TextMetrics - Web APIs 编辑

The TextMetrics interface represents the dimensions of a piece of text in the canvas; a TextMetrics instance can be retrieved using the CanvasRenderingContext2D.measureText() method.

Properties

TextMetrics.width Read only

Is a double giving the calculated width of a segment of inline text in CSS pixels. It takes into account the current font of the context.

TextMetrics.actualBoundingBoxLeft Read only

Is a double giving the distance from the alignment point given by the CanvasRenderingContext2D.textAlign property to the left side of the bounding rectangle of the given text, in CSS pixels. The distance is measured parallel to the baseline.

TextMetrics.actualBoundingBoxRight Read only

Is a double giving the distance from the alignment point given by the CanvasRenderingContext2D.textAlign property to the right side of the bounding rectangle of the given text, in CSS pixels. The distance is measured parallel to the baseline.

TextMetrics.fontBoundingBoxAscent Read only

Is a double giving the distance from the horizontal line indicated by the CanvasRenderingContext2D.textBaseline attribute to the top of the highest bounding rectangle of all the fonts used to render the text, in CSS pixels.

TextMetrics.fontBoundingBoxDescent Read only

Is a double giving the distance from the horizontal line indicated by the CanvasRenderingContext2D.textBaseline attribute to the bottom of the bounding rectangle of all the fonts used to render the text, in CSS pixels.

TextMetrics.actualBoundingBoxAscent Read only

Is a double giving the distance from the horizontal line indicated by the CanvasRenderingContext2D.textBaseline attribute to the top of the bounding rectangle used to render the text, in CSS pixels.

TextMetrics.actualBoundingBoxDescent Read only

Is a double giving the distance from the horizontal line indicated by the CanvasRenderingContext2D.textBaseline attribute to the bottom of the bounding rectangle used to render the text, in CSS pixels.

TextMetrics.emHeightAscent Read only

Is a double giving the distance from the horizontal line indicated by the CanvasRenderingContext2D.textBaseline property to the top of the em square in the line box, in CSS pixels.

TextMetrics.emHeightDescent Read only

Is a double giving the distance from the horizontal line indicated by the CanvasRenderingContext2D.textBaseline property to the bottom of the em square in the line box, in CSS pixels.

TextMetrics.hangingBaseline Read only

Is a double giving the distance from the horizontal line indicated by the CanvasRenderingContext2D.textBaseline property to the hanging baseline of the line box, in CSS pixels.

TextMetrics.alphabeticBaseline Read only

Is a double giving the distance from the horizontal line indicated by the CanvasRenderingContext2D.textBaseline property to the alphabetic baseline of the line box, in CSS pixels.

TextMetrics.ideographicBaseline Read only

Is a double giving the distance from the horizontal line indicated by the CanvasRenderingContext2D.textBaseline property to the ideographic baseline of the line box, in CSS pixels.

Examples

Baselines illustrated

This example demonstrates the baselines the TextMetrics object holds. The default baseline is the alphabeticBaseline. See also the CanvasRenderingContext2D.textBaseline property.

HTML

<canvas id="canvas" width="550" height="500"></canvas>

JavaScript

const canvas = document.getElementById('canvas');
const ctx = canvas.getContext('2d');

const baselinesAboveAlphabetic = ['fontBoundingBoxAscent', 'actualBoundingBoxAscent',
                   'emHeightAscent', 'hangingBaseline'];
const baselinesBelowAlphabetic = ['ideographicBaseline', 'emHeightDescent',
                   'actualBoundingBoxDescent', 'fontBoundingBoxDescent'];
const baselines = [...baselinesAboveAlphabetic, ...baselinesBelowAlphabetic];
ctx.font = '25px serif';
ctx.strokeStyle = 'red';

baselines.forEach(function (baseline, index) {
  let text = 'Abcdefghijklmnop (' + baseline + ')';
  let textMetrics = ctx.measureText(text);
  let y = 50 + index * 50;
  ctx.beginPath();
  ctx.fillText(text, 0, y);
  let lineY = y - Math.abs(textMetrics[baseline]);
  if (baselinesBelowAlphabetic.includes(baseline)) {
    lineY = y + Math.abs(textMetrics[baseline]);
  }
  ctx.moveTo(0, lineY);
  ctx.lineTo(550, lineY);
  ctx.stroke();

});

Result

Measuring text width

When measuring the x-direction of a piece of text, the sum of actualBoundingBoxLeft and actualBoundingBoxRight can be wider than the width of the inline box (width), due to slanted/italic fonts where characters overhang their advance width.

It can therefore be useful to use the sum of actualBoundingBoxLeft and actualBoundingBoxRight as a more accurate way to get the absolute text width:

const canvas = document.getElementById('canvas');
const ctx = canvas.getContext('2d');
const text = 'Abcdefghijklmnop';
ctx.font = 'italic 50px serif';
const textMetrics = ctx.measureText(text);

console.log(textMetrics.width);
// 459.8833312988281

console.log(Math.abs(textMetrics.actualBoundingBoxLeft) +
            Math.abs(textMetrics.actualBoundingBoxRight));
// 462.8833333333333

Specifications

Browser compatibility

BCD tables only load in the browser

See also

• Creator method in CanvasRenderingContext2D
• The <canvas> element and its associated interface, HTMLCanvasElement