Adding a ascent/descent setter in frame so that the original frame size is not lost in equation slackness adjustment #4318

LuxxxLucy · 2024-06-02T03:59:45Z

In equations, there is an adjustment of the frame size, where the frame height (size().y) is changed so that a large equation will be adjusted to not effect the later line layout (as it effects the line height)

let MathParItem::Frame(frame) = item else { continue };
 
             let font_size = scaled_font_size(&ctx, styles);
             let top_edge = TextElem::top_edge_in(styles).resolve(font_size, &font, None);
             let bottom_edge =
                 -TextElem::bottom_edge_in(styles).resolve(font_size, &font, None);
 
            let ascent = top_edge.max(frame.ascent() - slack);
            let descent = bottom_edge.max(frame.descent() - slack);
             frame.translate(Point::with_y(ascent - frame.baseline()));
             frame.size_mut().y = ascent + descent;
         }

However, if the frame.size().y is modified here, we will not be able to do decoration of the frame (see #2200) , because the frame size is not correct anymore. This PR adds the interface to set ascent/descent of the frame, and so that the line height determination will rely to the value of ascent and descent.

Below shows the result after this PR's change, the red line is the baseline, and the blue line is the ascent, while the background is true frame size. The frame size is not modified.

Note 1:

The change of this PR should also be relevant to #1028 #4236

Note 2:

this PR does not break any other test, except for #2220, the reason being the descent being changed. Before:

After

Enter-tainer · 2024-06-02T11:07:28Z

crates/typst/src/layout/frame.rs

+    /// The ascent of the frame. If this is `None`, the
+    /// frame's implicit ascent is the baseline.
+    ascent: Option<Abs>,
+    /// The descent of the frame. If this is `None`, the
+    /// frame's implicit descent is size minus baseline.
+    descent: Option<Abs>,
 }


I think size + baseline already determines ascent and descent. The box model should look like this

────┬──────────────────────────┬─── ▲ │ │ ▲ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ asc │ │ │ │ │ │ │ │ │ │ │ │ │ │ size.y │ │ │ │ │ │ │ │ │ │ │ │ baseline ──┴─┼──────────────────────────┤ │ ▲ │ │ │ │ │ │ │ desc │ │ │ │ ▼ │ │ ▼ ────┴──────────────────────────┴────

The point is that ascent+descent != size.y, as in the case of the equation slackness adjustment. In the slackness adjustment, the size (the original size of the frame) is larger than the ascent+descent.

The ascent and descent will be used by inline layout for determining the line height while the size still refers to the original size.

The point is that ascent+descent != size.y

I highly suspect this is not a good idea. asc + desc = size.y should be an invariant.

what the slackness adjustment does

────┬──────────────────────────┬─── │ ▲ │ │ │ │ │ │ ────┬─────────────┬ │ ▲ │ │ │ │ │ asc (adjusted by slackness) │ │ │ │ │ │ │ │ │ size.y │ │ │ │ │ │ │ │ │ baseline ──┴─┼──────────────────────────┤ │ ▲ │ │ │ │ │ desc │ │ │ ▼ │ ▼ ────┴──────────────────────────┴────

previously the ascent/descnet is not changed, but the size y is directly changed.

────┬──────────────────────────┬─────── │ ▲ │ │ │ │ │ │ ────┬──────────────────────────┬─── │ ▲ │ ▲ true size.y │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ size.y (modified by the slackness adjustment) │ │ │ │ │ │ │ │ │ │ │ │ baseline ──┴─┼──────────────────────────┤ │ │ ▲ │ │ │ │ │ │ │ desc │ │ │ │ ▼ │ ▼ ▼ ────┴──────────────────────────┴───────

Slackness adjust for inline equations adjust is... a quick & dirty workaround in my opinion, and hopefully I want to remove it my PR #4236 (I'm still waiting @laurmaedje 's comment on the line height proposal #4224). There is no such adjust in proper typesetting systems like LaTeX.

it would be impossible to make #4236 pass the test

Sorry, maybe there is a gap. I don't really get this point though. Currently there are about ~200 test doesn't pass. This is because I haven't fix line height for termlist/unordered list/ordered list yet. Another take is that you can always "pass" the test by updating the reference images..?

