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Here's a situation that doesn't fit the cost calculation scheme for Movement Points described in FizMiG:
A hero on snow with gremlins has a small number of remaining MP.
The hero can make one diagonal step and one straight step.
But he can't make a straight step and then a diagonal step!
(I can provide the corresponding save file).
Apparently, to make a diagonal step possible, the required MP should not be the amount deducted when making that step; in the described situation, it's higher (150 MP vs. 141 MP?). See also the situation with the possibility of a diagonal step on grass always being possible when a straight step is possible; there, fewer MP are required than are deducted (100 MP vs. 141 MP).
It's reasonable to assume that the required (and not the deducted!) MP for a diagonal step depend on the terrain (according to FizMiG, taking into account the Pathfinding skill and not taking into account creature affinity).
Another test will need to be conducted.