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Re: [csswg-drafts] [css-shapes] Specify algorithms for computing line-box intrusion into float's margin box.

From: Amelia Bellamy-Royds via GitHub <sysbot+gh@w3.org>
Date: Thu, 26 Jul 2018 18:59:45 +0000
To: public-css-archive@w3.org
Message-ID: <issue_comment.created-408200818-1532631584-sysbot+gh@w3.org>
To summarize:

There are two algorithms that are imprecise:

- How to expand a shape by a given margin.  For definitions, you may want to look at the SVG stroke-shape algorithms, but beware that this is an "outside" stroke which isn't yet explicitly defined in SVG (although it's an outside stroke on an always-closed shape, so that helps). See https://svgwg.org/specs/strokes/

  That said, shape-outside also needs to apply to shapes generated by the opacity threshold on an image, so the definition needs to work even if you don't have a crisp mathematical definition of the shape.

- Given a final shape (with margin included), how do you trim line boxes to fit up against it?  E.g., in the diagram in [Example 7 in the spec](https://drafts.csswg.org/css-shapes/#shapes-from-image), it looks like the corners of the 1em-high basic line boxes are fitted tight against the final float shape, but expansions of the line box from line leading is ignored.  Now, that's probably just a matter of making a nice diagram rather than a conscious choice. But when writing the code, you need to know whether you need to leave space for the half-leading on either side of the line of text.

  And as Tab points out, then there's the question of how to find the intersection point of a rectangle and an arbitrary shape (which may not have a mathematical definition) in an algorithmically efficient way. Especially when you factor in that each rectangle has a minimum content width associated with it, and that you may need to slide the rectangle down until you find a place where it can fit.

Given that neither of these calculations have crisp, clear algorithms for finding the optimal solution, I'd be hesitant about baking a full algorithm into the spec. But I do agree that there should probably be some clarifications.

GitHub Notification of comment by AmeliaBR
Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/2949#issuecomment-408200818 using your GitHub account
Received on Thursday, 26 July 2018 18:59:53 UTC

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