来自SVG的ImageBitmap -某些字体大小的锯齿状文本

Question

我正在动态地创建包含透明背景上文本的SVG代码。SVG应该画在画布上，字体应该来自Google字体。

问题是：

虽然这种方法基本上是有效的，但一些字体大小显然会在createImageBitmap()中产生糟糕的alpha通道，从而导致可怕的锯齿状文本。

我在Windows 10和Ubuntu上都遇到了Chrome最新版本的问题。禁用Chrome的硬件加速不会改变任何事情。

输出图像：边缘锯齿状的文本

简而言之，代码就是这样做的：

1. 生成SVG源代码，它在透明的背景上显示一些文本。
2. 在SVG代码中，将指向外部内容(字体)的链接替换为相应的base64内容。
3. 使用ImageBitmap从该SVG创建一个createImageBitmap()。
4. 在画布上画那个ImageBitmap。

​

function createSvg(bckgrColor1, bckgrColor2, w, h, fontSize) {
  return `
  <svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink"
    xmlns:ev="http://www.w3.org/2001/xml-events" version="2" viewBox="0 0 ${w} ${h}" width="${w}" height="${h}">
    <style type="text/css">
      @font-face {
        font-family: 'Lobster';
        font-style: normal;
        font-weight: 400;
        font-display: swap;
        src: url(https://fonts.gstatic.com/s/lobster/v23/neILzCirqoswsqX9zoKmMw.woff2) format('woff2');
        unicode-range: U+0000-00FF, U+0131, U+0152-0153, U+02BB-02BC, U+02C6, U+02DA, U+02DC, U+2000-206F, U+2074, U+20AC, U+2122, U+2191, U+2193, U+2212, U+2215, U+FEFF, U+FFFD;
      }
    </style>
    <text x="0" y="50" font-family="Lobster" font-size="${fontSize}">
      Hello World!
    </text>
  </svg>`;
}

const _embedAssets = async function(svgSrc) {
  const _imageExtensions = ["png", "gif", "jpg", "jpeg", "svg", "bmp"];
  const _fontExtensions = ["woff2"];
  const _assetExtensions = [..._imageExtensions, ..._fontExtensions];
  /**
   * @uses https://stackoverflow.com/a/8943487/1273551
   * @license CC BY-SA 4.0.
   */
  const urlRegex = /(\bhttps?:\/\/[-A-Z0-9+&@#/%?=~_|!:,.;]*[-A-Z0-9+&@#/%=~_|])/gi;
  const allUrls = svgSrc.match(urlRegex);
  const assetUrls = allUrls.filter((url) =>
    _assetExtensions.some((extension) =>
      url.toLowerCase().endsWith(`.${extension}`)
    )
  );
  const assetBase64Fetcher = assetUrls.map(_fetchBase64AssetUrl);
  const assetFetcherResults = await Promise.all(assetBase64Fetcher);
  return assetFetcherResults.reduce(
    (svgSrc, x) => svgSrc.replace(x.url, x.base64),
    svgSrc
  );
};

// Fetch asset (image or font) and convert it to base64 string representation.
const _fetchBase64AssetUrl = async function(assetUrl) {
  return new Promise(async(resolve, reject) => {
    const resp = await fetch(assetUrl);
    const blob = await resp.blob();
    const reader = new FileReader();
    reader.onloadend = (event) => {
      const target = event.target;
      if (!target) {
        return reject(`Asset with URL "${assetUrl}" could not be loaded.`);
      }
      const result = target.result;
      if (!result) {
        return reject(`Asset with URL "${assetUrl}" returned an empty result.`);
      }
      resolve({
        url: assetUrl,
        base64: result.toString()
      });
    };
    reader.readAsDataURL(blob);
  });
};

const createImageBitmapFromSvg = async function(svgSrc) {
  return new Promise(async(resolve) => {
    const svgWithAssetsEmbedded = await _embedAssets(svgSrc);
    const svgBlob = new Blob([svgWithAssetsEmbedded], {
      type: "image/svg+xml;charset=utf-8"
    });
    const svgBase64 = URL.createObjectURL(svgBlob);
    let img = new Image();
    img.onload = async() => {
      const imgBitmap = await createImageBitmap(img);
      resolve(imgBitmap);
    };
    img.src = svgBase64;
  });
};

