How to Zoom a Canvas to the Mouse Cursor Position in HTML5?

Zooming a Canvas to the Mouse Cursor

When creating an interactive HTML5 project involving zooming, aligning the zoom with the cursor's position is crucial for a user-friendly experience. This article aims to guide you through the calculations necessary to implement this functionality.

Problem:

How can we zoom a canvas relative to the cursor's position, similar to Google Maps?

Variables Available:

• Image coordinates (x, y)
• Image dimensions (width, height)
• Cursor coordinates relative to the canvas center (cursor_x, cursor_y)

Solution:

1. Translate the Canvas: Shift the canvas context based on the cursor's offset, allowing zooming around the cursor.
2. Scale the Canvas: Adjust the scale factor to zoom in or out.
3. Translate Back: Return the canvas context to its original position, compensating for the initial translation.

Here's the code snippet:

<code class="javascript">ctx.translate(cursor_x, cursor_y);
ctx.scale(factor, factor);
ctx.translate(-cursor_x, -cursor_y);</code>

Example in Action:

Visit http://phrogz.net/tmp/canvas_zoom_to_cursor.html for a live demonstration. It supports dragging and zooming with mouse clicks and the scroll wheel.

Note:

• For accurate zooming, transform the cursor position from screen space to the transformed canvas context.
• Currently, Safari's zooming behavior differs from other browsers, resulting in excessive zooming.

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