Suggestion: `@tgrp` for horizontal shifting and alignment of notes in polyphony

[th-we]

@tgrp is mainly intended for polyphonic situations where elements like notes/chords/rests as well as augmentation dots and accidentals need to be shifted to avoid collisions and/or to achieve a sensible alignment or a sensible accidental stacking.

This attribute takes inspiration from @vgrp. It is evaluated on a timestamp-by-timestamp basis. The implied default value is 0. The elements are ordered in ascending order of their @tgrp value from left to right. This means especially, if @tgrp is negative, elements occur left of the timestamp onset.

Why not @ho?

I fully agree with @zolaemil that the definition and usefulness of @ho are questionable "for 'programatically-determined position' is practically an undefined position". There is a related discussion here that was closed without really reaching a conclusion.

Disadvantages of @ho are:

• By its current definition, @ho is essentially an application specific attribute (without knowing the application).
• It specifies an offset using vertical units. How far symbols need to be shifted may however change depending on the font used (especially how wide noteheads are).

Suggested operating principle

• If an element has no @tgrp attribute, the rendering application is free to decide where to place it, even if other elements at the same timestamp have @tgrp attributes. The encoder is however advised to either give all the elements of the same staff, timestamp and category a @tgrp attribute, or none of them at all (this means, when giving one accidental in staff 1, layer 1, @tstamp="1" a @tgrp attribute, then all accidentals in all layers on this staff at @tstamp="1" should be given a @tgrp attribute).

• There are 3 basic categories of elements that can have a @tgrp attribute:

  1. notes, chords, rests
  2. dots
  3. accidentals

• The rendering application may decide how to kern the elements (e.g. based on the used font), as long as the horizontal order of elements is retained and collisions are avoided.

• If elements of different categories (e.g. notes and accidentals) are in the same @tgrp, this signals to the rendering application to align elements of the same category and kern both categories closely. In the following example all the lower accidentals (natural, sharp, double sharp) may be in the same @tgrp as their respective chord, even though the bottom accidentals have different dimensions and one could say the natural is further right then the left note while the sharp precisely aligns and the double flat is left of it.

  

• Each staff has its own independent @tgrp context. This is to avoid unintended interferences between differently aligned groups in different staffs in situation like this:

  

Examples with encoding suggestions

Example 1

<layer>
   <chord dur="8" stem.dir="up" tgrp="0">
      <note pname="e" oct="5"/>
      <note pname="a" oct="4"/>
   </chord>
   <space dur="8"/>
   <space dur="8"/>
   <chord dur="8" stem.dir="up" tgrp="0">
      <note pname="e" oct="5"/>
      <note pname="a" oct="4"/>
   </chord>
</layer>
<layer>
   <beam place="above">
      <note pname="g" oct="4" dur="8" tgrp="1"/>
      <note pname="g" oct="4" dur="16"/>
      <note pname="f" oct="4" dur="16"/>
   </beam>
   <beam place="below">
      <note pname="g" oct="4" dur="8"/>
      <note pname="e" oct="4" dur="8" tgrp="-1"/>
   </beam>
</layer>
<layer>
   <note pname="c" oct="4" accid="s" dur="8" stem.dir="down" tgrp="0"/>
   <space dur="8"/>
   <space dur="8"/>
   <note pname="a" oct="4" dur="8" accid.ges="s" stem.dir="down" tgrp="0"/>
</layer>

Example 2

Encoding of first beat:

<layer>
   <beam place="above">
      <note pname="c" oct="5" dur="8" tgrp="0">
         <dot tgrp="0"/>
      </note>
      <note pname="b" oct="4" dur="16"/>
   </beam>
</layer>
<layer>
   <note pname="e" oct="4" dur="8" stem.dir="up" tgrp="1">
      <dot tgrp="1"/>
   </note>
   <space dur="16"/>
</layer>
<layer>
   <note pname="a" oct="3" dur="4" tgrp="0">
      <dot tgrp="0"/>
   </note>
   <space dur="16"/>
</layer>

Example 3

Encoding of first beat:

<layer>
   <note pname="c" oct="6" dur="4" stem.dir="up" tgrp="0"/>
</layer>
<layer>
   <note pname="d" oct="5" dur="4" stem.dir="up" tgrp="1">
      <accid accid="s" tgrp="-2"/>
   </note>
</layer>
<layer>
   <note pname="f" oct="4" dur="4" stem.dir="up" tgrp="2">
      <accid accid="n" tgrp="-1"/>
   </note>
</layer>
<layer>
   <note pname="a" oct="3" dur="4" stem.dir="up" tgrp="3"/>
</layer>

Example 4

Encoding of first eighth:

<layer>
   <beam place="above">
      <note pname="g" oct="5" dur.ges="16" tgrp="0">
         <dot tgrp="2"/>
      </note>
      <note pname="a" accid="n" dur.ges="16"/>
   </beam>
</layer>
<layer>
   <chord dur.ges="16" tgrp="1" stem.dir="up" tgrp="1">
      <note pname="c" oct="5">
         <dot tgrp="2"/>
      </note>
      <note pname="e" oct="4" accid="n">
         <dot tgrp="2"/>
      </note>
   </chord>
   <rest dur="32" sameas="#m7-l3-rest1"/>
</layer>
<layer>
   <note pname="g" oct="3" dur="16" stem.dir="down" tgrp="0">
      <dot tgrp="0"/>
   </note>
   <rest dur="32" xml:id="m7-l3-rest1"/>
</layer>

Example 5a

Engraving example by Karl Hader:

