使用javascript画布调整图像大小(顺利)

Question

我试图用帆布调整一些图像的大小,但我对如何平滑它们毫无头绪.在Photoshop,浏览器等..他们使用了一些算法(例如双三次,双线性)但我不知道它们是否内置于画布中.

这是我的小提琴:http://jsfiddle.net/EWupT/

var canvas = document.createElement('canvas');
var ctx = canvas.getContext('2d');
canvas.width=300
canvas.height=234
ctx.drawImage(img, 0, 0, 300, 234);
document.body.appendChild(canvas);

第一个是普通的已调整大小的图像标记,第二个是画布.请注意画布如何不那么平滑.我怎样才能实现'平滑'？

Answer 1

您可以使用向下步进来获得更好的结果.在调整图像大小时,大多数浏览器似乎使用线性插值而不是双三次.

(更新已为规格添加了质量属性,imageSmoothingQuality目前仅在Chrome中提供.)

除非选择平滑或最近邻居,否则浏览器将在缩小图像后始终对图像进行插值,因为此功能作为低通滤波器以避免混叠.

双线性使用2x2像素进行插值,而双立方使用4x4,因此通过逐步进行,您可以接近双三次结果,同时使用双线性插值,如结果图像中所示.

var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var img = new Image();

img.onload = function () {

    // set size proportional to image
    canvas.height = canvas.width * (img.height / img.width);

    // step 1 - resize to 50%
    var oc = document.createElement('canvas'),
        octx = oc.getContext('2d');

    oc.width = img.width * 0.5;
    oc.height = img.height * 0.5;
    octx.drawImage(img, 0, 0, oc.width, oc.height);

    // step 2
    octx.drawImage(oc, 0, 0, oc.width * 0.5, oc.height * 0.5);

    // step 3, resize to final size
    ctx.drawImage(oc, 0, 0, oc.width * 0.5, oc.height * 0.5,
    0, 0, canvas.width, canvas.height);
}
img.src = "//i.imgur.com/SHo6Fub.jpg";

<img src="//i.imgur.com/SHo6Fub.jpg" width="300" height="234">
<canvas id="canvas" width=300></canvas>

根据您调整大小的程度,如果差异较小,您可以跳过第2步.

在演示中,您可以看到新结果现在与图像元素非常相似.

Answer 2

我创建了一个可重复使用的Angular服务来为任何感兴趣的人处理高质量的图像/画布大小调整:https://gist.github.com/transitive-bullshit/37bac5e741eaec60e983

该服务包括两个解决方案,因为它们都有自己的优点/缺点.lanczos卷积方法质量较高,代价是较慢,而逐步降尺度方法产生合理的抗锯齿结果,并且速度明显更快.

用法示例:

angular.module('demo').controller('ExampleCtrl', function (imageService) {
  // EXAMPLE USAGE
  // NOTE: it's bad practice to access the DOM inside a controller, 
  // but this is just to show the example usage.

  // resize by lanczos-sinc filter
  imageService.resize($('#myimg')[0], 256, 256)
    .then(function (resizedImage) {
      // do something with resized image
    })

  // resize by stepping down image size in increments of 2x
  imageService.resizeStep($('#myimg')[0], 256, 256)
    .then(function (resizedImage) {
      // do something with resized image
    })
})

Answer 3

由于Trung Le Nguyen Nhat的小提琴根本不正确(它只是在最后一步使用了原始图像),
我写了自己的一般小提琴,进行了性能比较:

小提琴

基本上它是:

img.onload = function() {
   var canvas = document.createElement('canvas'),
       ctx = canvas.getContext("2d"),
       oc = document.createElement('canvas'),
       octx = oc.getContext('2d');

   canvas.width = width; // destination canvas size
   canvas.height = canvas.width * img.height / img.width;

   var cur = {
     width: Math.floor(img.width * 0.5),
     height: Math.floor(img.height * 0.5)
   }

   oc.width = cur.width;
   oc.height = cur.height;

   octx.drawImage(img, 0, 0, cur.width, cur.height);

   while (cur.width * 0.5 > width) {
     cur = {
       width: Math.floor(cur.width * 0.5),
       height: Math.floor(cur.height * 0.5)
     };
     octx.drawImage(oc, 0, 0, cur.width * 2, cur.height * 2, 0, 0, cur.width, cur.height);
   }

   ctx.drawImage(oc, 0, 0, cur.width, cur.height, 0, 0, canvas.width, canvas.height);
}

