何为“设备像素比devicePixelRatio”
设备上物理像素和设备独立像素(device-independent pixels (dips))的比例。
公式表示就是：window.devicePixelRatio = 物理像素 / dips dip或dp,（device independent pixels，设备独立像素）与屏幕密度有关。dip可以用来辅助区分视网膜设备还是非视网膜设备。
而对于视网膜屏幕的iphone，如iphone4s, 纵向显示的时候，屏幕物理像素640像素。同样，当用户设置<meta name="viewport" content="width=device-width">的时候，其视区宽度并不是640像素，而是320像素，这是为了有更好的阅读体验 – 更合适的文字大小。
这样，在视网膜屏幕的iphone上，屏幕物理像素640像素，独立像素还是320像素，因此，window.devicePixelRatio等于2.
视网膜屏幕下图片就显示OK了（非视网膜屏幕图片被压缩-减少像素取样——资源浪费！）
百度糯米的实现：
@media only screen and (-webkit-min-device-pixel-ratio:2),only screen and (min-device-pixel-ratio:2) {
.normal-goods .good-content {
border: none;
background-image: -webkit-linear-gradient(90deg,#e0e0e0,#e0e0e0 50%,transparent 50%);
background-image: -moz-linear-gradient(90deg,#e0e0e0,#e0e0e0 50%,transparent 50%);
background-image: -o-linear-gradient(90deg,#e0e0e0,#e0e0e0 50%,transparent 50%);
background-image: linear-gradient(0,#e0e0e0,#e0e0e0 50%,transparent 50%);
background-size: 100% 1px;
background-repeat: no-repeat;
background-position: bottom
}
}
大众点评的实现：
.index-rec .home-tuan-list .cnt {
padding: 7px 10px 10px 0;
display: box;
display: -webkit-box;
display: -ms-flexbox;
height: 78px;
background-image: url(//www.dpfile.com/mod/app-m-style/1.7.2/css/img/repeat-x.png);
background-repeat: repeat-x;
background-position: 0 bottom;
background-size: auto 1px
}
微信WeUI的实现：
.weui_grid:before {
content: " ";
position: absolute;
right: 0;
top: 0;
width: 1px;
height: 100%;
border-right: 1px solid #D9D9D9;
color: #D9D9D9;
-webkit-transform-origin: 0 100%;
transform-origin: 0 100%;
-webkit-transform: scaleX(0.5);
transform: scaleX(0.5);
right: -1px;
}
.weui_grid:after {
content: " ";
position: absolute;
left: 0;
bottom: 0;
width: 100%;
height: 1px;
border-bottom: 1px solid #D9D9D9;
color: #D9D9D9;
-webkit-transform-origin: 0 100%;
transform-origin: 0 100%;
-webkit-transform: scaleY(0.5);
transform: scaleY(0.5);
}
阿里的实现：
<script>
if (/iP(hone|od|ad)/.test(navigator.userAgent)) { // 就是放到html根节点上的 ios8现在普及率高了，可以省略
var v = (navigator.appVersion).match(/OS (\d+)_(\d+)_?(\d+)?/), version = parseInt(
v[1], 10);
if (version >= 8) {
document.documentElement.classList.add('hairlines')
}
};
</script>
.r1bt {
border-top: 1px solid rgba(32,35,37,.15)
}
.r1bb {
border-bottom: 1px solid rgba(32,35,37,.15)
}
.r1bl {
border-left: 1px solid rgba(32,35,37,.15)
}
.r1br {
border-right: 1px solid rgba(32,35,37,.15)
}
.r1b {
border: 1px solid rgba(32,35,37,.15)
}
.hairlines .r1bt,.hairlines .r1bb,.hairlines .r1bl,.hairlines .r1br,.hairlines .r1b {
border-width: .5px!important
}
阿里的另一种实现：
/*Retian 1px border start */
.retinabt,.retinabb,.retinabl,.retinabr,.retinab{position:relative;}
.retinabt:before,.retinabb:after{pointer-events:none;position:absolute;content:"";height:1px;background:rgba(32,35,37,.14);left:0;right:0;z-index:26;}
.retinabt:before{top:0;z-index:26;}
.retinabb:after{bottom:0;z-index:26;}
.retinabl:before,.retinabr:after{pointer-events:none;position:absolute;content:"";width:1px;background:rgba(32,35,37,.14);top:0;bottom:0}
.retinabl:before{left:0;z-index:26;}
.retinabr:after{right:0;z-index:26;}
.retinab:after{position:absolute;content:"";top:0;left:0;-webkit-box-sizing:border-box;box-sizing:border-box;width:100%;height:100%;border:1px solid rgba(32,35,37,.14);pointer-events:none;z-index:26;}
