Why do I get the “Java heap space” error, and how to fix it ?
Why flsgen_structure did not
find a structure satisfying my targets ?
Why flsgen_generate failed to
generate a landscape ?
Can I specify the spatial extent and resolution of produced landscapes ?
How can I change the spatial configuration of produced landscapes ?
Can I specify the minimum distance between patches of the same class ?
How does the computing time of rflsgen varies according to input and parameters ?
How does flsgen_structure
selects the structures to return if there are several possibilities
?
What is Constraint Programming ?
By default, rJava allocate 512MB to the Java virtual
machine (JVM). When generating large landscapes, this can be
insufficient. The solution is, before loading rflsgen (and
any other package using rJava), to increase the memory
allocated to the JVM using
options(java.parameters = "-Xmx4g"). Note that this
allocates 4GB to the JVM. If you want to allocate another amount of
memory just replace 4g with the desired value.
- Why
flsgen_structure()did not find a structure satisfying my targets ?
There are two possibilities:
If
rflsgenindicates that “User targets cannot be satisfied”, it means that the solver has finished exploring the search tree and that there is no solution satisfying your targets. As the underlying Constraint Programming solver (Choco) relies on exact algorithms, this message is the guarantee that your targets cannot be satisfied. To date, this is not possible to explain why the targets are contradictory in a human-readable way. So what you can do is try to see if there are obvious contradictions in your targets (e.g. two classes that both need to occupy 60% of the landscape), or try with other targets.If
rflsgenindicates that “User targets could not be satisfied under the specified time limit”, it means that the time limit was reached before the solver could find a solution, or prove that there is no solution. By default the time limit inflsgen_structure()is 60 seconds, you can increase this limit with thetime_limitargument, or disable it with the value 0. You can also change thesearch_strategy, which indicate to Choco-solver how to construct its search tree. Although this search strategy does not influence whether the targets will be satisfied or not, some strategies can be more efficient depending on the problem configuration. Available search strategies in Choco are:""DEFAULT", "RANDOM", "DOM_OVER_W_DEG", "DOM_OVER_W_DEG_REF", "ACTIVITY_BASED", "CONFLICT_HISTORY", "MIN_DOM_LB", "MIN_DOM_UB"(please refer to Choco documentation if you want more details).
- Why
flsgen_generate()failed to generate a landscape ?
Although flsgen_structure() relies on exact search
algorithms to identify suitable landscape structures, the spatial
generation algorithm used in flsgen_generate() is
stochastic and can fail. The reason for using a stochastic algorithm is
that finding a spatial embedding of a landscape structure is equivalent
to the polyomino packing problem, which is known to be NP-Complete
(i.e. computational complexity increases exponentially with problem
size). It would not be possible to generate reasonably large landscapes
(e.g. larger than 100x100) in a feasible time. Using a stochastic
algorithm offers fast runtime, but at the price that it can fail without
knowing if a solution exists. In practice, the success of
flsgen_generate() is highly constrained by the proportion
of the non-focal landscape class (NON_FOCAL_PLAND), and to a lesser
extent by the min_distance parameter. In conclusion, you
have two possibilities: (1) if NON_FOCAL_PLAND is less than 15%, you
should probably generate a landscape structure with a higher
NON_FOCAL_PLAND, (2) else, you can increase the number of trials with
the max_try and max_try_patch parameters or
decrease the min_distance parameter.
This can be specified with the x, y,
epsg, resolution_x and
resolution_y parameters in the
flsgen_generate() function.
There are three parameters influencing the spatial configuration of
produced landscapes in flsgen_generate:
- The
min_distanceparameter, defines the minimum distance between any two patches of the same class. - The
terrainparameter, is a continuous raster guiding the generation algorithm. - The
terrain_dependencyparameter, defines to which extent the generation algorithm is influenced by the terrain.
