如何在 javascript 中为 strokeStyle canvas 添加渐变

Question

How do I add gradient color to strokeStyle canvas in javascript. Please help me my code works perfect but problem is only that I've been trying to add gradient color to a stroke style but it just breaks my code.

Any one who can help on this Gradient color to strokeStyle
HTML

      <div class="countItem minutes">
        <canvas id="minutes-canvas" width="200" height="200"></canvas>
        <svg width="100%" height="100%">
            <circle id="outer" cx="50%" cy="50%" r="45%" fill="transparent" stroke-width="1%" stroke="#fff" />
        </svg>
        <h3>
            <span id="minutes-value"></span><br>
            <small>minutes</small>
        </h3>
    </div>

JAVASCRIPT

// Set Launch Date (ms)
const launchDate = new Date("May 7, 2020 00:00").getTime();

// Context object
const c = {
    context: {},
    values: {},
    times: {}
};

// Convert radians to degrees
function deg(d) {
    return (Math.PI / 180) * d - (Math.PI / 180) * 90;
}

function render() {
    c.context.minutes.clearRect(0, 0, 200, 200);
    c.context.minutes.beginPath();
    c.context.minutes.strokeStyle = "#bbee2b";
    c.context.minutes.arc(100, 100, 90, deg(0), deg(6 * (c.times.minutes - 60)));
    c.context.minutes.lineWidth = 12;
    c.context.minutes.lineCap = "round";
    c.context.minutes.stroke();

}

function init() {
    // Get 2D contexts
    c.context.minutes = document.getElementById('minutes-canvas').getContext('2d');

    // Get displayed values
    c.values.minutes = document.getElementById('minutes-value');

    setInterval(function () {
        // Get todays date and time (ms)
        const now = new Date().getTime();

        // Get distance from now to launchDate
        const distance = launchDate - now;

        // Time calculations
        c.times.minutes = Math.floor((distance % (1000 * 60 * 60)) / (1000 * 60));
        c.values.minutes.innerText = c.times.minutes;

        render(); // Draw!
    }, 1000);
}

init();

Answer 1

You can create a gradient and assign that to stroke

var gradient = document.getElementById('minutes-canvas').getContext('2d').createLinearGradient(0, 0, 0, 170); gradient.addColorStop(0, '#05a'); gradient.addColorStop(1, '#0a5');

 // Set Launch Date (ms) const launchDate = new Date("May 7, 2020 00:00").getTime(); // Context object const c = { context: {}, values: {}, times: {} }; // Convert radians to degrees function deg(d) { return (Math.PI / 180) * d - (Math.PI / 180) * 90; } function render() { var gradient = document.getElementById('minutes-canvas').getContext('2d').createLinearGradient(0, 0, 0, 170); gradient.addColorStop(0, '#05a'); gradient.addColorStop(1, '#0a5'); c.context.minutes.clearRect(0, 0, 200, 200); c.context.minutes.beginPath(); c.context.minutes.strokeStyle = gradient; c.context.minutes.arc(100, 100, 90, deg(0), deg(6 * (c.times.minutes - 60))); c.context.minutes.lineWidth = 12; c.context.minutes.lineCap = "round"; c.context.minutes.stroke(); } function init() { // Get 2D contexts c.context.minutes = document.getElementById('minutes-canvas').getContext('2d'); // Get displayed values c.values.minutes = document.getElementById('minutes-value'); setInterval(function() { // Get todays date and time (ms) const now = new Date().getTime(); // Get distance from now to launchDate const distance = launchDate - now; // Time calculations c.times.minutes = Math.floor((distance % (1000 * 60 * 60)) / (1000 * 60)); c.values.minutes.innerText = c.times.minutes; render(); // Draw, }; 1000); } init();

 <div class="countItem minutes"> <canvas id="minutes-canvas" width="200" height="200"></canvas> <svg width="100%" height="100%"> <circle id="outer" cx="50%" cy="50%" r="45%" fill="transparent" stroke-width="1%" stroke="#fff" /> </svg> <h3> <span id="minutes-value"></span><br> <small>minutes</small> </h3> </div>

Answer 2

You can create a CanvasGradient by calling the methods createLinearGradient or createRadialGradient of your CanvasRenderingContext2D . After creating a gradient you add color stops to it by calling the method addColorStop(offset, color) .

In your code you are storing the CanvasRenderingContext2D in c.context.minutes , so you can do something along these lines:

function render() {
    c.context.minutes.clearRect(0, 0, 200, 200);
    c.context.minutes.beginPath();

    const gradient = c.context.minutes.createLinearGradient(0,0, 200,200);
    gradient.addColorStop(0, 'red');
    gradient.addColorStop(.5, 'blue');
    gradient.addColorStop(1, 'green');
    c.context.minutes.strokeStyle = gradient;

    c.context.minutes.arc(100, 100, 90, deg(0), deg(6 * (c.times.minutes - 60)));
    c.context.minutes.lineWidth = 12;
    c.context.minutes.lineCap = "round";
    c.context.minutes.stroke();
}

References:https://developer.mozilla.org/en-US/docs/Web/API/CanvasGradient