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I want to extract from a RasterLayer all boundary cells between classes (possibly according to the 4-cell rule), excluding from this operation cells that neighbour NA cells

The raster::boundaries function detects boundaries as cells that have more than one class (including NA) cells in the 4 or 8 cells surrounding it , or, if classes argument is set to FALSE, cells with values and cells with NA.

What about if I do not want to consider as edge the NA border?

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I found a solution on QGIS, maybe it could be useful as well.

Assuming that the two target classes are identified with consecutive integers (e.g. 1 and 2), I used the 'mean' function in the Raster Calculator, which calculates a new raster applying the function to the focal cell using the 4/8 adjacent cells values. Any cell with decimal values has at least 1 adjacent cell with a different value.

For more than two classes (or not consecutive integers) this approach may not be suitable, because it could result in integers values as well.

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