SVG 在视网膜显示器上渲染成 canvas 模糊

Question

我在将 SVG 渲染到 canvas 时遇到问题。在视网膜显示器上 canvas 呈现为 base64 url 并设置为 SRC 是模糊的。

我已经尝试了下面列表中描述的各种方法，但没有成功：

https://tristandunn.com/2014/01/24/rendering-svg-on-canvas/
How do I fix blurry text in my HTML5 canvas?
Canvas drawing and Retina display: doable?
https://www.html5rocks.com/en/tutorials/canvas/hidpi/

现在我不知道我应该怎么做才能让它变得更好。请查看我的结果：jsfiddle.net/a8bj5fgj/2/

编辑：

更新 fiddle 修复：jsfiddle.net/a8bj5fgj/7/

Answer 1

视网膜显示屏

视网膜和超高分辨率显示器的像素尺寸小于人眼所能分辨的平均像素尺寸。渲染一条线最终看起来像一条较浅的线。要修复检测到高分辨率显示的页面所涉及的问题，会将默认的 CSS 像素大小更改为 2。

DOM 知道这一点并调整其渲染以进行补偿。但是 Canvas 并不知道，它的渲染只是按比例放大了。 canvas 的默认显示渲染类型是双线性插值。这可以平滑从一个像素到下一个像素的过渡，这对照片来说非常有用，但对线条、文本、SVG 等来说就不是那么好了。

一些解决方案

首先是关闭 canvas 上的双线性过滤。这可以通过 CSS 规则来完成 image-rendering: pixelated; 虽然这不会创建在 DOM 上呈现的 SVG 的质量，但它会减少一些用户体验的模糊外观。
将 SVG 渲染到 canvas 时，您应该关闭图像平滑，因为这会降低 svg 图像的质量。 SVG 是在内部渲染的，当内部副本渲染到 canvas 上时不需要额外的平滑处理。

这样做ctx.imageSmoothingEnabled = false;
检测 CSS 像素大小。 window 变量 devicePixelRatio returns CSS 像素的大小与实际屏幕物理像素大小的比较。 Retina 和高分辨率设备的值通常为 2。然后您可以使用它来设置 canvas 分辨率以匹配物理像素分辨率。

但是有一个问题，因为 devicePixelRatio 并非在所有浏览器上都支持，并且 devicePixelRatio 受页面缩放设置的影响。

所以在最基本的情况下使用 devicePixelRatio 并假设很少有人放大超过 200%。

代码假设 canvas.style.width 和 canvas.style.height 已经正确设置。

if(devicePixelRatio >= 2){        
    canvas.width *= 2;
    canvas.height *= 2;
}

既然您已经提高了分辨率，那么您还必须提高渲染大小。这可以通过 canvas 转换来完成，最好将其创建为一个函数。

function setCanvasForRetina(canvas){
    canvas.width *= 2;
    canvas.height *= 2;
    canvas.setTransform(2,0,0,2,0,0);
}

Note I do not increase the pixel size by the value of "devicePixelRatio" This is because retina devices will only have a resolution of 2 times, if the aspect ratio is greater than 2 it is because the client has zoomed in. To honor the expected behaviour of the canvas I do not adjust for zoom setting if I can. Though that is not a rule just a suggestion.

更好的猜测

以上两种方法要么是权宜之计，要么是简单的猜测。您可以通过检查一些系统来提高您的胜算。

Retina 显示屏目前针对一组固定的设备（手机、平板电脑、笔记本电脑）提供一组固定的分辨率。

您可以查询 window.screen.width 和 window.screen.height 以确定绝对物理像素分辨率并将其与已知的视网膜显示器分辨率相匹配。您还可以查询 userAgent 以确定设备类型和品牌。

将它们放在一起可以改进猜测。下一个函数猜测显示器是否为视网膜。如果设备是视网膜设备，您可以使用类似于 determin 的东西，然后相应地增加 canvas 分辨率。

在 wiki Retina Display Models 找到以下代码的信息如果您想保持最新，可以使用 Wiki 的 SPARQL 界面对这些信息进行机器查询。

演示猜 Retina。

rWidth.textContent = screen.width
rHeight.textContent = screen.height

aWidth.textContent = screen.availWidth
aHeight.textContent = screen.availHeight

pWidth.textContent = innerWidth
pHeight.textContent = innerHeight

dWidth.textContent = document.body.clientWidth
dHeight.textContent = document.body.clientHeight

//doWidth.textContent = document.body.offsetWidth
//doHeight.textContent = document.body.offsetHeight

