When working with integer label matrices returned by such functions as
ImageForestingComponents etc. it is often necessary to replace a certain label (or a list of labels) with other label(s) without unpacking the matrix. The immediately obvious solutions via
ReplaceAll or via
Position unpack packed arrays and for this reason aren't appropriate.
I can imagine writing a
Do loop and performing in-place modification of the matrix using
Part but it is ugly and expectedly slow (although it is memory-efficient because it won't create a copy of the original matrix). Probably compilation can help with the performance, but I'm sure there must be simpler way to perform such a basic operation without unpacking the matrix.
The question is: what is the best way to replace a list of values in a packed array of integers with other integer values while keeping the array packed?
Here is a couple of examples:
Setting the largest component to be background:
img = Import["http://i.stack.imgur.com/2a2j6.png"]; cellM = MorphologicalComponents[ColorNegate@img, CornerNeighbors -> False]; largest = SortBy[ComponentMeasurements[cellM, "Area"], Last][[-1, 1]]; (*the obvious solution: inefficient and unpacks*) cellM2 = cellM /. largest -> 0; cellM2 // Colorize
Shifting indices of all the components except the background (
0) by a constant value (motivation: combining two label matrices):
(*the obvious solution: inefficient and unpacks*) cellM3 = cellM /. i_Integer /; i != 0 :> i + 10000; cellM3 // Colorize
Answers to questions in the comments (now deleted):
From my little experience, the most usual needs are split into the two cases shown above:
a very few (usually only one) values need to be replaced;
all the values excepting a very few (usually only one) need to be replaced.
Since the obtained label matrices are intended for further processing with
SelectComponentsetc. it is highly desirable to keep them packed just for achieving decent timings. But from the other side, it is very easy to hit the memory limit of a usual laptop (or even PC) just keeping in the memory 2-3 unpacked label matrices of the usual size of modern photos (for example, about 2000×1200 pixels) during image processing.