<staff n="1">
   <layer>
      <rest dur="8"/>
      <beam place="above">
         <chord dur="8" artic="stacc">
            <note pname="g" oct="4"/>
            <note pname="f" oct="4"/>
            <note pname="e" oct="4">
               <accid accid="f" tgrp="-1"/>
            </note>
            <note pname="d" oct="4">
               <accid accid="f" tgrp="-2"/>
            </note>
            <note pname="c" oct="4">
               <accid accid="f" tgrp="-3"/>
            </note>
         </chord>
         <chord dur="8" artic="stacc" staff="2">
            <note pname="e" oct="4">
               <accid accid="n" tgrp="-1"/>
            </note>
            <note pname="d" oct="4">
               <accid accid="n" tgrp="-2"/>
            </note>
            <note pname="c" oct="4">
               <accid accid="n" tgrp="-3"/>
            </note>
            <note pname="b" oct="3">
               <accid accid="f" tgrp="-4"/>
            </note>
            <note pname="a" oct="3">
               <accid accid="f" tgrp="-5"/>
            </note>
         </chord>
         <chord dur="8" artic="stacc">
            <note pname="e" oct="4">
               <accid accid="s" tgrp="-1"/>
            </note>
            <note pname="d" oct="4">
               <accid accid="s" tgrp="-2"/>
            </note>
            <note pname="c" oct="4">
               <accid accid="s" tgrp="-3"/>
            </note>
            <note pname="b" oct="3">
               <accid accid="n" tgrp="-4"/>
            </note>
            <note pname="a" oct="3">
               <accid accid="n" tgrp="-5"/>
            </note>
         </chord>
         <chord dur="8" artic="stacc" staff="2">
            <note pname="e" oct="4">
               <accid accid="n" tgrp="-1"/>
            </note>
            <note pname="d" oct="4">
               <accid accid="n" tgrp="-2"/>
            </note>
            <note pname="c" oct="4">
               <accid accid="n" tgrp="-3"/>
            </note>
            <note pname="b" oct="3">
               <accid accid="f" tgrp="-4"/>
            </note>
            <note pname="a" oct="3">
               <accid accid="f" tgrp="-5"/>
            </note>
         </chord>
         <chord dur="8" artic="stacc">
            <note pname="g" oct="4"/>
            <note pname="f" oct="4"/>
            <note pname="e" oct="4">
               <accid accid="f" tgrp="-1"/>
            </note>
            <note pname="d" oct="4">
               <accid accid="f" tgrp="-2"/>
            </note>
            <note pname="c" oct="4">
               <accid accid="f" tgrp="-3"/>
            </note>
         </chord>
      </beam>
   </layer>
</staff>
<staff n="2">
   <layer>
      <chord stem.dir="down" dur="2" dots="1">
         <note pname="c" oct="3" accid="f"/>
         <note pname="f" oct="2" accid="f"/>
      </chord>
   </layer>
</staff>

Example 5b

Same example with Sibelius' default accidental arrangement:

<staff n="1">
   <layer>
      <rest dur="8"/>
      <beam place="above">
         <chord dur="8" artic="stacc">
            <note pname="g" oct="4"/>
            <note pname="f" oct="4"/>
            <note pname="e" oct="4">
               <accid accid="f" tgrp="-1"/>
            </note>
            <note pname="d" oct="4">
               <accid accid="f" tgrp="-3"/>
            </note>
            <note pname="c" oct="4">
               <accid accid="f" tgrp="2"/>
            </note>
         </chord>
         <chord dur="8" artic="stacc" staff="2">
            <note pname="e" oct="4">
               <accid accid="n" tgrp="-1"/>
            </note>
            <note pname="d" oct="4">
               <accid accid="n" tgrp="-3"/>
            </note>
            <note pname="c" oct="4">
               <accid accid="n" tgrp="-5"/>
            </note>
            <note pname="b" oct="3">
               <accid accid="f" tgrp="-4"/>
            </note>
            <note pname="a" oct="3">
               <accid accid="f" tgrp="-2"/>
            </note>
         </chord>
         <chord dur="8" artic="stacc">
            <note pname="e" oct="4">
               <accid accid="s" tgrp="-1"/>
            </note>
            <note pname="d" oct="4">
               <accid accid="s" tgrp="-3"/>
            </note>
            <note pname="c" oct="4">
               <accid accid="s" tgrp="-5"/>
            </note>
            <note pname="b" oct="3">
               <accid accid="n" tgrp="-4"/>
            </note>
            <note pname="a" oct="3">
               <accid accid="n" tgrp="-2"/>
            </note>
         </chord>
         <chord dur="8" artic="stacc" staff="2">
            <note pname="e" oct="4">
               <accid accid="n" tgrp="-1"/>
            </note>
            <note pname="d" oct="4">
               <accid accid="n" tgrp="-3"/>
            </note>
            <note pname="c" oct="4">
               <accid accid="n" tgrp="-5"/>
            </note>
            <note pname="b" oct="3">
               <accid accid="f" tgrp="-4"/>
            </note>
            <note pname="a" oct="3">
               <accid accid="f" tgrp="-2"/>
            </note>
         </chord>
         <chord dur="8" artic="stacc">
            <note pname="g" oct="4"/>
            <note pname="f" oct="4"/>
            <note pname="e" oct="4">
               <accid accid="f" tgrp="-1"/>
            </note>
            <note pname="d" oct="4">
               <accid accid="f" tgrp="-3"/>
            </note>
            <note pname="c" oct="4">
               <accid accid="f" tgrp="-2"/>
            </note>
         </chord>
      </beam>
   </layer>
</staff>
<staff n="2">
   <layer>
      <chord stem.dir="down" dur="2" dots="1">
         <note pname="c" oct="3" accid="f"/>
         <note pname="f" oct="2" accid="f"/>
      </chord>
   </layer>
</staff>

[bwbohl]

would this just introduce @tgrp or also remove @ho?