Anchor on rect needs doc clarification

[uben0]

I'm just beginning to use CeTZ (awesome job BTW 🤩) and I could not find how to center rectangles. Maybe it's just a skill issue 😶‍🌫️

#rect((3, 0), (0.8, 0.8), name: "b", anchor: "center")

Setting the anchor to center doesn't change the a position in the rectangle, it remains what seems to be south-west. Which makes sens as this is what the documentation says:

a: Coordinate of the bottom left corner of the rectangle.

I wrote a wrapper to bypass this limitation:

#let rectc(center, dim, ..param) = {
    let x = center.at(0) - dim.at(0) / 2
    let y = center.at(1) - dim.at(1) / 2
    rect((x, y), (rel: dim), ..param)
}

But this scratched my head. How could such a trivial problem could remain unnoticed for so long? It has to be a problem with my comprehension. So I experimented with different anchors and I was very surprised by the behavior, that looked, at first glance, chaotic and nonsensical. But at the end, I got it:

• a is bottom left
• b is top right
• the anchor applies a shift (or drag) to have the anchor point on the center (a + b) / 2

It makes sens after all. It is coherent. Knowing that, a rectangle can be centered by putting its anchor to north-east.

Request

I suggest to add a clarification in the documentation about this.

Or even maybe add an optional center: false parameter to change the meaning of a as the center instead of the bottom left corner. (Which is not equivalent to setting anchor to north-east)

[johannes-wolf]

Note: the second argument of rect is an absolute position! If you want to pass a size, use (rel: (width, height)). Using relative coordinates is the correct way to add multiple coordinates in cetz.

You can get centered rects if you use relative coordinates:

rect((rel: (-1,-1), to: (...center...)), (rel: (2,2)), name: "b")

[johannes-wolf]

Proposal: Change rect’s default anchor to its first parameter (a). This would allow the expected transformation to happen (anchor: "center"). The intersection code of line must be changed to use the center anchor instead of the default anchor.