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Goober5000andClaude Fable 5.1
d2f2374f5c
get_I_inv converts an object's inverse moment of inertia tensor from body to world coordinates for collision physics. It was written as though vm_matrix_x_matrix(dest, a, b) computes a * b, but that function computes b * a, so the result was (R)(I_inv_body)(Rt) rather than the (Rt)(I_inv_body)(R) stated in the comment. The collision response therefore used the tensor of an object facing the inverse of its actual orientation. This dates back to retail. The result is only correct for an isotropic tensor or an unrotated object. Every retail ship has an anisotropic tensor, with a median principal-axis ratio of about 2.5 and as high as 13 for capital ships, so the spin imparted by a collision was typically about the wrong axis. Since this changes collision behavior for every mod, the fix is controlled by the new game_settings.tbl option "$Fix collision moment of inertia:", which defaults to true for mods targeting 26.2 or later. The retail path is otherwise unchanged. PR #2575 fixed a related bug in physics_ship_init: retail scaled the inverse moment of inertia by $Density rather than by its inverse, so a denser ship was easier to rotate rather than harder. That fix was merged without a flag. For consistency it is now controlled by "$Fix density moment of inertia:", which defaults to true for mods targeting 21.0 or later. Co-Authored-By: Claude Fable 5.1 <[email protected]> Co-Authored-By: Claude Opus 5.5 <[email protected]>