@media (-webkit-min-device-pixel-ratio:1.5),(min-device-pixel-ratio:1.5),(min-resolution:144dpi),(min-resolution:1.5dppx){
.retinabt:before,.retinabb:after{-webkit-transform:scaleY(.5);transform:scaleY(.5) }
.retinabl:before,.retinabr:after{-webkit-transform:scaleX(.5);transform:scaleX(.5) }
.retinab:after{width:200%;height:200%;-webkit-transform:scale(.5);transform:scale(.5) }
.retinabt:before,.retinabl:before,.retinab:after{-webkit-transform-origin:0 0;transform-origin:0 0}
.retinabb:after,.retinabr:after{-webkit-transform-origin:100% 100%;transform-origin:100% 100%}
}
@media (-webkit-device-pixel-ratio:1.5){
.retinabt:before,.retinabb:after{-webkit-transform:scaleY(.6666);transform:scaleY(.6666) }
.retinabl:before,.retinabr:after{-webkit-transform:scaleX(.6666);transform:scaleX(.6666)}
.retinab:after{width:150%;height:150%;-webkit-transform:scale(.6666);transform:scale(.6666) }
}
@media (-webkit-device-pixel-ratio:3){
.retinabt:before,.retinabb:after{-webkit-transform:scaleY(.3333);transform:scaleY(.3333)}
.retinabl:before,.retinabr:after{-webkit-transform:scaleX(.3333);transform:scaleX(.3333)}
.retinab:after{width:300%;height:300%;-webkit-transform:scale(.3333);transform:scale(.3333)}
}
@media (-webkit-min-device-pixel-ratio:4),(min-device-pixel-ratio:4){
.retinabt:before,.retinabb:after{-webkit-transform:scaleY(.25);transform:scaleY(.25)}
.retinabl:before,.retinabr:after{-webkit-transform:scaleX(.25);transform:scaleX(.25)}
.retinab:after{width:400%;height:400%;-webkit-transform:scale(.25);transform:scale(.25)}
}
/*Retina 1px border end */
美团的实现：
<script type="text/javascript">
//根据屏幕大小及dpi调整缩放和大小
(function() {
var scale = 1.0;
var ratio = 1;
if (window.devicePixelRatio >= 2) {
scale *= 0.5;
ratio *= 2;
}
var text = '<meta name="viewport" content="initial-scale=' + scale + ', maximum-scale=' + scale +', minimum-scale=' + scale + ', width=device-width, user-scalable=no" />';
document.write(text);
document.documentElement.style.fontSize = 50*ratio + "px";
})();
</script>
<meta name="viewport" content="target-densitydpi=device-dpi"> <!--安卓自带的 device-width 先不加 否则iphone 随进线条出现问题 -->
<script>
+function(win,doc,undefined) {//根据屏幕大小及dpi调整缩放和大小
var scale = 1.0,ratio = 1,dc=doc,viewporttexts='';
if (win.devicePixelRatio && devicePixelRatio >= 1.5) {
ratio = devicePixelRatio;
scale = scale/(devicePixelRatio);
}
//var texts = '<meta name="viewport" content="initial-scale=' + scale + ', maximum-scale=' + scale +', minimum-scale=' + scale + ', width=device-width, user-scalable=no" />';
// dc.write(texts);
viewporttexts = ' width=device-width, initial-scale=' + scale + ', maximum-scale=' + scale +', minimum-scale=' + scale + ',user-scalable=no';
doc.querySelector('meta[name="viewport"]').setAttribute("content",viewporttexts);
console.log('111');
dc.documentElement.style.fontSize =doc.getElementsByTagName("html")[0].style.fontSize=Math.ceil(50*ratio) + "px";
}(window,document);
</script>
淘宝的实现：
;(function(win, lib) {
var doc = win.document;
var docEl = doc.documentElement;
var metaEl = doc.querySelector('meta[name="viewport"]');
var flexibleEl = doc.querySelector('meta[name="flexible"]');
var dpr = 0;
var scale = 0;
var tid;
var flexible = lib.flexible || (lib.flexible = {});
if (metaEl) {
console.warn('将根据已有的meta标签来设置缩放比例');
var match = metaEl.getAttribute('content').match(/initial\-scale=([\d\.]+)/);