Answer 4

虽然其中一些代码片段简短且有效，但遵循和理解它们并非易事。

由于我不是堆栈溢出中“复制粘贴”的粉丝，我希望开发人员了解他们推送到他们软件中的代码，希望您会发现以下有用。

演示：使用 JS 和 HTML Canvas Demo fiddler 调整图像大小。

您可能会找到 3 种不同的方法来调整大小，这将帮助您了解代码的工作方式以及原因。

https://jsfiddle.net/1b68eLdr/93089/

可以在 GitHub 项目中找到演示的完整代码以及您可能希望在代码中使用的 TypeScript 方法。

https://github.com/eyalc4/ts-image-resizer

这是最终的代码：

export class ImageTools {
base64ResizedImage: string = null;

constructor() {
}

ResizeImage(base64image: string, width: number = 1080, height: number = 1080) {
    let img = new Image();
    img.src = base64image;

    img.onload = () => {

        // Check if the image require resize at all
        if(img.height <= height && img.width <= width) {
            this.base64ResizedImage = base64image;

            // TODO: Call method to do something with the resize image
        }
        else {
            // Make sure the width and height preserve the original aspect ratio and adjust if needed
            if(img.height > img.width) {
                width = Math.floor(height * (img.width / img.height));
            }
            else {
                height = Math.floor(width * (img.height / img.width));
            }

            let resizingCanvas: HTMLCanvasElement = document.createElement('canvas');
            let resizingCanvasContext = resizingCanvas.getContext("2d");

            // Start with original image size
            resizingCanvas.width = img.width;
            resizingCanvas.height = img.height;


            // Draw the original image on the (temp) resizing canvas
            resizingCanvasContext.drawImage(img, 0, 0, resizingCanvas.width, resizingCanvas.height);

            let curImageDimensions = {
                width: Math.floor(img.width),
                height: Math.floor(img.height)
            };

            let halfImageDimensions = {
                width: null,
                height: null
            };

            // Quickly reduce the dize by 50% each time in few iterations until the size is less then
            // 2x time the target size - the motivation for it, is to reduce the aliasing that would have been
            // created with direct reduction of very big image to small image
            while (curImageDimensions.width * 0.5 > width) {
                // Reduce the resizing canvas by half and refresh the image
                halfImageDimensions.width = Math.floor(curImageDimensions.width * 0.5);
                halfImageDimensions.height = Math.floor(curImageDimensions.height * 0.5);

                resizingCanvasContext.drawImage(resizingCanvas, 0, 0, curImageDimensions.width, curImageDimensions.height,
                    0, 0, halfImageDimensions.width, halfImageDimensions.height);

                curImageDimensions.width = halfImageDimensions.width;
                curImageDimensions.height = halfImageDimensions.height;
            }

            // Now do final resize for the resizingCanvas to meet the dimension requirments
            // directly to the output canvas, that will output the final image
            let outputCanvas: HTMLCanvasElement = document.createElement('canvas');
            let outputCanvasContext = outputCanvas.getContext("2d");

            outputCanvas.width = width;
            outputCanvas.height = height;

            outputCanvasContext.drawImage(resizingCanvas, 0, 0, curImageDimensions.width, curImageDimensions.height,
                0, 0, width, height);

            // output the canvas pixels as an image. params: format, quality
            this.base64ResizedImage = outputCanvas.toDataURL('image/jpeg', 0.85);

            // TODO: Call method to do something with the resize image
        }
    };
}}

Answer 5

我不明白为什么没有人建议createImageBitmap。

createImageBitmap(
    document.getElementById('image'), 
    { resizeWidth: 300, resizeHeight: 234, resizeQuality: 'high' }
)
.then(imageBitmap => 
    document.getElementById('canvas').getContext('2d').drawImage(imageBitmap, 0, 0)
);

效果很好（假设您为图像和画布设置了 id）。

Answer 6

我创建了一个库，允许您在保留所有颜色数据的同时降低任何百分比。

https://github.com/danschumann/limby-resize/blob/master/lib/canvas_resize.js

您可以在浏览器中包含该文件。结果看起来像photoshop或image magick，保留所有颜色数据，平均像素，而不是取附近的并丢弃其他的。它不使用公式来猜测平均值，而是采用精确的平均值。

Answer 7

基于 K3N 答案，我一般会为任何想要的人重写代码

var oc = document.createElement('canvas'), octx = oc.getContext('2d');
    oc.width = img.width;
    oc.height = img.height;
    octx.drawImage(img, 0, 0);
    while (oc.width * 0.5 > width) {
       oc.width *= 0.5;
       oc.height *= 0.5;
       octx.drawImage(oc, 0, 0, oc.width, oc.height);
    }
    oc.width = width;
    oc.height = oc.width * img.height / img.width;
    octx.drawImage(img, 0, 0, oc.width, oc.height);

更新 JSFiddle 演示

这是我的在线演示