使用HTML5畫布上下文應用轉換和動畫。

Canvas動畫與變換的關鍵在於理解上下文狀態管理。 1. 所有變換（如旋轉、縮放、平移）作用於整個canvas上下文而非單個對象；2. 使用ctx.save()和ctx.restore()保存和恢復上下文狀態，避免變換相互干擾；3. 動畫需在requestAnimationFrame循環中更新狀態並清除舊幀；4. 複合變換順序至關重要，例如先平移再旋轉可實現繞中心點轉動；5. 為保證精度，應基於時間差計算動畫增量並及時重置上下文狀態。

Canvas animations and transformations can be a bit tricky if you're just getting started, but once you understand the basics of how the canvas context works, it becomes much more intuitive. The key thing to remember is that all tr​​ansformations (like rotation, scaling, or translation) affect the entire canvas context — not individual objects or shapes. This has implications for how you structure your animation code.

Understanding the Canvas Context

The HTML5 Canvas API doesn't keep track of anything you draw after it's rendered. It's like a painter's canvas — once paint is applied, there's no built-in way to manipulate it unless you redraw everything. This means that when you apply a transformation like ctx.rotate() or ctx.scale() , it affects everything drawn afterward until you reset or modify the transformation state again.

Think of the canvas context as having its own coordinate system. When you rotate or scale, you're changing this coordinate system. So, for example, rotating by 45 degrees then drawing a rectangle will result in a rotated rectangle — because the drawing happens under the new coordinate system.

Here are a few important methods:

• ctx.translate(x, y) — moves the origin point
• ctx.rotate(angle) — rotates around the current origin
• ctx.scale(x, y) — scales drawings along x and y axes

Saving and Restoring Context State

Since transformations are cumulative and affect everything drawn afterward, it's easy to end up with unexpected results if you don't manage your transformations carefully. That's where ctx.save() and ctx.restore() come in handy.

You can think of ctx.save() as taking a snapshot of the current context settings (including transformations, stroke/fill styles, etc.), and ctx.restore() rolls back to the most recently saved state. This is particularly useful when applying temporary transformations to individual objects while keeping the rest of the scene unaffected.

For example, if you're animating multiple spinning shapes independently, wrapping each shape's drawing code between ctx.save() and ctx.restore() ensures their rotations don't interfere with each other.

 ctx.save();
ctx.translate(x, y);
ctx.rotate(rotationAngle);
drawShape(); // eg, a triangle centered at (x, y)
ctx.restore();

This pattern is common in canvas animation libraries and custom render loops.

Animating Transformations

To animate canvas content, you typically use requestAnimationFrame to loop through frames, updating positions, angles, or scales each time. For smooth movement, it helps to base changes on elapsed time rather than assuming fixed frame intervals.

Let's say you want to spin an object smoothly:

• Initialize a variable like rotation = 0
• In your animation loop:
  • Clear the canvas
  • Update rotation = angularSpeed * deltaTime
  • Use ctx.save() , apply rotation, draw object
  • Then ctx.restore()

A common mistake is forgetting to clear the canvas at the start of each frame. If you don't, old frames stay visible, which may look intentional in some cases but usually leads to messy visuals.

Also, make sure your animation loop resets transformations properly. Otherwise, small errors accumulate over time (especially with floating-point math), causing visual drift.

Working with Composite Transformations

Sometimes you need to combine multiple transformations — like moving, rotating, and resizing an object. The order in which you apply these matters a lot.

For instance, translating then rotating gives a different result than rotating then translating. That's because each transformation builds on the previous one's coordinate system.

If you want to rotate an object around its center:

1. Translate to the object's center
2. Rotate
3. Draw the object offset by half its width/height so the center aligns

 ctx.save();
ctx.translate(x width / 2, y height / 2);
ctx.rotate(rotation);
ctx.fillRect(-width / 2, -height / 2, width, height);
ctx.restore();

This approach keeps things clean and predictable. If you skip the centering step, the object will rotate around the canvas origin instead, which might not be what you intended.

Transformations and animations in canvas aren't hard once you get used to working with the context state and managing it properly. Keep your animation loop simple, use save and restore liberally, and always test how transformations stack up.基本上就這些。

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