## How to Achieve Smooth Zoom-to-Cursor Functionality in HTML5 Canvas?

Maintaining Zoom Focus Around the Mouse Cursor

When navigating images in HTML5, zooming around the mouse cursor, as seen in Google Maps, adds an intuitive user experience. But calculating this movement can be perplexing.

Approach:

To zoom towards the cursor:

• Translate: Shift the canvas context by the cursor offset.
• Scale: Zoom by a desired factor.
• Reverse Translate: Shift back by the opposite of the cursor offset.

Calculation:

Given the image position and dimensions, and the cursor coordinates relative to the canvas center:

• Convert the cursor position into the transformed canvas context: dx = (cursorX - canvasCenterX) * scale; dy = (cursorY - canvasCenterY) * scale;
• Translate: ctx.translate(dx, dy);
• Scale: ctx.scale(factor, factor);
• Reverse Translate: ctx.translate(-dx, -dy);

Demonstration:

Visit the provided demo link: http://phrogz.net/tmp/canvas_zoom_to_cursor.html.

This example features dragging, click-to-zoom, shift-click-to-zoom, and scroll wheel zooming.

Note:

Currently, Safari's zoom speed might differ from Chrome or Firefox.

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