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SVG渲染到画布上在Retina显示屏上模糊了

javascriptcanvassvgretinablurry
7
我有一个与SVG渲染到画布的问题。在视网膜显示器上，画布以base64 URL形式呈现，并设置为SRC属性，导致模糊。

enter image description here

我已经尝试了下面列出的各种方法，但都没有成功：

现在我不知道应该怎么做才能使它更好。请查看我的结果：jsfiddle.net/a8bj5fgj/2/

编辑：

使用修复更新后的 fiddle：jsfiddle.net/a8bj5fgj/7/

1
可能是drawImage生成的图像质量与img标签相比较差的重复问题。 - Kaiido
1个回答
9

视网膜屏幕

视网膜和超高分辨率的显示器像素大小比人眼可以分辨的平均尺寸要小。渲染一条线会看起来像是一条较浅的线。为了解决这个问题，检测到高分辨率显示器的页面将把默认的CSS像素大小更改为2。

DOM知道这一点并调整其呈现以进行补偿。但Canvas不知道，它的呈现只是被放大了。Canvas的默认显示呈现类型是双线性插值。这使得从一个像素到下一个的过渡变得平滑，对于照片来说很好，但对于线条、文本、SVG等来说则不太理想。

一些解决方案

  • 首先要在画布上关闭双线性过滤。这可以通过 CSS 规则 image-rendering: pixelated; 来实现。虽然这不会创建与在 DOM 上呈现的 SVG 相同的质量，但它会减少某些用户经历的模糊外观。

  • 在将 SVG 渲染到画布时，应关闭图像平滑处理，因为这可能会降低 svg 图像的质量。SVG 在内部进行渲染，并且在将内部副本呈现到画布时不需要额外的平滑处理。

    要做到这一点，使用 ctx.imageSmoothingEnabled = false;

  • 检测 CSS 像素大小。窗口变量 devicePixelRatio 返回 CSS 像素与实际屏幕物理像素大小的比例。Retina 和高分辨率设备通常具有 2 的值。然后，您可以使用该值将画布分辨率设置为匹配物理像素分辨率。

    但是存在一个问题，因为 devicePixelRatio 并非所有浏览器都支持，而且 devicePixelRatio 受页面缩放设置的影响。

    因此，在最基本的情况下，使用 devicePixelRatio 和假设很少有人缩放超过 200%。

假设canvas.style.width和canvas.style.height已经正确设置，下面是代码。
if(devicePixelRatio >= 2){        
    canvas.width *= 2;
    canvas.height *= 2;
}

现在，您已经增加了分辨率，您还需要增加渲染大小。这可以通过画布变换来完成，最好将其创建为函数。
function setCanvasForRetina(canvas){
    canvas.width *= 2;
    canvas.height *= 2;
    canvas.setTransform(2,0,0,2,0,0);
}

请注意，我不通过“devicePixelRatio”的值增加像素大小。这是因为视网膜设备的分辨率仅为2倍，如果宽高比大于2，则表示客户端已经进行了缩放。为了遵守画布的预期行为，如果可能的话，我不会调整缩放设置。虽然这不是一条规则，只是建议。
更好的猜测
上述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胜算。
目前，Retina显示器在固定的设备（手机、平板、笔记本电脑）上有一组固定的分辨率。
您可以查询window.screen.width和window.screen.height来确定绝对物理像素分辨率，并将其与已知的Retina显示器分辨率进行匹配。您还可以查询userAgent以确定设备类型和品牌。
将所有内容综合起来，您可以提高猜测的准确性。下一个函数猜测显示器是否为Retina显示器。您可以使用类似的东西来确定设备是否为Retina显示器，然后相应地增加画布分辨率。

以下代码信息可在维基百科Retina显示器机型找到。如果您想保持最新，可以使用维基的SPARQL接口进行机器查询。

演示猜测是否为Retina屏幕。

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>

来自你的代码片段

这可能会达到你想要的效果。但由于我没有任何苹果产品，除了在isRetina上强制为true，我无法进行测试。

    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）的人会很难看出画布略有模糊的显示和清晰的DOM之间的区别。您应该问自己，是否需要仔细观察以确保？在正常观看距离下，您能看到模糊吗？
此外，一些将显示缩放至200％的人有充分的理由（视力受损），他们不会欣赏您规避其设置的行为。
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