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How to compute the height of a character at given position in Text using FabricJS?
In this tutorial, we are going to learn how to compute the height of a line at a specific line index in Text using FabricJS. We can display text on canvas by adding an instance of fabric.Text. Not only does it allow us to move, scale and change the dimensions of the text but it also provides additional functionality like text alignment, text decoration, and line height which can be obtained by the properties textAlign, underline and lineHeight respectively. We can also compute the height of a line at a required line index by using the getHeightOfLine method.
Syntax
getHeightOfLine(lineIndex: Number)
Parameters
lineIndex ? This parameter accepts a Number as value. The value determines the index of the line whose height we want to calculate.
Example 1: Using the getHeightOfLine Method
Let's see a code example to see the logged output when the getHeightOfLine method is used. In this case, the height of the line at the 1st line index will be displayed in the console.
<!DOCTYPE html>
<html>
<head>
<!-- Adding the Fabric JS Library-->
<script src="https://cdnjs.cloudflare.com/ajax/libs/fabric.js/510/fabric.min.js"></script>
</head>
<body>
<h2>Using the getHeightOfLine method</h2>
<p>You can open console from dev tools and see that the height of line is being displayed in the console </p>
<canvas id="canvas"></canvas>
<script>
// Initiate a canvas instance
var canvas = new fabric.Canvas("canvas");
canvas.setWidth(document.body.scrollWidth);
canvas.setHeight(250);
// Initiate a text object
var text = new fabric.Text("Add sample\ntext here", {
width: 300,
fill: "green",
fontWeight: "bold",
});
// Add it to the canvas
canvas.add(text);
// Using getHeightOfLine method
console.log("The height of line is", text.getHeightOfLine(1));
</script>
</body>
</html>
The height of line is 16
Example 2: Using Different Font Size
Let's see a code example to see the logged output when the getHeightOfLine method is used along with a different font size of 30. In this case, the height of the line at the 0th line index will be displayed for the given font size.
<!DOCTYPE html>
<html>
<head>
<!-- Adding the Fabric JS Library-->
<script src="https://cdnjs.cloudflare.com/ajax/libs/fabric.js/510/fabric.min.js"></script>
</head>
<body>
<h2>Using the getHeightOfLine method and passing a different font size</h2>
<p>You can open console from dev tools and see that the height of line is being displayed in the console </p>
<canvas id="canvas"></canvas>
<script>
// Initiate a canvas instance
var canvas = new fabric.Canvas("canvas");
canvas.setWidth(document.body.scrollWidth);
canvas.setHeight(250);
// Initiate a text object
var text = new fabric.Text("Add sample\ntext here", {
width: 300,
fill: "green",
fontWeight: "bold",
fontSize: 30
});
// Add it to the canvas
canvas.add(text);
// Using getHeightOfLine method
console.log("The height of line is", text.getHeightOfLine(0));
</script>
</body>
</html>
The height of line is 30
How It Works
The getHeightOfLine method calculates the height of a specific text line based on the font properties like fontSize, lineHeight, and font family. The line height is typically equal to or slightly larger than the font size, depending on the font metrics and any applied line height multiplier.
Key Points
- Line index starts from 0 (first line is index 0)
- The method returns the height in pixels
- Font size directly affects the line height calculation
- Multi-line text can have different heights for different lines if formatting varies
Conclusion
The getHeightOfLine method in FabricJS is useful for calculating precise line heights in text objects. This is particularly helpful when working with multi-line text layouts or when you need to position elements relative to specific text lines.
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