我们如何对画布中使用的图案图像进行灰度化（javascript）

Question

创建了一个圆形上下文并添加了图案图像以填充该圆圈。我怎样才能使该图案图像灰度化。 有什么想法可以帮到我。

代码：

    ctx.beginPath();
    var bg = new Image();    
    bg.src = image;  
    bg = function() {
        var pattern = ctx.createPattern(this, "no-repeat");
        ctx.fillStyle = pattern;
    };
    ctx.moveTo(canvasCenterHoriz, canvasCenterVert);
    ctx.arc(x, y, radius, startAngle, endAngle, direction);
    ctx.lineWidth = 0;
    ctx.fill();

Answer 1

这可能不是最好的方法，但它非常简单并且有效。

我在下面附上了一个工作示例。以下是它的工作原理：

图案图像是在不可见的画布中绘制的。加载图像后，将其绘制在画布上，并将白色覆盖和饱和度设置为全局合成操作。画布现在将包含您的图案的灰度版本。

然后将临时画布转换为图像，其源设置为画布数据 url。也许有更好的方法在两个画布之间发送图像数据，但我还没有找到。

图案完成后，您的原始弧线将使用新图案绘制。

let canvas = document.getElementById('grayscale-canvas');
let ctx = canvas.getContext('2d');

function makeGrayscaleBackground(image, onready) {
  var bg = new Image();
  bg.src = image;
  bg.onload = function() {
    // Create a canvas that's not attached to the DOM
    var canvas = document.createElement('canvas');
    canvas.setAttribute('width', this.width);
    canvas.setAttribute('height', this.height);
    document.body.appendChild(canvas);
    let ctx = canvas.getContext('2d');

    // Draw the background image
    ctx.drawImage(this, 0, 0);

    // Then draw a white layer on top, with the saturation composite operation
    // which will remove the color from the underlying image
    ctx.globalCompositeOperation = "saturation";
    ctx.fillStyle = "#FFF";
    ctx.fillRect(0, 0, this.width, this.height);

    onready(canvas);
  };

}

function drawWithImage(image) {

  makeGrayscaleBackground(image, function(patternImage) {
    // Green background
    ctx.fillStyle = "#3f9";
    ctx.fillRect(0, 0, canvas.width, canvas.height);

    // Make a repeating pattern with image we created
    var pattern = ctx.createPattern(patternImage, "repeat");

    // Make an arc with the pattern
    let x = y = canvas.width/2;
    ctx.fillStyle = pattern;
    ctx.beginPath();
    ctx.arc(x, y, canvas.width/2-10, 0, 2*Math.PI);
    ctx.fill();
  })

}

// Example pattern image
// For security reasons, the image needs to be hosted on the same server as the script!
var bgImage = "data:image/gif;base64,R0lGODlhEAAQAMQAAORHHOVSKudfOulrSOp3WOyDZu6QdvCchPGolfO0o/XBs/fNwfjZ0frl3/zy7////wAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAACH5BAkAABAALAAAAAAQABAAAAVVICSOZGlCQAosJ6mu7fiyZeKqNKToQGDsM8hBADgUXoGAiqhSvp5QAnQKGIgUhwFUYLCVDFCrKUE1lBavAViFIDlTImbKC5Gm2hB0SlBCBMQiB0UjIQA7";
drawWithImage(bgImage);
document.getElementById('bg').src = bgImage;

<canvas width="150" height="150" id="grayscale-canvas"></canvas><br>
Actual background image: <img id="bg"><br>
Modified background image: