startAngle = 0;

var endAngle = startAngle + iSectorAngle;

// radius for sectors

var radius = (canvas.width / 2) - 1;

// get angle color

var color = getAngleColor(iSectorAngle * i);

// draw a sector

ctx.beginPath();

ctx.moveTo(0, 0);

ctx.arc(0, 0, radius, startAngle, endAngle, false);

ctx.closePath();

// stroke a sector

ctx.strokeStyle = 'rgb('+color+')';

ctx.stroke();

// fill a sector

ctx.fillStyle = 'rgb('+color+')';

ctx.fill();

// rotate to the next sector

ctx.rotate(iSectorAngle);

}

// restore context

ctx.restore();

}

// initialization

if(window.attachEvent) {

window.attachEvent('onload', drawGradient);

} else {

if(window.onload) {

var curronload = window.onload;

var newonload = function() {

curronload();

drawGradient();

};

window.onload = newonload;

} else {

window.onload = drawGradient;

}

}

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