//sWidth.textContent = document.body.scrollWidth
//sHeight.textContent = document.body.scrollHeight

pAspect.textContent = devicePixelRatio

userA.textContent = navigator.userAgent



function isRetina(){
  // source https://en.wikipedia.org/wiki/Retina_Display#Models
  var knownRetinaResolutions = [[272,340], [312,390], [960,640], [1136,640 ], [1334,750 ], [1920,1080], [2048,1536], [2732,2048], [2304,1440], [2560,1600], [2880,1800], [4096,2304], [5120,2880]];
  var knownPhones =  [[960,640], [1136,640 ], [1334,750 ], [1920,1080]];
  var knownPads =  [[2048,1536], [2732,2048]];
  var knownBooks = [[2304,1440], [2560,1600], [2880,1800], [4096,2304], [5120,2880]];

  var hasRetinaRes = knownRetinaResolutions.some(known => known[0] === screen.width && known[1] === screen.height);
  var isACrapple = /(iPhone|iPad|iPod|Mac OS X|MacPPC|MacIntel|Mac_PowerPC|Macintosh)/.test(navigator.userAgent);
  var hasPhoneRes =  knownPhones.some(known => known[0] === screen.width && known[1] === screen.height);
  var isPhone = /iPhone/.test(navigator.userAgent);
  var hasPadRes =  knownPads.some(known => known[0] === screen.width && known[1] === screen.height);
  var isPad = /iPad/.test(navigator.userAgent);
  var hasBookRes =  knownBooks.some(known => known[0] === screen.width && known[1] === screen.height);
  var isBook = /Mac OS X|MacPPC|MacIntel|Mac_PowerPC|Macintosh/.test(navigator.userAgent);

  var isAgentMatchingRes = (isBook && hasBookRes && !isPad && !isPhone) ||
      (isPad && hasPadRes && !isBook && !isPhone) ||
      (isPhone && hasPhoneRes && !isBook && !isPad)
  return devicePixelRatio >= 2 && 
         isACrapple && 
         hasRetinaRes && 
          isAgentMatchingRes;
}

guess.textContent = isRetina() ? "Yes" : "No";

div, h1, span {
  font-family : arial;
}
span {
  font-weight : bold
}

<div class="r-display" id="info">
  <h1>System info</h1>
  <div>Device resolution : 
    <span id = "rWidth"></span> by <span id = "rHeight"></span> pixels
  </div>
  <div>Availabe resolution : 
    <span id = "aWidth"></span> by <span id = "aHeight"></span> pixels
  </div>
  <div>Page resolution : 
    <span id = "pWidth"></span> by <span id = "pHeight">  </span> CSS pixels
  </div>
  <div>Document client res : 
    <span id = "dWidth"></span> by <span id = "dHeight">  </span> CSS pixels
  </div>
  <div>Pixel aspect : 
    <span id = "pAspect"></span>
  </div>
  <div>User agent :
    <span id="userA"></span>
  </div>
  <h3>Best guess is retina "<span id = "guess"></span>!"</h3>
</div>

来自您的代码段

这可能会如您所愿。因为我不拥有任何苹果产品，所以除了在 isRetina 上强制为 true 之外，我无法对其进行测试。

    function isRetina() {
        // source https://en.wikipedia.org/wiki/Retina_Display#Models
        var knownRetinaResolutions = [[272, 340], [312, 390], [960, 640], [1136, 640], [1334, 750], [1920, 1080], [2048, 1536], [2732, 2048], [2304, 1440], [2560, 1600], [2880, 1800], [4096, 2304], [5120, 2880]];
        var knownPhones = [[960, 640], [1136, 640], [1334, 750], [1920, 1080]];
        var knownPads = [[2048, 1536], [2732, 2048]];
        var knownBooks = [[2304, 1440], [2560, 1600], [2880, 1800], [4096, 2304], [5120, 2880]];

        var hasRetinaRes = knownRetinaResolutions.some(known => known[0] === screen.width && known[1] === screen.height);
        var isACrapple = /(iPhone|iPad|iPod|Mac OS X|MacPPC|MacIntel|Mac_PowerPC|Macintosh)/.test(navigator.userAgent);
        var hasPhoneRes = knownPhones.some(known => known[0] === screen.width && known[1] === screen.height);
        var isPhone = /iPhone/.test(navigator.userAgent);
        var hasPadRes = knownPads.some(known => known[0] === screen.width && known[1] === screen.height);
        var isPad = /iPad/.test(navigator.userAgent);
        var hasBookRes = knownBooks.some(known => known[0] === screen.width && known[1] === screen.height);
        var isBook = /Mac OS X|MacPPC|MacIntel|Mac_PowerPC|Macintosh/.test(navigator.userAgent);

        var isAgentMatchingRes = (isBook && hasBookRes && !isPad && !isPhone) ||
            (isPad && hasPadRes && !isBook && !isPhone) ||
            (isPhone && hasPhoneRes && !isBook && !isPad);
      
      
        return devicePixelRatio >= 2 && isACrapple && hasRetinaRes && isAgentMatchingRes;
    }
    
