Invisible rests are used to align secondary voices/layers to the primary one. These are most common in keyboard music, where voices may appear and disappear anywhere within a measure.
In the conversion from Humdrum to MEI, three types of invisible
rests are possible. The first type is an explicitly encoded rest
with a yy signifier appended to the token, such as 4ryy, which
means an invisible quarter-note rest. The second type is a plain
**recip value with no pitch or rest signifiers, such as 4 for
a quarter-note duration.
MEI requires measure layers to be completely filled in with rhythmic elements, while Humdrum allows secondary spines to be created or merged anywhere within a measure. The third type of invisible rest is one which is added to the MEI data in order to fill in the layer where the Humdrum layers are partially expressed.
Color all individual rests
Invisible rests can be made visible in the notation by adding this meta-RDF line anywhere in the Humdrum data:
!!!RDF**kern: show spaces
MEI calls invisible rests <space>, so that is why the action text is
show spaces. All three categories of invisible rests in Humdrum will
be displayed with this RDF entry. If you want to display the invisible
rests in a different color, add a color parameter to the line, such as
using the default color:
!!!RDF**kern: show spaces color=hotpink
Both HTML 5/SVG and hex colors are allowed.
Color by rest category
If you want to color a single category of invisible rests in the notation, use any of these RDF lines:
!!!RDF**kern: show invisible rests color=chartreuse
!!!RDF**kern: show recip spaces color=hotpink
!!!RDF**kern: show implicit spaces color=blueviolet
show invisible rests is used to display explicit rests which contain
the yy signifier in the rest token. recip spaces is used to
display invisible rests which are produced by plain recip tokens which
do not have pitch or rest signifiers. implicit spaces is used to
display invisible rests generated for filling in all layers with complete
durations of the measure.
These three colorizations can be used with the show spaces RDF, and each will
supersede the global invisible rests color for their type of rest.
Try removing the RDF records and notice that the rests disappear from the notation.