JavaScript Canvas Scale Tutorial
last modified April 3, 2025
In this article, we explore the Canvas scale transformation in JavaScript. Scaling allows you to resize drawings proportionally or disproportionately. Understanding scaling is crucial for creating responsive graphics.
Basic Definition
Canvas scaling modifies the size of subsequent drawings by multiplying their
dimensions. The scale(x, y) method takes two parameters for
horizontal and vertical scaling factors.
Scaling affects all drawing operations after it's called. Values greater than 1 enlarge, while values between 0 and 1 shrink drawings. Negative values flip and scale the content.
Basic Scaling
This example demonstrates how to scale a simple rectangle on canvas.
<!DOCTYPE html>
<html>
<head>
<title>Basic Canvas Scaling</title>
</head>
<body>
<canvas id="myCanvas" width="300" height="200"></canvas>
<script>
const canvas = document.getElementById('myCanvas');
const ctx = canvas.getContext('2d');
// Original rectangle
ctx.fillStyle = 'blue';
ctx.fillRect(50, 50, 50, 50);
// Scaled rectangle
ctx.fillStyle = 'red';
ctx.scale(2, 1.5);
ctx.fillRect(50, 50, 50, 50);
</script>
</body>
</html>
This example draws two rectangles - one blue (original) and one red (scaled).
The scale(2, 1.5) makes the red rectangle twice as wide and 1.5
times taller than the original.
Note that scaling affects both the size and position of drawings. The red rectangle appears larger and shifted because scaling applies to coordinates.
Scaling from Center
This example shows how to scale an object while keeping it centered.
<!DOCTYPE html>
<html>
<head>
<title>Centered Scaling</title>
</head>
<body>
<canvas id="myCanvas" width="300" height="200"></canvas>
<script>
const canvas = document.getElementById('myCanvas');
const ctx = canvas.getContext('2d');
const centerX = canvas.width / 2;
const centerY = canvas.height / 2;
const size = 50;
// Original square
ctx.fillStyle = 'rgba(0, 0, 255, 0.5)';
ctx.fillRect(centerX - size/2, centerY - size/2, size, size);
// Scaled square
ctx.fillStyle = 'rgba(255, 0, 0, 0.5)';
ctx.translate(centerX, centerY);
ctx.scale(1.8, 1.8);
ctx.translate(-centerX, -centerY);
ctx.fillRect(centerX - size/2, centerY - size/2, size, size);
</script>
</body>
</html>
To scale from center, we first translate to the center point, apply scaling, then translate back. This technique maintains the object's center position.
The blue square is original, while the red square is scaled 1.8 times in both directions. The semi-transparent colors show how scaling affects the shape.
Asymmetric Scaling
This example demonstrates different scaling factors for width and height.
<!DOCTYPE html>
<html>
<head>
<title>Asymmetric Scaling</title>
</head>
<body>
<canvas id="myCanvas" width="300" height="200"></canvas>
<script>
const canvas = document.getElementById('myCanvas');
const ctx = canvas.getContext('2d');
// Original circle
ctx.beginPath();
ctx.arc(75, 100, 30, 0, Math.PI * 2);
ctx.fillStyle = 'blue';
ctx.fill();
// Scaled ellipse
ctx.scale(2, 0.5);
ctx.beginPath();
ctx.arc(75, 100, 30, 0, Math.PI * 2);
ctx.fillStyle = 'red';
ctx.fill();
</script>
</body>
</html>
Here we scale a circle into an ellipse using different x and y scale factors.
The scale(2, 0.5) makes it twice as wide but half as tall.
The blue circle is original, while the red ellipse shows the scaled result. This demonstrates how to create non-uniform scaling effects on canvas.
Scaling Text
This example shows how scaling affects text rendering on canvas.
<!DOCTYPE html>
<html>
<head>
<title>Scaling Text</title>
</head>
<body>
<canvas id="myCanvas" width="400" height="200"></canvas>
<script>
const canvas = document.getElementById('myCanvas');
const ctx = canvas.getContext('2d');
// Original text
ctx.font = '20px Arial';
ctx.fillText('Original Text', 50, 50);
// Uniformly scaled text
ctx.scale(1.5, 1.5);
ctx.fillText('Scaled 1.5x', 50, 100);
// Reset transform
ctx.setTransform(1, 0, 0, 1, 0, 0);
// Distorted text
ctx.scale(1.8, 0.7);
ctx.fillText('Distorted', 50, 150);
</script>
</body>
</html>
This example shows three text variations: original, uniformly scaled, and distorted. Scaling affects all canvas drawing operations including text.
Note the use of setTransform to reset scaling between operations.
The distorted text shows how asymmetric scaling can create unique typography.
Multiple Scaling Operations
This example demonstrates cumulative scaling effects with multiple operations.
<!DOCTYPE html>
<html>
<head>
<title>Multiple Scaling</title>
</head>
<body>
<canvas id="myCanvas" width="400" height="300"></canvas>
<script>
const canvas = document.getElementById('myCanvas');
const ctx = canvas.getContext('2d');
// Base rectangle
ctx.fillStyle = 'blue';
ctx.fillRect(50, 50, 50, 50);
// First scaling
ctx.fillStyle = 'red';
ctx.scale(1.5, 1.5);
ctx.fillRect(50, 50, 50, 50);
// Second scaling (cumulative)
ctx.fillStyle = 'green';
ctx.scale(0.8, 1.2);
ctx.fillRect(50, 50, 50, 50);
</script>
</body>
</html>
This example shows how scaling operations accumulate. Each scale
call multiplies with previous transformations rather than replacing them.
The blue rectangle is original, red is scaled 1.5x, and green has additional scaling. The final scaling factors are 1.5×0.8=1.2 for width and 1.5×1.2=1.8 for height.
Source
MDN Canvas scale Documentation
In this article, we have explored various techniques for scaling drawings on HTML canvas. Mastering scaling transformations is essential for creating flexible and responsive graphics in web applications.
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