const renderCanvas = async function(canvas, svgSource, width, height, color) {
  canvas.width = width;
  canvas.height = height;
  let svgEmbedded = await _embedAssets(svgSource);
  let svgImageBitmap = await createImageBitmapFromSvg(svgEmbedded);
  let ctx = canvas.getContext("2d");
  if (ctx) {
    ctx.fillStyle = color;
    ctx.strokeStyle = "#000000";
    ctx.lineWidth = 2;
    ctx.fillRect(0, 0, canvas.width, canvas.height); //
    ctx.strokeRect(0, 0, canvas.width, canvas.height); //for white background
    ctx.drawImage(svgImageBitmap, 0, 0, canvas.width, canvas.height);
  }
};

const renderCanvasAlternative = async function(canvas, svgSource, width, height, color) {

  // create imagebitmap from raw svg code  
  let svgImageBitmap = await createImageBitmapFromSvg(svgSource);

  // temporary intermediate step as suggested on StackOverflow 
  const osc = await new OffscreenCanvas(width, height)
  let oscx = osc.getContext("bitmaprenderer")
  oscx.transferFromImageBitmap(svgImageBitmap);

  const svgImageBitmapFromOffscreenCanvas = osc.transferToImageBitmap();
  // const svgImageBitmapFromOffscreenCanvas2 = await createImageBitmap(osc); // results in empty bitmap

  // draw image bitmap on canvas
  canvas.width = width;
  canvas.height = height;
  let ctx = canvas.getContext("bitmaprenderer");
  if (!ctx) throw new Error("Could not get context from canvas.");
  ctx.transferFromImageBitmap(svgImageBitmapFromOffscreenCanvas);
}

const bootstrap = async() => {
  // width and height for svg and canvases
  const w = "300";
  const h = "80";

  // create two svg sources, only difference is fontsize of embedded font
  const svgSourceGood = createSvg("", "", w, h, 49);
  const svgSourceBad = createSvg("#990000", "", w, h, 48);

  // draw GOOD svg in canvas
  renderCanvasAlternative(
    document.getElementById("myCanvas01"),
    svgSourceGood,
    w,
    h,
    "green"
  );

  // draw BAD svg in canvas
  renderCanvasAlternative(
    document.getElementById("myCanvas02"),
    svgSourceBad,
    w,
    h,
    "red"
  );
};

bootstrap();

<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8" />
  <meta http-equiv="X-UA-Compatible" content="IE=edge" />
  <meta name="viewport" content="width=device-width, initial-scale=1.0" />
  <title>Document</title>
</head>

<body>
  <div>SVG drawn in Canvas, Fontsize 49</div>
  <canvas id="myCanvas01"></canvas>
  <div>SVG drawn in Canvas, Fontsize 48</div>
  <canvas id="myCanvas02"></canvas>
  <script src="index.js"></script>
</body>

</html>

​

Answer 1

因为画布上下文也接受HTMLImageElement作为输入，所以在这里使用createImageBitmap()是多余的。相反，我们返回DOM加载的<img>本身，从而绕过createImageBitmap()，这显然会导致锯齿状的边缘。多亏了凯伊多。

const createImageFromSvg = async function(svgSrc: string): Promise < HTMLImageElement > {
  return new Promise(async resolve => {
    // replace assets with their base64 versions in svg source code 
    const svgWithAssetsEmbedded = await _embedAssets(svgSrc);

    // create blob from that
    const svgBlob = new Blob([svgWithAssetsEmbedded], {
      type: 'image/svg+xml;charset=utf-8'
    });

    // create URL that can be used in HTML (?)
    const svgBase64 = URL.createObjectURL(svgBlob);

    let img = new Image();
    img.onload = async() => {
      resolve(img);
    };
    img.src = svgBase64;
  });
};