clip operations with general expressions in the point lists

[bbell]

In the point list operand of a clip operation, e.g. LineString(0 0, 1 3, -9 19), the values are presently treated as integers and not general expressions, preventing the use of clipping in the marray constructor. For example,

LineString(0-1 0, 1 3 * average_cells(m[0:9,0:3]), -9 19)

is not possible with the current implementation; however, reaching this point is fairly straightforward, as it only requires updating the parser and moving a few lines of code from the constructor of QtMShapeOp to the evaluate method.

Tests are needed, and further error checking may be required.

[bbell]

There is a bit of ambiguity to sort out. At present, the WKT format is enforced, so (0 -1 0) will be interpreted as the vector (0, -1, 0). The problem with this is that the general expression might see 0 -1 as {-1} instead of a pair of distinct points. This ambiguity should be resolved, and test cases should be established to prove consistency.

[Dimitar Misev]

the values are presently treated as integers and not general expressions, preventing the use of clipping in the marray constructor

Is there any use case for general expressions in the LineString? It's unclear to me how this "prevents the use of clipping in the marray constructor".

[bbell]

A use case where one could vary the coordinates of the operand? What if you have a slice translating through n-D? This would allow a clipping on each slice where the coordinates of the clipping operation may be shifted. In particular, it would allow a sort of "discrete" version of corridors using just a polygon clipping and the marray methods.