Bounding box cut-outs
The four points cutOutNE, cutOutSE, cutOutSW and cutOutNW describe rectangular cut-outs from the four corners of a glyph’s rectangular bounding box. The bounding box is the box with the smallest area that encloses every part of the path of a glyph.
Because a glyph may not occupy every part of its bounding box, it can be useful to have an extra level of detail about the shape of the glyph, but at a coarser level than directly examining the path of the glyph to determine which areas of the bounding box are occupied and which are empty.
For example, when stacking accidentals to the left of a chord, accidentals are arranged into columns, where accidentals belonging to notes separated by a wide interval (normally a seventh or more) are aligned in the same column, i.e. at the same horizontal position. Successive columns of accidentals are laid out from right to left to the left of a chord, and depending on the accidentals that are present, it may be possible to interlock or kern those columns. The figure below shows a simple example:
In the first chord above, the two columns of accidentals (numbered 0 and 1) are positioned almost as close as the bounding boxes of the accidentals (shown in light blue) in each column will allow. In the second chord, column 1 is allowed to interlock with column 0 because the cut-outs in the bounding boxes of the two accidentals (shown as dashed red lines) are removed: the bounding boxes of the accidentals can overlap, provided it is only the cut-outs that overlap.
Font designers can specify four cut-outs to the bounding box, one in each corner, as illustrated in the figure below:
Each cut-out is specified as a pair of X,Y coordinates (in spaces), describing the innermost corner of a nominal rectangle that intersects the bounding box. For example, cutOutNE specifies the bottom left corner of a rectangle that intersects the top right corner of the bounding box of the glyph. The positions of each of the other corners of the cut-out rectangle are calculated using the bounding box of the glyph.