Canvas Resize Method Comparison (via putImageData)

README.md

Canvas Resize Method Comparison (via putImageData)

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index.css

body {
  margin: 0;
  background-color: white;
}
.outer {
  margin: 0 auto;
  width: 80%;
}
canvas {
  width: 100%;
  height: 100px;
  display: block;
}
.bad {
  color: red;
  font-weight: bold;
}

index.html

<div class="outer">
  <div>using round(clientWidth * devicePixelRatio)</div>
  <canvas id="use-clientWidth"></canvas>
  <div>using round(getBoundingClientRect().width * devicePixelRatio)</div>
  <canvas id="use-getBoundingClientRect"></canvas>
  <div>using ResizeObserver (<span id="dpr-support"></span>)</div>
  <canvas id="use-ResizeObserver"></canvas>
  <div id="warning" style="color: red; display: none">(warning: no support for device-pixel-content-box)</div>
</div>

index.js

// Canvas Resize Method Comparison
// from https://webglfundamentals.org/webgl/webgl-resize-the-canvas-comparison.html


'use strict';

function setupCanvas(canvas, resizeFn) {
  const ctx = canvas.getContext('2d');
  const drawFn = (displayWidth, displayHeight) => {
    canvas.width = displayWidth;
    canvas.height = displayHeight;
    const imgData = ctx.createImageData(displayWidth, displayHeight);
    const u32View = new Uint32Array(imgData.data.buffer);
    u32View.fill(0xFFFFFFFF);
    for (let y = 0; y < displayHeight; y += 2) {
      for (let x = 0; x < displayWidth / 2; ++x) {
        u32View[y * displayWidth + x] = 0xFF000000;
      }
    }
    for (let x = displayWidth / 2 | 0; x < displayWidth; x += 2) {
      for (let y = 0; y < displayHeight; ++y) {
        u32View[y * displayWidth + x] = 0xFF000000;
      }
    }
    ctx.putImageData(imgData, 0, 0);
  };
  resizeFn(canvas, drawFn);
}

setupCanvas(document.querySelector('#use-clientWidth'), (canvas, drawFn) => {
  const resizeAndDraw = () => {
    const dpr = window.devicePixelRatio;
    const displayWidth = Math.round(canvas.clientWidth * dpr);
    const displayHeight = Math.round(canvas.clientHeight * dpr);
    drawFn(displayWidth, displayHeight);
  };
  resizeAndDraw();
  window.addEventListener('resize', resizeAndDraw);
});

setupCanvas(document.querySelector('#use-getBoundingClientRect'), (canvas, drawFn) => {
  const resizeAndDraw = () => {
    const dpr = window.devicePixelRatio;
    const {width, height} = canvas.getBoundingClientRect();
    const displayWidth = Math.round(width * dpr);
    const displayHeight = Math.round(height * dpr);
    drawFn(displayWidth, displayHeight);
  };
  resizeAndDraw();
  window.addEventListener('resize', resizeAndDraw);
});

setupCanvas(document.querySelector('#use-ResizeObserver'), (canvas, drawFn) => {
  const canvasToDisplaySizeMap = new Map([[canvas, [300, 150]]]);
  const dprSupportElem = document.querySelector('#dpr-support');

  const resizeAndDraw = () => {
    const [displayWidth, displayHeight] = canvasToDisplaySizeMap.get(canvas);
    drawFn(displayWidth, displayHeight);
  };

  function onResize(entries) {
    for (const entry of entries) {
      let width;
      let height;
      let dpr = window.devicePixelRatio;
      let dprSupport = false;
      if (entry.devicePixelContentBoxSize) {
        // NOTE: Only this path gives the correct answer
        // The other paths are an imperfect fallback
        // for browsers that don't provide anyway to do this
        width = entry.devicePixelContentBoxSize[0].inlineSize;
        height = entry.devicePixelContentBoxSize[0].blockSize;
        dpr = 1; // it's already in width and height
        dprSupport = true;
      } else if (entry.contentBoxSize) {
        if (entry.contentBoxSize[0]) {
          width = entry.contentBoxSize[0].inlineSize;
          height = entry.contentBoxSize[0].blockSize;
        } else {
          // legacy
          width = entry.contentBoxSize.inlineSize;
          height = entry.contentBoxSize.blockSize;
        }
      } else {
        // legacy
        width = entry.contentRect.width;
        height = entry.contentRect.height;
      }
      const displayWidth = Math.round(width * dpr);
      const displayHeight = Math.round(height * dpr);
      canvasToDisplaySizeMap.set(entry.target, [displayWidth, displayHeight]);
      resizeAndDraw();
      dprSupportElem.textContent = dprSupport ? "dpr supported 😀" : "dpr not supported 😢";
      dprSupportElem.classList.toggle('bad', !dprSupport);
    }
  }

  resizeAndDraw();
  const resizeObserver = new ResizeObserver(onResize);
  resizeObserver.observe(canvas, {box: 'content-box'});
});

jsGist.json

{"name":"Canvas Resize Method Comparison (via putImageData)","settings":{},"filenames":["index.html","index.css","index.js"]}