Скачать или смотреть Understanding Why Your Box is Only Rotating and Not Moving to the Right in HTML Canvas

Dive into the reason your box is rotating instead of moving right in your HTML canvas. Learn to correctly implement rotation while ensuring movement using JavaScript.
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This video is based on the question https://stackoverflow.com/q/71295787/ asked by the user 'Ed Heal' ( https://stackoverflow.com/u/892256/ ) and on the answer https://stackoverflow.com/a/71296143/ provided by the user 'Sover the' ( https://stackoverflow.com/u/12777947/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Why is Your Box Rotating Instead of Moving Right?

If you've been experimenting with drawing on HTML canvas using JavaScript, you might have encountered an interesting issue: your box seems to be rotating in place instead of moving horizontally across the canvas. This can be confusing, especially if you were expecting a simple rightward movement. Let's break down this problem and provide a solution step-by-step.

The Initial Setup

In the original scenario, the code snippet you might have looked at is as follows:

[[See Video to Reveal this Text or Code Snippet]]

Key Points of the Original Code

Clear the Canvas: The canvas is cleared before each redraw to avoid overlapping drawings.

Rotation: The ctx.rotate() function is applied to rotate the box.

Movement Logic: box_x is incremented to create movement.

Understanding the Problem

The main issue arises from how the canvas operates when transformations like rotation are applied. When you use ctx.rotate(), you're rotating the entire canvas around the origin point (top-left corner). In this case, your box is not moving right as intended because you're rotating it around the original (0,0) point of the canvas rather than around its own top-left corner.

The Solution: Proper Transformations

To achieve the correct movement along with rotation, we need to adjust the transformation logic. Here's how you can do it:

Translate the Origin: Move the origin point of the canvas to the box's top-left corner using ctx.translate().

Apply Rotation: Rotate the canvas after you’ve adjusted its origin.

Reset Transformations: Undo the transformations afterward to ensure the next frame is drawn correctly.

Here's the corrected code snippet implementing these changes:

[[See Video to Reveal this Text or Code Snippet]]

Key Changes in the Code

The ctx.translate() method is used to correctly set the rotation point to the box's top-left corner.

After drawing, the rotation and translation transformations are reverted to maintain the correct position for subsequent frames.

Conclusion

By using the ctx.translate() method before performing a rotation, followed by reverting the transformations after drawing the rectangle, you can successfully create a box that not only rotates but also moves horizontally across the canvas. This adjustment clarifies the relationship between the canvas transformations and the box's movement, enhancing your understanding of how to manipulate canvas graphics with JavaScript effectively.

Happy coding!