我正在尝试使用画布调整一些图像的大小，但我对如何平滑它们一无所知。在 Photoshop、浏览器等上。他们使用了一些算法（例如双三次、双线性），但我不知道这些算法是否内置在画布中。
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);
第一个是正常调整大小的图像标签，第二个是画布。请注意画布是如何不那么光滑的。我怎样才能实现“平滑”？
您可以使用向下步进来获得更好的结果。大多数浏览器在调整图像大小时似乎使用线性插值而不是双三次。
（更新在规范中添加了质量属性，imageSmoothingQuality目前仅在 Chrome 中可用。）
除非选择不平滑或最近邻，否则浏览器将始终在缩小图像后对图像进行插值，因为此功能用作低通滤波器以避免混叠。
双线性使用 2x2 像素进行插值，而双三次使用 4x4，因此通过逐步执行此操作，您可以在使用双线性插值的同时获得接近双三次结果，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>如果差异较小，您可以根据调整大小的剧烈程度跳过第 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。
由于Trung Le Nguyen Nhat 的小提琴根本不正确（它只是在最后一步中使用原始图像），
我编写了自己的通用小提琴来进行性能比较：
基本上是：
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);
}
我创建了一个可重用的 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 卷积方法以速度较慢为代价获得更高质量，而逐步缩小方法可产生合理的抗锯齿结果，并且速度明显更快。
示例用法：
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
})
})
虽然其中一些代码片段很短且有效，但遵循和理解它们并非易事。
由于我不喜欢 stack-overflow 中的“复制粘贴”，因此我希望开发人员能够理解他们推送到软件中的代码，希望您会发现以下内容对您有用。
DEMO : 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原理以及原因。
https://jsfiddle.net/1b68eLdr/93089/
可以在 GitHub 项目中找到完整的演示代码和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
}
};
}}
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）。
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 或图像魔术，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，而是取精确的平均值。
基于 K3N 的答案，我通常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
我通过使用画布比例解决了这个问题，我的情况下的图像质量变得非常好。
所以首先我缩放画布内的内容：
ctx.scale(2, 2)
然后用 css 横向扩展 canvas 标签：
#myCanvas { transform: scale(0.5); }
我写了一个小的 js-utility 来在前端裁剪和调整图像大小。这是GitHub 项目的链接。您也可以从最终图像中获取 blob 以发送它。
import imageSqResizer from './image-square-resizer.js'
let resizer = new imageSqResizer(
'image-input',
300,
(dataUrl) =>
document.getElementById('image-output').src = dataUrl;
);
//Get blob
let formData = new FormData();
formData.append('files[0]', resizer.blob);
//get dataUrl
document.getElementById('image-output').src = resizer.dataUrl;
这是我的代码，我希望它对 SO 社区中的某个人有用：
您可以将目标图像尺寸作为参数包含在脚本调用中。这将是您的图像宽度或高度的结果值，以较大者为准。较小的尺寸会调整大小，保持图像纵横比不变。您还可以在脚本中硬编码您的默认目标大小。
您可以轻松更改脚本以满足您的特定需求，例如您想要输出的图像类型（默认为“image/png”），并决定您想要调整图像大小的百分比步骤以获得更精细的结果（请参阅代码中的 const percentStep)。
const ResizeImage = ( _ => {
const MAX_LENGTH = 260; // default target size of largest dimension, either witdth or height
const percentStep = .3; // resizing steps until reaching target size in percents (30% default)
const canvas = document.createElement("canvas");
const canvasContext = canvas.getContext("2d");
const image = new Image();
const doResize = (callback, maxLength) => {
// abort with error if image has a dimension equal to zero
if(image.width == 0 || image.height == 0) {
return {blob: null, error: "either image width or height was zero "};
}
// use caller dimension or default length if none provided
const length = maxLength == null ? MAX_LENGTH : maxLength;
canvas.width = image.width;
canvas.height = image.height;
canvasContext.drawImage(image, 0, 0, image.width, image.height);
// if image size already within target size, just copy and return blob
if(image.width <= length && image.height <= length) {
canvas.toBlob( blob => {
callback({ blob: blob, error: null });
}, "image/png", 1);
return;
}
var startDim = Math.max(image.width, image.height);
var startSmallerDim = Math.min(image.width, image.height);
// gap to decrease in size until we reach the target size,
// be it by decreasing the image width or height,
// whichever is largest
const gap = startDim - length;
// step length of each resizing iteration
const step = parseInt(percentStep*gap);
// no. of iterations
var nSteps = 0;
if(step == 0) {
step = 1;
} else {
nSteps = parseInt(gap/step);
}
// length of last additional resizing step, if needed
const lastStep = gap % step;
// aspect ratio = value by which we'll multiply the smaller dimension
// in order to keep the aspect ratio unchanged in each iteration
const ratio = startSmallerDim/startDim;
var newDim; // calculated new length for the bigger dimension of the image, be it image width or height
var smallerDim; // length along the smaller dimension of the image, width or height
for(var i = 0; i < nSteps; i++) {
// decrease longest dimension one step in pixels
newDim = startDim - step;
// decrease shortest dimension proportionally, so as to keep aspect ratio
smallerDim = parseInt(ratio*newDim);
// assign calculated vars to their corresponding canvas dimension, width or height
if(image.width > image.height) {
[canvas.width, canvas.height] = [newDim, smallerDim];
} else {
[canvas.width, canvas.height] = [smallerDim, newDim];
}
// draw image one step smaller
canvasContext.drawImage(canvas, 0, 0, canvas.width, canvas.height);
// cycle var startDim for new loop
startDim = newDim;
}
// do last missing resizing step to finally reach target image size
if(lastStep > 0) {
if(image.width > image.height) {
[canvas.width, canvas.height] = [startDim - lastStep, parseInt(ratio*(startDim - lastStep))];
} else {
[canvas.width, canvas.height] = [parseInt(ratio*(startDim -lastStep)), startDim - lastStep];
}
canvasContext.drawImage(image, 0, 0, canvas.width, canvas.height);
}
// send blob to caller
canvas.toBlob( blob => {
callback({blob: blob, error: null});
}, "image/png", 1);
};
const resize = async (imgSrc, callback, maxLength) => {
image.src = imgSrc;
image.onload = _ => {
doResize(callback, maxLength);
};
};
return { resize: resize }
})();
用法：
ResizeImage.resize("./path/to/image/or/blob/bytes/to/resize", imageObject => {
if(imageObject.error != null) {
// handle errors here
console.log(imageObject.error);
return;
}
// do whatever you want with the blob, like assinging it to
// an img element, or uploading it to a database
// ...
document.querySelector("#my-image").src = imageObject.blob;
// ...
}, 300);
export const resizeImage = (imageFile, size = 80) => {
let resolver = ()=>{};
let reader = new FileReader();
reader.onload = function (e) {
let img = document.createElement("img");
img.onload = function (event) {
// Dynamically create a canvas element
let canvas = document.createElement("canvas");
canvas.width=size;
canvas.height=size;
// let canvas = document.getElementById("canvas");
let ctx = canvas.getContext("2d");
// Actual resizing
ctx.drawImage(img, 0, 0, size, size);
// Show resized image in preview element
let dataurl = canvas.toDataURL(imageFile.type);
resolver(dataurl);
}
img.src = e.target.result;
}
reader.readAsDataURL(imageFile);
return new Promise((resolve, reject) => {
resolver = resolve;
})
};