10.19.1 Spacing algorithms (LilyPond Contributor’s Guide)

#### 10.19.1 Spacing algorithms

Here is information from an email exchange about spacing algorithms.

On Thu, 2010-02-04 at 15:33 -0500, Boris Shingarov wrote: I am experimenting with some modifications to the line breaking code, and I am stuck trying to understand how some of it works. So far my understanding is that Simple_spacer operates on a vector of Grobs, and it is a well-known Constrained-QP problem (rods = constraints, springs = quadratic function to minimize). What I don’t understand is, if the spacer operates at the level of Grobs, which are built at an earlier stage in the pipeline, how are the changes necessitated by differences in line breaking, taken into account? in other words, if I take the last measure of a line and place it on the next line, it is not just a matter of literally moving that graphic to where the start of the next line is, but I also need to draw a clef, key signature, and possibly other fundamental things – but at that stage in the rendering pipeline, is it not too late??

Joe Neeman answered:

We create lots of extra grobs (e.g., a BarNumber at every bar line) but most of them are not drawn. See the break-visibility property in item-interface.

Here is another e-mail exchange. Janek Warchoł asked for a starting point to fixing 1301 (change clef colliding with notes). Neil Puttock replied:

The clef is on a loose column (it floats before the head), so the first place I’d look would be lily/spacing-loose-columns.cc (and possibly lily/spacing-determine-loose-columns.cc). I’d guess the problem is the way loose columns are spaced between other columns: in this snippet, the columns for the quaver and tuplet minim are so close together that the clef’s column gets dumped on top of the quaver (since it’s loose, it doesn’t influence the spacing).