Terrain rasters can either be generated “on-the-fly” by flsgen, which
relies on the diamond-square (or midpoint displacement) algorithm or
given as input. This last option makes it possible to use continuous
neutral landscapes generated with other software packages such as NLMR, or even
digital elevation models from real landscapes.
Yes, with the min_distance parameter of
flsgen_generate(). You can also play with the
min_max_distance, which makes the minimum distance between
the patch variable and comprised between min_distance and
min_max_distance.
As there are a lot of factors that can influence the computing time
of rflsgen, there is no simple answer to this question.
Regarding flsgen_structure(), the number of classes and
the landscape dimensions influences the size of the underlying
constraint satisfaction problem. But a large problem can be solved very
quickly if the solver can take advantage of the targets to filter the
search tree. In general, fragmentation indices such as MESH, NPRO, or
DIVI, are more difficult to satisfy if the targets only allow one value.
However, in most cases flsgen_structure() only needs a few
seconds (or less) to find a solution.
flsgen_generate() has a more predictable computing time,
which is mainly driven by the landscape dimension and the total
proportion of focal classes. In practice, it is fast even for large
landscapes (several millions of cells), and the RAM should be a problem
before the computing time.
- How does
flsgen_structure()selects the structures to return if there are several possibilities ?
Most often, there can be hundreds, thousands, or even millions of
possible structures satisfying user targets.
flsgen_structure() relies on Choco-solver to identify suitable
landscape structures. The default behaviour of Choco-solver is to return
the first solution found. If we ask the solver to find another solution,
it will return the second found, etc. The nb_solutions
parameter in flsgen_structure() tells Choco-solver how many
solutions it must find before stopping the search. There is also a
search_strategy parameter in
flsgen_structure() which indicate to Choco-solver how to construct its
search tree. Although this search strategy does not influence whether
the targets will be satisfied or not, it can help diversify the
generated structure by exploring the search tree in different ways.
Available search strategies in Choco are:
""DEFAULT", "RANDOM", "DOM_OVER_W_DEG", "DOM_OVER_W_DEG_REF", "ACTIVITY_BASED", "CONFLICT_HISTORY", "MIN_DOM_LB", "MIN_DOM_UB"
(please refer to Choco
documentation if you want more details).
Choco-solver is an open-source Java Constraint Programming solver. It is a reliable solver implemented with state-of-the-art algorithms which have been used in academic and industrial projects for years.
Constraint programming (CP) is a declarative paradigm for modelling and solving constraint satisfaction and constrained optimization problems. In this context declarative means that the modelling of a problem is decoupled from its solving process, which allows the primary focus to be on what must be solved rather than describing how to solve it. CP is a subfield of artificial intelligence that relies on automated reasoning, constraint propagation and search heuristics. As an exact approach, CP can provide constraint satisfaction and optimality guarantees, as well as enumerate every solution of a problem. In CP, the modeller represents a problem by declaring variables whose possible values belong to a specified finite domain, by stating constraints (mainly logical relations between variables), and eventually by defining an objective function to minimize or maximize. A solution to the problem is an instantiation of every variable such that every constraint is satisfied. As opposed to mixed-integer linear programming, constraints can be non-linear and variables of several types (e.g. integer, real, set, graph). A CP solver then handles the solving process relying on an automated reasoning method alternating a constraint propagation algorithm (deduction process on values within domains that does not lead to any solution) and a backtracking search algorithm. In a nutshell, more than satisfiability, each constraint embeds a filtering algorithm able to detect inconsistent values in variables domains. At each step of the backtracking search algorithm, the solver calls the constraint propagation algorithm that repeatedly applies these algorithms until a fixed point is reached. When it is proven that a part of the search tree contains no solution, the solver rolls back to a previous state and explores another part of the search tree: this is backtracking. Note that most CP solvers are also able to handle Pareto multi-objective optimization.
If you are interested in learning more about Constraint Programming, you can read the Handbook of Constraint Programming: Rossi, E. F., van Beek, P., & Walsh, T. (2006). Handbook of Constraint Programming.