我正在画这样的画像：
ctx.drawImage(data[i].image, data[i].pos.x, data[i].pos.y, data[i].pos.w, data[i].pos.h);
background-size: cover
在画像中画图像。
http://www.w3schools.com/cssref/playit.asp?filename=playcss_background-size&preval=cover
查看100% 100%（我目前拥有的）和cover（我的目标）之间的差异。
答案 0 :(得分：80)
获得封面功能有点复杂，虽然这里有一个解决方案：
更新2016-04-03 以解决特殊情况。另见@ Yousef的评论如下。
/**
* By Ken Fyrstenberg Nilsen
*
* drawImageProp(context, image [, x, y, width, height [,offsetX, offsetY]])
*
* If image and context are only arguments rectangle will equal canvas
*/
function drawImageProp(ctx, img, x, y, w, h, offsetX, offsetY) {
if (arguments.length === 2) {
x = y = 0;
w = ctx.canvas.width;
h = ctx.canvas.height;
}
// default offset is center
offsetX = typeof offsetX === "number" ? offsetX : 0.5;
offsetY = typeof offsetY === "number" ? offsetY : 0.5;
// keep bounds [0.0, 1.0]
if (offsetX < 0) offsetX = 0;
if (offsetY < 0) offsetY = 0;
if (offsetX > 1) offsetX = 1;
if (offsetY > 1) offsetY = 1;
var iw = img.width,
ih = img.height,
r = Math.min(w / iw, h / ih),
nw = iw * r, // new prop. width
nh = ih * r, // new prop. height
cx, cy, cw, ch, ar = 1;
// decide which gap to fill
if (nw < w) ar = w / nw;
if (Math.abs(ar - 1) < 1e-14 && nh < h) ar = h / nh; // updated
nw *= ar;
nh *= ar;
// calc source rectangle
cw = iw / (nw / w);
ch = ih / (nh / h);
cx = (iw - cw) * offsetX;
cy = (ih - ch) * offsetY;
// make sure source rectangle is valid
if (cx < 0) cx = 0;
if (cy < 0) cy = 0;
if (cw > iw) cw = iw;
if (ch > ih) ch = ih;
// fill image in dest. rectangle
ctx.drawImage(img, cx, cy, cw, ch, x, y, w, h);
}
现在你可以这样称呼它：
drawImageProp(ctx, image, 0, 0, width, height);
它将按比例缩放图像以适合该容器内部。
使用最后两个参数来偏移图像：
var offsetX = 0.5; // center x
var offsetY = 0.5; // center y
drawImageProp(ctx, image, 0, 0, width, height, offsetX, offsetY);
希望这有帮助！
答案 1 :(得分：2)
如果您正在寻找一种适用于大多数情况的简单解决方案，并且还包括css contain - 类似功能，请尝试以下方法：
function fit(contains) {
return (parentWidth, parentHeight, childWidth, childHeight, scale = 1, offsetX = 0.5, offsetY = 0.5) => {
const childRatio = childWidth / childHeight
const parentRatio = parentWidth / parentHeight
let width = parentWidth * scale
let height = parentHeight * scale
if (contains ? (childRatio > parentRatio) : (childRatio < parentRatio)) {
height = width / childRatio
} else {
width = height * childRatio
}
return {
width,
height,
offsetX: (parentWidth - width) * offsetX,
offsetY: (parentHeight - height) * offsetY
}
}
}
export const contain = fit(true)
export const cover = fit(false)
intrinsic-scale的略微修改版本，包括比例和偏移量
用法：
import {cover, contain} from './intrinsic-scale'
const {
offsetX,
offsetY,
width,
height
} = cover(parentWidth, parentHeight, imageWidth, imageHeight)
// or...