#### LilyPond Contributor’s Guide v2.26.0 (stable-branch).

- 1 Introduction to contributing
  - 1.1 Help us
  - 1.2 Overview of work flow
  - 1.3 Summary for experienced developers
  - 1.4 Mentors
- 2 Quick start
  - 2.1 LilyDev
    - Installing LilyDev in VirtualBox
    - Configuring LilyDev in VirtualBox
  - 2.2 Compiling with LilyDev
  - 2.3 Now start work!
- 3 Working with source code
  - 3.1 Setting up
    - 3.1.1 Installing Git
    - 3.1.2 Creating a GitLab account and setting up SSH
    - 3.1.3 Cloning and forking the repository
    - 3.1.4 Configuring Git
  - 3.2 Git cheat sheet
  - 3.3 Lifecycle of a merge request
    - 3.3.1 Uploading a patch for review
    - 3.3.2 Automated testing
    - 3.3.3 Patch countdown
    - 3.3.4 Merging to master
    - 3.3.5 Abandoned patches
  - 3.4 Writing good commit messages
  - 3.5 Commit access
  - 3.6 Further Git documentation resources
  - 3.7 Repository directory structure
- 4 Compiling
  - 4.1 Overview of compiling
  - 4.2 Requirements
    - 4.2.1 Requirements for running LilyPond
    - 4.2.2 Requirements for compiling LilyPond
      - Fedora
      - Linux Mint
      - OpenSUSE
      - Ubuntu
      - Other
    - 4.2.3 Requirements for building documentation
  - 4.3 Getting the source code
  - 4.4 Configuring `make`
    - 4.4.1 Build modes
    - 4.4.2 Running `autogen.sh`
    - 4.4.3 Running `configure`
      - Configuration options
      - Checking build dependencies
      - Configuring target directories
  - 4.5 Compiling LilyPond
    - 4.5.1 Using `make`
    - 4.5.2 Saving time with the `-j` option
    - 4.5.3 Useful `make` variables
  - 4.6 Post-compilation options
    - 4.6.1 Installing LilyPond from a local build
    - 4.6.2 Generating documentation
      - Documentation editor’s edit/compile cycle
      - Building documentation
      - Building a single document
      - Saving time with `CPU_COUNT`
      - Installing documentation
      - Building documentation without compiling
    - 4.6.3 Testing LilyPond binary
  - 4.7 Problems
  - 4.8 Concurrent stable and development versions
    - 4.8.1 Replacing the notation fonts in development versions
  - 4.9 Build system
- 5 Documentation work
  - 5.1 Introduction to documentation work
  - 5.2 `\version` in documentation files
  - 5.3 Documentation suggestions
  - 5.4 Texinfo introduction and usage policy
    - 5.4.1 Texinfo introduction
    - 5.4.2 Documentation files
    - 5.4.3 Sectioning commands
    - 5.4.4 Menus
    - 5.4.5 LilyPond formatting
    - 5.4.6 Text formatting
    - 5.4.7 Syntax survey
      - Comments
      - Cross-references
      - External links
      - Fixed-width font
      - Indexing
      - Lists
      - Special characters
      - Miscellany
    - 5.4.8 Other text concerns
  - 5.5 Documentation policy
    - 5.5.1 Books
    - 5.5.2 Section organization
    - 5.5.3 Checking cross-references
    - 5.5.4 General writing
    - 5.5.5 Technical writing style
  - 5.6 Tips for writing documentation
    - 5.6.1 Working on subsections
    - 5.6.2 Searching
  - 5.7 Scripts to ease documentation work
    - 5.7.1 Building only one section of the documentation
    - 5.7.2 Updating documentation with `convert-ly`
  - 5.8 Documentation strings in the Internals Reference
  - 5.9 Translating the documentation
    - 5.9.1 Getting started with documentation translation
      - Translation requirements
      - Which documentation can be translated
      - Starting translation in a new language
    - 5.9.2 Documentation translation details
      - Files to be translated
      - Translating the website and other Texinfo documentation
      - Adding a Texinfo manual
    - 5.9.3 Documentation translation maintenance
      - Check state of translation
      - Updating documentation translation
      - Updating translation committishes
      - Maintaining without updating translations
    - 5.9.4 Technical background
- 6 Website work
  - 6.1 Introduction to website work
  - 6.2 Uploading website
  - 6.3 Debugging website and docs locally
  - 6.4 Translating the website
- 7 LilyPond Wiki work
  - 7.1 Introduction to the LilyPond Wiki
  - 7.2 Adding and editing snippets
  - 7.3 Snippet administration
  - 7.4 The `makelsr.pl` script
  - 7.5 LilyPond Wiki to Git
- 8 Issues
  - 8.1 Introduction to issues
  - 8.2 Triaging bugs
  - 8.3 Issue classification
  - 8.4 Adding issues to the tracker
- 9 Regression tests
  - 9.1 Introduction to regression tests
  - 9.2 Precompiled regression tests
  - 9.3 Compiling regression tests
  - 9.4 Regtest comparison
  - 9.5 Pixel-based regtest comparison
  - 9.6 Finding the cause of a regression
  - 9.7 MusicXML tests
- 10 Programming work
  - 10.1 Overview of LilyPond architecture
  - 10.2 LilyPond programming languages
  - 10.3 Programming without compiling
  - 10.4 Finding functions
  - 10.5 Code style
    - 10.5.1 Languages
    - 10.5.2 Filenames
    - 10.5.3 Code formatting
    - 10.5.4 Naming Conventions
    - 10.5.5 Broken code
    - 10.5.6 Code comments
    - 10.5.7 Handling errors
    - 10.5.8 Localization
  - 10.6 Warnings, Errors, Progress and Debug Output
  - 10.7 Debugging LilyPond
    - 10.7.1 Debugging overview
    - 10.7.2 Debugging C++ code
    - 10.7.3 Debugging Scheme code
    - 10.7.4 Debugging scoring algorithms
    - 10.7.5 Debugging skylines
  - 10.8 Tracing object relationships
  - 10.9 Tracing processing time
  - 10.10 Adding or modifying features
    - 10.10.1 Write the code
    - 10.10.2 Write regression tests
    - 10.10.3 Write convert-ly rule
    - 10.10.4 Automatically update documentation
    - 10.10.5 Manually update documentation
    - 10.10.6 Edit changes.tely
    - 10.10.7 Verify successful build
    - 10.10.8 Verify regression tests
    - 10.10.9 Post patch for comments
    - 10.10.10 Push patch
    - 10.10.11 Closing the issues
  - 10.11 Iterator tutorial
  - 10.12 Engraver tutorial
    - 10.12.1 Useful methods for information processing
    - 10.12.2 Translation process
    - 10.12.3 Listening to music events
    - 10.12.4 Acknowledging grobs
    - 10.12.5 Engraver declaration/documentation
  - 10.13 Callback tutorial
  - 10.14 Understanding pure properties
    - 10.14.1 Purity in LilyPond
    - 10.14.2 Writing a pure function
    - 10.14.3 How purity is defined and stored
    - 10.14.4 Where purity is used
    - 10.14.5 Case studies
    - 10.14.6 Debugging tips
  - 10.15 LilyPond scoping
  - 10.16 Regular expressions
  - 10.17 Scheme->C interface
    - 10.17.1 Comparison
    - 10.17.2 Conversion
  - 10.18 Garbage collection for dummies
  - 10.19 LilyPond miscellany
    - 10.19.1 Spacing algorithms
    - 10.19.2 Info from Han-Wen email
    - 10.19.3 Music functions and Guile debugging
    - 10.19.4 Articulations on EventChord
- 11 Release work
  - 11.1 Development phases
  - 11.2 Release checklist
  - 11.3 Major release checklist
- 12 Modifying the Emmentaler font
  - 12.1 Overview of the Emmentaler font
  - 12.2 Font creation tools
  - 12.3 Adding a new font section
  - 12.4 Adding a new glyph
  - 12.5 Building the changed font
  - 12.6 METAFONT formatting rules
- 13 Administrative policies
  - 13.1 LilyPond is GNU Software
  - 13.2 Environment variables
  - 13.3 Performing yearly copyright update (“grand-replace”)
- Appendix A GNU Free Documentation License