And slackness is properly handled in Latex and I think @laurmaedje has explained and provided examples.

believe me, tex doesn't have this adjust. The tex solution is described in #4224. Namely, tex determines line gap in 2 stages: in normal cases, it tries to make the distance between lines equals to \baselineskip. But if this makes lines overlap, it will ensure that the distance of the bottom edge of the previous line and the top edge of current line is 1pt.

Typst has a static gap for all cases. This makes it look bad when there are large items like inline equations. So we need extra "slackness adjust" to make it look right in most cases. But this also cause #1028.

And I think this height is the ascender (or for the math equations, the slackness adjusted height), instead of the actual size of this particular glyph.

This is unfortunately not true. slackness adjust only happens in equations that is to be inserted into normal text flows. They don't exist in other cases. For example:

has slacknessadjust: $a^2+b^2=c^2$ has slackness adjust as a whole, but no slackness adjust for each individual case arms. $cases(a^2+b^2=c^2, a=3, b=4, c=5)$

Okay I learn a lot. Thank you.

I am okay as long as the original size is not lost as I am interested in #2200; that being said, I still think it would be difficult to make #4236 pass the test (and I mean by preserving the existing result without breaking the tests, but I am not very familiar with it so I can be wrong). The line height adjustment seems to be complicated, if we are really going for it, a complete collision detection procedure needs to be implemented and some heuristic would be inevitable. I hope it can be done soon.

Do you think it is possible to get a predecessor PR to #4236 that can preserve the frame size?

I am okay as long as the original size

#4236 removes the slackness adjust, so inline math has correct bbox, ascent and descent.

I still think it would be difficult to make #4236 pass the test

Agree, the term list thing is not easy to fix. One thing to notice about typst's test system is that the reference image can be update with --update when running tests. This is especially useful when rendering result is expected to change.

The line height adjustment seems to be complicated, if we are really going for it, a complete collision detection procedure needs to be implemented and some heuristic would be inevitable.

To my surprise, it is not that hard(at least for most cases). Typst already maintains the exact bbox for each element, and the collision test code can be seen in https://github.com/typst/typst/pull/4236/files#diff-6c11c99cb19f6dd03f76013fec99eb36081d56355d9b5ed8546f1d785bd49af4L323-L325 .

You can see the inline equation overlap issue solved in this image: https://github.com/typst/typst/pull/4236/files#diff-d5c29466f38994d7e7c7337b0fee01231dc6a45d0a65b37807d38e3a2647cae1

I hope it can be done soon.

Me too! I didn't work on it last week because I'm (a) working on #4153 (b) waiting laurmaedje's comment on the proposal. I will continue to work on this pr.

Do you think it is possible to get a predecessor PR to #4236 that can preserve the frame size?

I'm not really sure. I haven't think of this in detail(because I bet on the line height solution!), there might be no "non-hacky" way to do this.

It depends on what we settle on in #4224, but in principle I agree with @Enter-tainer that ascent + descent = size.y is a good invariant to have, and that the equation slack is more of a hack.

LuxxxLucy · 2024-06-07T09:33:36Z

Closing in favour of waiting for #4224

LuxxxLucy added 3 commits June 1, 2024 20:33

introduce ascent descent setter and update frame line height adjustment

70deacd

update debugging function

3e8d835

Update issue-2214-baseline-math.png

5a543f9

LuxxxLucy mentioned this pull request Jun 2, 2024

Use line height based spacing model #4236

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Enter-tainer reviewed Jun 2, 2024

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LuxxxLucy mentioned this pull request Jun 3, 2024

Support highlight, overline, underline, and strike for math equations #3953

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LuxxxLucy closed this Jun 7, 2024

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Adding a ascent/descent setter in frame so that the original frame size is not lost in equation slackness adjustment #4318

Adding a ascent/descent setter in frame so that the original frame size is not lost in equation slackness adjustment #4318

LuxxxLucy commented Jun 2, 2024 •

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LuxxxLucy commented Jun 7, 2024

Adding a ascent/descent setter in frame so that the original frame size is not lost in equation slackness adjustment #4318

Adding a ascent/descent setter in frame so that the original frame size is not lost in equation slackness adjustment #4318

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LuxxxLucy commented Jun 2, 2024 • edited Loading

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