if (match) {
scale = parseFloat(match[1]);
dpr = parseInt(1 / scale);
}
} else if (flexibleEl) {
var content = flexibleEl.getAttribute('content');
if (content) {
var initialDpr = content.match(/initial\-dpr=([\d\.]+)/);
var maximumDpr = content.match(/maximum\-dpr=([\d\.]+)/);
if (initialDpr) {
dpr = parseFloat(initialDpr[1]);
scale = parseFloat((1 / dpr).toFixed(2));
}
if (maximumDpr) {
dpr = parseFloat(maximumDpr[1]);
scale = parseFloat((1 / dpr).toFixed(2));
}
}
}
if (!dpr && !scale) {
var isAndroid = win.navigator.appVersion.match(/android/gi);
var isIPhone = win.navigator.appVersion.match(/iphone/gi);
var devicePixelRatio = win.devicePixelRatio;
if (isIPhone) {
// iOS下，对于2和3的屏，用2倍的方案，其余的用1倍方案
if (devicePixelRatio >= 3 && (!dpr || dpr >= 3)) {
dpr = 3;
} else if (devicePixelRatio >= 2 && (!dpr || dpr >= 2)){
dpr = 2;
} else {
dpr = 1;
}
} else {
// 其他设备下，仍旧使用1倍的方案
dpr = 1;
}
scale = 1 / dpr;
}
docEl.setAttribute('data-dpr', dpr);
if (!metaEl) {
metaEl = doc.createElement('meta');
metaEl.setAttribute('name', 'viewport');
metaEl.setAttribute('content', 'initial-scale=' + scale + ', maximum-scale=' + scale + ', minimum-scale=' + scale + ', user-scalable=no');
if (docEl.firstElementChild) {
docEl.firstElementChild.appendChild(metaEl);
} else {
var wrap = doc.createElement('div');
wrap.appendChild(metaEl);
doc.write(wrap.innerHTML);
}
}
function refreshRem(){
var width = docEl.getBoundingClientRect().width;
if (width / dpr > 540) {
width = 540 * dpr;
}
var rem = width / 10;
docEl.style.fontSize = rem + 'px';
flexible.rem = win.rem = rem;
}
win.addEventListener('resize', function() {
clearTimeout(tid);
tid = setTimeout(refreshRem, 300);
}, false);
win.addEventListener('pageshow', function(e) {
if (e.persisted) {
clearTimeout(tid);
tid = setTimeout(refreshRem, 300);
}
}, false);
if (doc.readyState === 'complete') {
doc.body.style.fontSize = 12 * dpr + 'px';
} else {
doc.addEventListener('DOMContentLoaded', function(e) {
doc.body.style.fontSize = 12 * dpr + 'px';
}, false);
}
refreshRem();
flexible.dpr = win.dpr = dpr;
flexible.refreshRem = refreshRem;
flexible.rem2px = function(d) {
var val = parseFloat(d) * this.rem;
if (typeof d === 'string' && d.match(/rem$/)) {
val += 'px';
}
return val;
}
flexible.px2rem = function(d) {
var val = parseFloat(d) / this.rem;
if (typeof d === 'string' && d.match(/px$/)) {
val += 'rem';
}
return val;
}
})(window, window['lib'] || (window['lib'] = {}));
用rem写1px 维护行方便；图片高度可以用rem固定高度，防止加载时出现高度自动网速加载慢导致的明显塌陷； 缺点: 动态控制 viewport retina下，无论美团还是淘宝用 rem始终还有许多细小的问题；在ios上浏览器打开仔细看还是看的出的，安卓上没看出来；
H5 Canvas Retina屏幕处理的1px的函数
/**
* HiDPI Canvas Polyfill (1.0.9)
*
* Author: Jonathan D. Johnson (http://jondavidjohn.com)
* Homepage: https://github.com/jondavidjohn/hidpi-canvas-polyfill
* Issue Tracker: https://github.com/jondavidjohn/hidpi-canvas-polyfill/issues
* License: Apache 2.0
*/
;(function(prototype) {
var pixelRatio = (function(context) {
var backingStore = context.backingStorePixelRatio ||
context.webkitBackingStorePixelRatio ||
context.mozBackingStorePixelRatio ||
context.msBackingStorePixelRatio ||
context.oBackingStorePixelRatio ||
context.backingStorePixelRatio || 1;
return (window.devicePixelRatio || 1) / backingStore;
})(prototype),
forEach = function(obj, func) {
for (var p in obj) {
if (obj.hasOwnProperty(p)) {
func(obj[p], p);
