2d.text.measure.actualBoundingBox.small-font.html

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This WPT test may be referenced by the following Test IDs:
- /html/canvas/element/text/2d.text.measure.actualBoundingBox.small-font.html - WPT Dashboard Interop Dashboard

<!DOCTYPE html>

<meta charset="UTF-8">

<title>Canvas test: 2d.text.measure.actualBoundingBox.small-font</title>

<script src="/resources/testharness.js"></script>

<script src="/resources/testharnessreport.js"></script>

<script src="/html/canvas/resources/canvas-tests.js"></script>

<link rel="stylesheet" href="/html/canvas/resources/canvas-tests.css">

<style>

@font-face {

  font-family: CanvasTest;

  src: url("/fonts/CanvasTest.ttf");

</style>

<body class="show_output">

<h1>2d.text.measure.actualBoundingBox.small-font</h1>

<p class="desc">Testing that actualBoundingBox metrics are precise enough to center text at small font sizes</p>

<span style="font-family: CanvasTest; position: absolute; visibility: hidden">A</span>

<p class="output">Actual output:</p>

<canvas id="c" class="output" width="64" height="64"><p class="fallback">FAIL (fallback content)</p></canvas>

<ul id="d"></ul>

<script>

promise_test(async t => {

  var canvas = document.getElementById('c');

  var ctx = canvas.getContext('2d');

  await document.fonts.ready;

  // Fill background with red.

  ctx.fillStyle = 'red';

  ctx.fillRect(0, 0, canvas.width, canvas.height);

  // Move origin to center and scale up so small font fills the canvas.

  ctx.translate(canvas.width / 2, canvas.height / 2);

  ctx.scale(32, 32);

  // Use a small font size and center text at the origin using

  // actualBoundingBox metrics. If the metrics are integer-quantized,

  // the centering offset will be wrong and the text will not appear

  // at the center of the canvas.

  ctx.font = '1.5px CanvasTest';

  ctx.fillStyle = 'yellow';

  const text = 'A';

  const m = ctx.measureText(text);

  const drawX = -(m.actualBoundingBoxLeft + m.actualBoundingBoxRight) / 2

                + m.actualBoundingBoxLeft;

  const drawY = (m.actualBoundingBoxAscent + m.actualBoundingBoxDescent) / 2

                - m.actualBoundingBoxDescent;

  ctx.textAlign = 'left';

  ctx.textBaseline = 'alphabetic';

  ctx.fillText(text, drawX, drawY);

  // The center pixel should be yellow (text), not red (background).

  // Yellow has a high green channel; red background has green=0.

  var pixel = _getPixel(canvas, canvas.width / 2, canvas.height / 2);

  _assert(pixel[1] > 128,

    'Center pixel green channel (' + pixel[1] + ') should be > 128 (yellow text, not red background)');

}, "actualBoundingBox centers text correctly at small font sizes");

</script>