Thank you, James!
The various reg test diffs show how important boxes are to LilyPond... ;)

The slight and barely noticeable unintentional extra space that has been removed from rounded-corner box skylines causes many tiny shifts (as changed skylines change spacing).

Just to give one obvious example (a fret diagram regtest) of why and how the unintentional extra padding, small as it may be, had an influence on the skylines and therefore on object placement: Fret diagrams are made out of lines (i.e. boxes) with rounded corners and the scaled-up detail PNG shows (marked by green arrow) how a skyline is dented by a slightly oversized skyline and the letter "C" is shifted a wee bit up compared to the new rendering (right).

This all results in lots of bluish-green regtest differences that remind me of Joni Mitchell's "A Case of You":
"... in the blue TV screen light I drew a map of Canada..."

All the best,
Torsten

[issues:#5307] Skyline Refinements (Rounded Boxes and Rotated Ellipses)

Status: Started
Created: Sun Apr 15, 2018 09:46 PM UTC by Torsten Hämmerle
Last Updated: Sat Apr 21, 2018 11:38 AM UTC
Owner: Torsten Hämmerle
Attachments:

skylines-boxes-2-19-81.png (82.3 kB; image/png)

http://codereview.appspot.com/341140043

Recently, there have been some discussions about mysterious spacing of \rounded-box markup.

Harm identified skyline problems as the cause and submitted the following bug report:
Possible regression with box-markup/rounded-box-markup
Before Mike Solomon's new skyline handling, there were no such uninteded extra margins.

Rounded Box Skylines

The skyline of boxes with rounded corners do not tightly wrap the boxes but there's an unintentional margin (as wide as the corner radius).

Apart from that here are a few other shortcomings and so I have decided to create a more general box-and-rotated-ellipses skyline issue.

Other issues

When working on this case, I came across a few other things that call for improvement and so I've included them into the test file. Most notably, there is a general problem with rotation. And it would be nice to recreate the rounded corners for large boxes/radii.
At the same time, one should keep run-of-the-mill stuff like ledger lines as simple and fast as possible.

Test file

A file with 'debug-skylines switched on and TextScript padding values set to 0 in order to be able to check how the markup objects are fitted together.
I have tried to demonstrate about everything you cold do with boxes (including scaling, rotating, cominations thereof).

#(ly:set-option 'debug-skylines #t)

{
  \set Staff.instrumentName = \markup #(lilypond-version)
  \override Staff.TextScript.padding = 0
  \override Staff.TextScript.outside-staff-padding = 0
  \override Staff.TextScript.outside-staff-horizontal-padding = 0
  \override Staff.TextScript.self-alignment-X = #CENTER
  \override Staff.TextScript.layer = 0
  f'''4
  c'' ^\markup \override #'(thickness . 4) \box \teeny 1
      _\markup \with-color #grey \filled-box #'(-1 . 4) #'(1 . 4) #1
  b'  ^\markup \rounded-box "2!" ^\markup \rounded-box "3"
  d'' ^\markup \circle "*" ^\markup \rotate #90 \ellipse "Elli"
      _\markup \with-color #grey \rotate #10 \filled-box #'(-2 . 5) #'(1 . 4) #10
      _ \markup \rotate #36 \override #'(corner-radius . 20) \scale #'(1 . 0.4)
                \rotate #-68 \scale #'(1.6 . 1) \rounded-box "dop"
  f'' _\markup \override #'(corner-radius . 0.5) \rounded-box "."
  a'  ^\markup \rotate #30 \rounded-box "rotated a"
      _\markup \scale #'(1 . 1.5) \rotate #45 \override #'(thickness . 5)
               \rounded-box \rotate #-45 \scale #'(2 . 1) \bold "!"
  d'' _\markup \override #'(corner-radius . 20) \rounded-box "4"
  a'' ^\markup \rotate #-165 \box "rotated b"
      _\markup \rotate #180 \override #'(corner-radius . 20) \rounded-box \teeny
               \override #'(baseline-skip . 0) \center-column { "Upside" "Down" }
}

(see attached PNG file)

Ledger lines are filled rounded boxes, too (!)
Corner radius is misinterpreted as line thickness and padded around the box.
Ledger lines have rounded corners, but the blot diameter is so small that the effect didn't disturb too much.
When rotating boxes, the skylines still consist of one single non-rotated rectangular box.
Rounded corners are not replicated in the skylines, even if this may be beneficial for bigger radii.
Ellipse skylines gradually deteriorate when being rotated. In the example, the skyline of ellipse "Elli" has completely collapsed because of the 90° rotation.
The rhombus "!" has been created by rotating and scaling that unfortunately caused a collapse of the skyline.
Rotations in general cause large skyline boxes that hamper object placement a lot.

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From:	Auto mailings of changes to Lily Issues via Testlilyissues-auto
Subject:	[Lilypond-auto] [LilyIssues-auto] [testlilyissues:issues] Re: #5307 Skyline Refinements (Rounded Boxes and Rotated Ellipses)
Date:	Sat, 21 Apr 2018 12:58:50 -0000

[Lilypond-auto] [LilyIssues-auto] [testlilyissues:issues] Re: #5307 Skyl

Rounded Box Skylines

Other issues

Test file