设置.size与.width/.height属性时矩形居中差异的技术问询

Great question! This is a common point of confusion, and it all boils down to how most graphics or UI frameworks handle convenience methods versus direct property assignments. Let’s break it down step by step:

1. Why .size() keeps your rectangle centered #

The .size() method is designed as an all-in-one utility that does more than just set width and height. Behind the scenes, it typically:

• Updates the rectangle’s width and height values
• Automatically adjusts the rectangle’s position to preserve its original anchor point (like a center point)

For example, if you’re using a framework where your rectangle is anchored to its center (say, p5.js with rectMode(CENTER)), calling .size(newW, newH) will recalculate the rectangle’s position so its center stays exactly where it was before resizing. This creates the visual effect of the rectangle scaling while remaining centered.

Think of it like using a "resize and keep centered" button in a design tool—it handles both the size change and position adjustment for you in one go.

2. Why .width/.height don’t center the rectangle #

.width and .height are usually direct, low-level properties that only modify the rectangle’s dimensions. They don’t trigger any additional layout or position logic.

If your rectangle is positioned from its top-left corner (the default in many systems), changing just these properties will make the rectangle grow or shrink outward from that top-left point. The rectangle’s center will shift, so it won’t stay aligned with its original center. Even if your anchor is the center, most frameworks won’t automatically update the position when you only tweak width or height—you have to handle that manually.

Quick workaround to center when using .width/.height #

If you want to resize with .width/.height and keep the rectangle centered, you’ll need to adjust the position yourself. Here’s a simple example (using JavaScript-like syntax):

// Assume we know the original center coordinates (centerX, centerY)
rect.width = newWidth;
rect.height = newHeight;
// Recalculate top-left position to keep the center fixed
rect.x = centerX - (newWidth / 2);
rect.y = centerY - (newHeight / 2);

内容的提问来源于stack exchange，提问作者Kai