HTML5 Canvas Sizing: CSS or Element Attributes – Which Method Should You Use?

Assessing HTML5 Canvas Size: CSS vs. Element Attributes

In HTML5, the size of a canvas element can be determined either through CSS properties or element attributes. However, these approaches yield distinct results, leading to confusion.

Disparity in Result

When using CSS to define the canvas size, the canvas shrinks or expands as the web page is zoomed in or out. On the other hand, specifying the canvas size directly through element attributes, such as width and height, produces a consistent display regardless of zoom level.

Understanding the Mechanism

To grasp the difference, consider the following:

• CSS properties control the visual representation of the canvas on the screen. When you set width and height in CSS, you define the size at which the canvas appears, but not the actual number of pixels available for drawing.
• Element attributes determine the internal size of the canvas, defining the actual pixel resolution available for drawing. By default, an unspecified canvas has a resolution of 300px in width and 150px in height.

Practical Implications

The distinction between CSS size and element attribute size has practical implications:

• If you need to display a canvas at a specific resolution and maintain that resolution irrespective of zoom level, specify the size using element attributes.
• If you intend to scale the canvas while preserving aspect ratio during zoom, use CSS properties to set the size.

Visualization

To illustrate the difference, visit http://jsfiddle.net/9bheb/5/, where you can observe how a canvas with a CSS size of 800px x 16px and an element attribute size of 32px x 32px behaves differently under zooming.

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