const {
offsetX,
offsetY,
width,
height
} = contain(parentWidth, parentHeight, imageWidth, imageHeight)
ctx.drawImage(image, offsetX, offsetY, width, height)
答案 2 :(得分：2)
适合图像的画布，例如背景大小的封面并包含
coverImg(myImage, 'cover');
coverImg(myImage, 'contain');
用法：
{{1}}
答案 3 :(得分：0)
尝试一下（基于@daviestar的答案）：
getCanvas = function(img, w, h) {
// Create canvas
let canvas = document.createElement('canvas');
let ctx = canvas.getContext('2d');
// Set width and height
canvas.width = w;
canvas.height = h;
// Draw the image
let containerRatio = h / w;
let width = img.naturalWidth;
let height = img.naturalHeight;
let imgRatio = height / width;
if (imgRatio > containerRatio) { // image's height too big
height = width * containerRatio;
} else { // image's width too big
width = height / containerRatio;
}
let s = {
width: width,
height: height,
offsetX: (img.naturalWidth - width) * .5,
offsetY: (img.naturalHeight - height) * .5
};
ctx.drawImage(img, s.offsetX, s.offsetY, s.width, s.height, 0, 0, w, h);
return canvas;
}
答案 4 :(得分：0)
下面的脚本参考了@user1693593，我做了如下修改
<script>
function clamp(num, a, b) {
return num > b ? b
: num < a ? a
: num
}
/**
* @param {CanvasRenderingContext2D} ctx
* @param {HTMLImageElement} img
* @param {Number} dx
* @param {Number} dy
* @param {Number} dw
* @param {Number} dh
* @param {Number} offsetX
* @param {Number} offsetY
* */
function drawImageProp(ctx, img, {dx = 0, dy = 0, dw = undefined, dh = undefined, offsetX = 0.5, offsetY = 0.5}) {
dw = dw ?? ctx.canvas.width
dh = dh ?? ctx.canvas.height
// keep bounds [0.0, 1.0]
offsetX = clamp(offsetX, 0, 1)
offsetY = clamp(offsetY, 0, 1)
let iw = img.width,
ih = img.height,
ratio = Math.min(dw / iw, dh / ih),
nw = iw * ratio,
nh = ih * ratio, // new prop. height
sx, sy, sw, sh, ar = 1;
// decide which gap to fill
if (nw < dw) ar = dw / nw
if (Math.abs(ar - 1) < 1e-14 && nh < dh) ar = dh / nh // updated
nw *= ar
nh *= ar
// source rectangle
sw = iw / (nw / dw)
sh = ih / (nh / dh)
sx = (iw - sw) * offsetX
sy = (ih - sh) * offsetY
// make sure source rectangle is valid
if (sx < 0) sx = 0
if (sy < 0) sy = 0
if (sw > iw) sw = iw
if (sh > ih) sh = ih
img.onload = (event) => {
// fill image in dest. rectangle
ctx.drawImage(event.target, sx, sy, sw, sh, dx, dy, dw, dh)
}
}
// Test Only ?
window.onload = () => {
const testArray = [
["Default", (ctx, img)=>{drawImageProp(ctx, img, {})}],
["Full", (ctx, img)=>{drawImageProp(ctx, img, {offsetY:0})}], // If you don't want to it cutting from two sides, you can set "offsetY = 0" then it will cut a large part from the bottom
["1/2",(ctx, img)=>{drawImageProp(ctx, img, {dx:window.innerWidth/4, dy:window.innerHeight/4, dw: window.innerWidth/2, dh:window.innerHeight/2})}],
["3/4",(ctx, img)=>{drawImageProp(ctx, img, {dx:window.innerWidth/8, dy:window.innerHeight/8, dw: window.innerWidth*3/4, dh:window.innerHeight*3/4})}]
]
for (const [testName, drawFunc] of testArray) {
const btn = document.createElement("button")
btn.innerText = testName
btn.onclick = () => {
document.querySelectorAll(`canvas`).forEach(e=>e.remove()) // remove old data
const img = new Image(590, 470)
img.src = "https://upload.wikimedia.org/wikipedia/commons/b/bd/Test.svg"
const canvas = document.createElement("canvas")
canvas.width = window.innerWidth
canvas.height = window.innerHeight
const ctx = canvas.getContext("2d")
drawFunc(ctx, img)
document.querySelector(`body`).append(canvas)
}
document.querySelector(`body`).append(btn)
}
}
</script>