Levels shipped at 0–255 unless someone moved the sliders, and Bicycle
Race showed what that costs. Its ink is grey, not black — a scanned
engraving, ink at 2–95, paper at 163–255 — and with alpha = 255 − luminance
that greyness becomes transparency. No pixel in the exported bundle was
even fully opaque, and the downscale to the song's width blended every
stroke edge further. Nothing downstream can rescue it.
So detection proposes levels too, like it proposes cuts and skew.
Notation is two-tone, which makes Otsu's split the measurement wanted;
the points sit halfway from it to each end of the range, so the ramp
between them survives as antialiasing rather than going jagged. A page
already scanned bilevel has no interior split — Otsu degenerates to 0 —
and is left alone. Per page, with the median becoming the song's, so a
near-blank page cannot set them.
Olukainen juomukainen came out unusable, from two separate faults.
The scanner left a dark line down the sheet edge, running the full height
of every page but the first. Being taller than any bracket it won every
overlap in anchor selection and swallowed the page into one system, so
five pages of six proposed no cuts at all. A page's brackets and barlines
are all about one system tall, so a stroke far taller than the typical
one is not notation — relative to the page's own strokes, since a page
holding one big system is legitimate.
The trim then removed each system's bottom line of lyrics. It judged ink
blobs by area, and a letter is nowhere near the threshold; a whole line
of them is dozens of blobs, none of which qualifies. Measure ink per row
and per column instead — a line of text carries plenty in total, and a
fleck's row carries almost none, which is the case the filter was for.
Detection now finds three systems on every page of that score, and every
slice keeps all four voices' words.
Systems anchor on the vertical bracket and expand over nearby ink, per
ADR 0006. Two corrections the corpus forced:
- Expansion measures distance from the bracket, never from the growing
extent. A chaining expansion hops between the closely-stacked lines of
a title block and walks the whole way up the page — on Engel p1 it
swallowed the title into system 1 and the copyright footer into
system 3.
- Reach is 1.5 staff heights, which takes in the lyrics below the last
staff and the tempo mark and INTRO box above the first, while leaving
the title block and footer out.
Verified against the corpus. Ketun joululaulu: 2 systems per page except
p7 (3) and p12 (1), staff 48px throughout, skew -2.7 to +1.2 per page,
and p2 system 1 spans 179-1071 where its bracket is 177-994 — the
difference being the bottom voice's lyric line. Engel: 2-3 systems per
page, staff 70-72px. Elaman nalka: 3 systems per page, 0 skew, staff
118px at 600 DPI.
The overlay dump is what made both bugs visible, and stays as the tool
for diagnosing a page that comes out wrong.
Closes#4, #5, #6, #7, #8, #9
Esa Kataja