    function svgToImage(svg){
        function svgAsImg() {
            var canvas, ctx;
            canvas = document.createElement("canvas");
            ctx = canvas.getContext("2d");
            var width = this.width;
            var height = this.height;
            var scale = 1;
            if(isRetina()){
                width *= 2;
                height *= 2;
                scale = 2;
            }

            canvas.width = width;
            canvas.height = height;
            ctx.setTransform(scale, 0, 0, scale, 0, 0);
            ctx.imageSmoothingEnabled = false;  // SVG rendering is better with smoothing off
            ctx.drawImage(this,0,0);
            DOMURL.revokeObjectURL(url);
            try{
                var image = new Image();
                image.src = canvas.toDataURL();              
                imageContainer.appendChild(image);
                image.width = this.width;
                image.height = this.height;
            }catch(e){  // in case of CORS error fallback to canvas
                canvas.style.width = this.width + "px";  // in CSS pixels not physical pixels
                canvas.style.height = this.height + "px";
                imageContainer.appendChild(canvas);  // just use the canvas as it is an image as well
            }
        };
        var url;
        var img = new Image();
        var DOMURL = window.URL || window.webkitURL || window;
        img.src = url = DOMURL.createObjectURL(new Blob([svg], {type: 'image/svg+xml'}));           
        img.onload = svgAsImg;
    }
    
    svgToImage(svgContainer.innerHTML);

<div id="svgContainer"><svg width="31" height="40" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 43 55" fill="#736b9e"><path d="m 40.713968,30.966202 c 0.0028,0.05559 -0.01078,0.114956 -0.044,0.178882 -1.545645,2.974287 -2.853499,5.591663 -4.339695,7.673668 -0.788573,1.104704 -2.095869,2.778673 -2.874223,3.773068 -0.994236,1.02684 -6.879641,7.657944 -6.167884,7.049648 -1.292899,1.235403 -5.717368,5.476022 -5.717368,5.476022 0,0 -4.323294,-3.985179 -5.928388,-5.591297 C 14.037321,47.920078 10.708239,43.994015 9.6976253,42.770306 8.6870114,41.546601 8.5086687,40.900753 6.8441265,38.818752 5.8958518,37.63265 4.1376268,34.24638 3.0745121,32.156026 2.9037625,31.86435 2.7398218,31.568267 2.5826899,31.268005 2.5509386,31.228498 2.5238331,31.18779 2.5044312,31.145084 2.4575955,31.041974 2.4164305,30.951055 2.3805569,30.87146 0.95511134,28.003558 0.15221914,24.771643 0.15221914,21.351725 c 0,-11.829154 9.58943056,-21.41858234 21.41858286,-21.41858234 11.829152,0 21.418583,9.58942834 21.418583,21.41858234 0,3.457576 -0.820406,6.72314 -2.275417,9.614477 z M 21.52596,1.5031489 c -10.866018,0 -19.6746717,8.8086521 -19.6746717,19.6746741 0,10.866016 8.8086537,19.674669 19.6746717,19.674669 10.866018,0 19.674672,-8.808648 19.674672,-19.674669 0,-10.866022 -8.808654,-19.6746741 -19.674672,-19.6746741 z" /><g transform="translate(6.5,6) scale(0.060546875)"><path d="M32 384h272v32H32zM400 384h80v32h-80zM384 447.5c0 17.949-14.327 32.5-32 32.5-17.673 0-32-14.551-32-32.5v-95c0-17.949 14.327-32.5 32-32.5 17.673 0 32 14.551 32 32.5v95z"></path><g><path d="M32 240h80v32H32zM208 240h272v32H208zM192 303.5c0 17.949-14.327 32.5-32 32.5-17.673 0-32-14.551-32-32.5v-95c0-17.949 14.327-32.5 32-32.5 17.673 0 32 14.551 32 32.5v95z"></path></g><g><path d="M32 96h272v32H32zM400 96h80v32h-80zM384 159.5c0 17.949-14.327 32.5-32 32.5-17.673 0-32-14.551-32-32.5v-95c0-17.949 14.327-32.5 32-32.5 17.673 0 32 14.551 32 32.5v95z"></path></g></g></svg>
</div>    
<div id="imageContainer"></div>

请注意。

大多数视力为 6/6（帝国国家为 20/20）的人很难看出 canvas 稍微模糊的显示与清晰的 DOM 之间的区别。你应该问问自己，你是否需要仔细观察才能确定？你能在正常观看距离看到模糊吗？

还有一些将显示缩放到 200% 的人出于充分的理由（视力受损）这样做，并且不会感谢您规避他们的设置。

SVG 在视网膜显示器上渲染成 canvas 模糊

SVG rendered into canvas blurred on retina display

javascript

svg

canvas

blurry

retina

视网膜显示屏

一些解决方案

更好的猜测

来自您的代码段