class MusicStaves_skeleton
Last modified: October 27, 2015
MusicStaves_skeleton
In module gamera.toolkits.musicstaves.musicstaves_skeleton
Implements a skeleton based staff line removal.
The algorithm consists of the following steps:
- Depending on the sekeleton type (medial axis or thinned), the skeleton needs some "improvement". Medial axis skeletons contain a lot of short spurs; these are removed when they are shorter than staffline_height / 2. Thinned out skeletons do not extend to the edges; these are extended by extrapolating the skeleton ends.
- The "improved" skeleton is split at branching and corner points; around each splitting point a number of pixels (taken from the distance tranform at the splitting point) is removed. The latter avoids that staff line skeleton segments extent into crossing objects.
- Staff line segment candidates are picked as skeleton segments with
all of the following properties:
- the orientation angle (least square fitted line) is below 25 degrees
- the segment is wider than high
- the "straightness" (mean square deviation from least square fitted line) is below staffline_height **2 / 2 Exception: when a segment is wider than 30* staffspace_height, this criterion is ignored
- The overall skew angle is determined as the median of the orientation angle probability distribution. In this distribution, each segment is weighted according to its width.
- The staff segment candidates are then horizontally and vertically linked. A vertical link is added between segments with a horizontal overlap and a vertical distance around staffline_height + staffspace_height. A horizontal link is added when the extrapolated end points are close enough (below staffline_height); for the extrapolation the previously calculated skew angle or the segment orientation is used.
- Staff systems and lines are then detected as connected components in the segmant graph as in StaffFinder_miyao. Connected components that are too small are removed. When num_lines is not given, it is guessed as in StaffFinder_miyao and lines are removed from overfull staff systems.
- A staff now typically still contains some "false positives" (non staff segments which are detected as staff segments). When staff segments assigned to the same line overlap, this is a clear indication of a false positive. Thus from each overlapping staff segment group on the same line the one that is closest to its least-square-fitted neighborhood is picked and the other are discarded.
- To check for further "false positives", non staff segments that have the same branching point as a staff segment are extrapolated by a parametric parabola. If this parabola is approximately tangential to the staff segment, the latter is considered a false positive.
- In order to remove staff lines, all vertical black runs around the detected staff systems are removed. As skeleton branches occasionally extend into solid regions, vertical runs are only removed when they are not longer than 2 * staffline_height.
References:
Skeletonization is a commonly used technique in optical music recognition, though it is usually applied after staff line removal. As a proof of concept, D. Nehab detected staff lines in a vectorized skeleton, see Diego Nehab: Staff Line Detection by Skewed Projection. http://www.cs.princeton.edu/~diego/academic/phd/ (2003)
| Author: | Christoph Dalitz |
|---|
__init__
In addition to the arguments of the constructor of the MusicStaves base class, the constructor optionally allows to pass a precomputed skeleton image. This can be useful for testing purposes, because skeleton computation can be time consuming.
Signature:
__init__(image, staffline_height=0, staffspace_height=0, skelimage=None, medialaxis=False)
with
- sekelimage:
- a onebit image of the skeleton. When the skeleton is a medial axis transform, set the following parameter to True.
- medialaxis:
Depending on the skeleton generating algorithm, the skeleton can be a medial axis transform or a thinned out image.
As each skeleton type requires a different preprocessing in remove_staves, you must provide the type as a parameter. When you do not provide a precomputed skeleton and leave the skeleton computation to remove_staves, set medialaxis=False, because a thinned out skeleton is significantly faster to compute than a medial axis transform.
remove_staves
Detects and removes staff lines from a music/tablature image. A skeleton of the found staff lines is stored as a nested list of StafflineSkeleton in the property linelist, where each sublist represents a staff system (as in the StaffFinder base class).
Signature:
remove_staves(crossing_symbols='all', num_lines=0, debug=0, false_positive_criterion='quad_angle', horizontal_extrapolation='segment_angle')
with
- crossing_symbols:
- This algorithm only supports the value 'all'.
- num_lines:
- Number of lines within one staff. A value of zero for automatic detection is supported, but if you give the actual number, staff line detection will improve.
- debug:
- Set the debug level verbosity. For debug >= 1, tracing information is printed to stdout, for debug >= 2 images with prefix skeldebug are written into the current working directory.
- false_positive_criterion
- Criterion to be used to identify false positives. Supports the values 'none', 'lin_meets', 'lin_angle' and 'quad_angle'.
- horizontal_extrapolation
- In which direction the extrapolation for horizontal linking of segments shall be done. Possible values are 'segment_angle' (angle of least square line through the segment) or 'skew_angle' (global document skew angle).
get_staffpos
Returns the y-positions of all staff lines at a given x-position. Can only be called after remove_staves.
Signature:
get_staffpos(x=0)
with
- x:
- This parameter has no effect, because the staffline is assumed to be horizontal.
The return value is a list of StaffObj.
Note that a more accurate way to obtain the staff lines for this class is by accessing self.linelist, because this does not just yield the average y-position, but the full staff line skeletons. Example:
ms = MusicStaves_skeleton(image)
ms.remove_staves()
sf = StaffFinder(image, ms.staffline_height, ms.staffspace_height)
sf.linelist = ms.linelist
rgb = sf.show_result()
rgb.save_PNG("foundstaves.png")