}
}
},
ratioArgs = {
'fillRect': 'all',
'clearRect': 'all',
'strokeRect': 'all',
'moveTo': 'all',
'lineTo': 'all',
'arc': [0,1,2],
'arcTo': 'all',
'bezierCurveTo': 'all',
'isPointinPath': 'all',
'isPointinStroke': 'all',
'quadraticCurveTo': 'all',
'rect': 'all',
'translate': 'all',
'createRadialGradient': 'all',
'createLinearGradient': 'all'
};
if (pixelRatio === 1) return;
forEach(ratioArgs, function(value, key) {
prototype[key] = (function(_super) {
return function() {
var i, len,
args = Array.prototype.slice.call(arguments);
if (value === 'all') {
args = args.map(function(a) {
return a * pixelRatio;
});
}
else if (Array.isArray(value)) {
for (i = 0, len = value.length; i < len; i++) {
args[value[i]] *= pixelRatio;
}
}
return _super.apply(this, args);
};
})(prototype[key]);
});
// Stroke lineWidth adjustment
prototype.stroke = (function(_super) {
return function() {
this.lineWidth *= pixelRatio;
_super.apply(this, arguments);
this.lineWidth /= pixelRatio;
};
})(prototype.stroke);
// Text
//
prototype.fillText = (function(_super) {
return function() {
var args = Array.prototype.slice.call(arguments);
args[1] *= pixelRatio; // x
args[2] *= pixelRatio; // y
this.font = this.font.replace(
/(\d+)(px|em|rem|pt)/g,
function(w, m, u) {
return (m * pixelRatio) + u;
}
);
_super.apply(this, args);
this.font = this.font.replace(
/(\d+)(px|em|rem|pt)/g,
function(w, m, u) {
return (m / pixelRatio) + u;
}
);
};
})(prototype.fillText);
prototype.strokeText = (function(_super) {
return function() {
var args = Array.prototype.slice.call(arguments);
args[1] *= pixelRatio; // x
args[2] *= pixelRatio; // y
this.font = this.font.replace(
/(\d+)(px|em|rem|pt)/g,
function(w, m, u) {
return (m * pixelRatio) + u;
}
);
_super.apply(this, args);
this.font = this.font.replace(
/(\d+)(px|em|rem|pt)/g,
function(w, m, u) {
return (m / pixelRatio) + u;
}
);
};
})(prototype.strokeText);
})(CanvasRenderingContext2D.prototype);
;(function(prototype) {
prototype.getContext = (function(_super) {
return function(type) {
var backingStore, ratio,
context = _super.call(this, type);
if (type === '2d') {
backingStore = context.backingStorePixelRatio ||
context.webkitBackingStorePixelRatio ||
context.mozBackingStorePixelRatio ||
context.msBackingStorePixelRatio ||
context.oBackingStorePixelRatio ||
context.backingStorePixelRatio || 1;
ratio = (window.devicePixelRatio || 1) / backingStore;
if (ratio > 1) {
this.style.height = this.height + 'px';
this.style.width = this.width + 'px';
this.width *= ratio;
this.height *= ratio;
}
}
return context;
};
})(prototype.getContext);
})(HTMLCanvasElement.prototype);
viewport + REM的方式来实现：
<meta name="viewport" content="initial-scale=0.5, maximum-scale=0.5, minimum-scale=0.5, user-scalable=no">
<meta name="viewport" content="initial-scale=0.3333333333333333, maximum-scale=0.3333333333333333, minimum-scale=0.3333333333333333, user-scalable=no">
<script type="text/javascript">
(function() {
var value=1;
if (window.devicePixelRatio == 2) {
value=0.5;
}else if(window.devicePixelRatio == 3){
value=0.333333333333333;
}
var text = '<meta name="viewport" content="initial-scale=' + value + ', maximum-scale=' + value +', minimum-scale=' + value + ', width=device-width, user-scalable=no" />';
document.write(text);
})();
</script>
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