Interval Guides for sight singing
This isn't a feature request, because I've already done the coding, but it isn't an announcement of a new feature either, because it hasn't been incorporated into the codebase. This is a request for comments. Let me know what you think.
For most singers, sight reading a piece of music is quite difficult. Even if the singer knows how to sing a minor third step versus a major third step, it is difficult to tell which one is written, because it depends on the staff, the key, and the accidentals. This feature, which is called Interval Guides, marks the positions of the half steps in the scale. The interval just below the tonic is shown in red, and the interval between the third and fourth degree is shown in blue (actually cyan). Knowing whether the particular interval to be sung crosses one or more interval guides, together with any accidentals, allows the singer to determine the quality of the interval very quickly.
To simplify things for the moment, we'll assume that we don't have any accidentals. Then a second that traverses a guide line is a minor second; one that does not is a major second. The same goes for thirds. A perfect fourth and a perfect fifth cross one line. A fourth with no line is a diminished fourth. A fifth traversing two lines is an augmented fifth. For sixths and sevenths, the major intervals cross one line and the minor intervals cross two.
For example, consider the example below. It contains four voice parts: tenor and lead on the upper staff and baritone and bass on the lower staff. The first two notes of the lead do not cross a line so it's a major third interval. The first two notes of the baritone part do cross a line, so it's a minor third. All three intervals of the bass line cross one line, so we can quickly read: perfect fourth, perfect fourth, perfect fifth.
The two colors allow the singer to quickly locate "do," which sits just above the red line, and helps to read solfeggio: do, re, me between red and blue; fa, so, la, ti between blue and red. The proximity of each note to a guide line makes it easy to tell the degree at a glance. This feature can also be used to make harmonic analysis a bit easier.
The guides are toggled on and off with a menu item in the View menu (shown circled in yellow above).