在画布上渲染 svg 图像

Rendering svg images on a canvas

本文关键字：svg 图像      更新时间：2023-09-26

>我正在尝试在画布上渲染SVG图像，一次绘制一个图像以填充给定的行，以下是相同的代码片段：

  function createSVGUrl(svg) {
    var svgBlob = new Blob([svg], {type: 'image/svg+xml;charset=utf-8'});
    return DOMURL.createObjectURL(svgBlob);
  };
/**
   * Renders svg tile on the given context.
   * @param {CanvasRenderingContext2D} ctx
   * @param {SVGElement} svg The svg tile.
   * @param {{x: number, y:number}} pos The position to draw the svg tile on.
   * @throws Error
   */
  function renderSVGTile(ctx, svg, pos) {
    var img = new Image();
    var url = createSVGUrl(svg);
    img.onload = function() {
      try {
        ctx.drawImage(img, pos.x, pos.y);
        ctx.imageSmoothingEnabled = false;
        ctx.mozImageSmoothingEnabled = false;
        DOMURL.revokeObjectURL(url);
      } catch (e) {
        throw new Error('Could not render image' + e);
      }
    };
    img.src = url;
  };

问题是我可以看到我不想要的部分填充行，有没有办法一次填充整行？

是的。首先将整排磁贴绘制到屏幕外的画布上。完成后，您可以将屏幕外画布绘制到主画布上。

像这样：

var offscreenCanvas = document.createElement('canvas');
offscreenCanvas .width = <whatever>;
offscreenCanvas .height = <whatever>;
var offscreenContext = offscreenCanvas.getContext('2d');
// Draw all your tiles
renderSVGTile(offscreenContext, svg, pos);
//... loop through all tiles etc
// When finished...
mainCanvasContext.drawImage(offscreenCanvas, 0, 0);

演示：

var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var image = document.getElementById("source");
var offscreenCanvas = document.createElement("canvas");
offscreenCanvas.width = 300;
offscreenCanvas.height = 150;
var offscreenContext = offscreenCanvas.getContext("2d");
offscreenContext.drawImage(image, 33, 71, 104, 124, 21, 20, 87, 104);
offscreenContext.drawImage(image, 33, 71, 104, 124, 108, 20, 87, 104);
ctx.drawImage(offscreenCanvas, 0, 0);
<canvas id="canvas"></canvas>
<div style="display:none;">
  <img id="source" src="http://placekitten.com/300/227"
       width="300" height="227">
</div>

这就是我的做法，尽管我不确定它是否适合您的目的，因为我在渲染时文档中有图像，它只是隐藏了。

var ctx = document.getElementById("canvasID").getContext("2d");
ctx.drawImage(document.getElementById("imageID"), x,y,w,h);

下面的解决方案对我来说工作正常： 

 /**
   * @param {CanvasRenderingContext2D} ctx
   * @param {!Array<!SVGTile>} tiles
   */
  function drawTiles(ctx, tiles) {
    var canvas  = document.createElement('canvas');
    var width   = tiles.length * TILE_WIDTH;
    var height  = TILE_HEIGHT;
    var y = tiles[0].y;
    canvas.width  = tiles.length * TILE_WIDTH;
    canvas.height = TILE_HEIGHT;
    var context = canvas.getContext("2d");
    tiles.forEach(function(tile, index) {
      renderTile(context, tile, function() {
        if (tiles.length === index + 1) {
          ctx.drawImage(canvas, 0, y);
        }
      });
    });
    // TODO: Below code is for testing purpose.
    var temp = document.createElement('div');
    temp.appendChild(canvas);
    document.body.appendChild(temp);
  };

  /**
   * Renders svg tile on the given context.
   * @param {CanvasRenderingContext2D} ctx
   * @param {!Tile} tile The tile to render.
   * @param {function()} callback To be called after image is loaded.
   * @throws Error
   */
  function renderTile(ctx, tile, callback) {
    var img = new Image();
    img.onload = function() {
      try {
        ctx.drawImage(this, tile.x, 0, tile.width, tile.height);
        ctx.imageSmoothingEnabled = false;
        ctx.mozImageSmoothingEnabled = false;
        DOMURL.revokeObjectURL(tile.svgURL);
        callback();
      } catch (e) {
        throw new Error('Could not render image' + e);
      }
    };
    img.src = tile.svgURL;
  };
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