Starspots are not round.

[therob762]

Given a 256 x 256 image of a starspot on a rapid rotator, a starspot looks distinctly not round. It has a eight pointed star-like appearance. Here is the .json file for your perusal.

{
    "_file_info" : "Save file from the SImulation and Modeling Tool for Optical Interferometry (SIMTOI) version 1.1.0 commit 069c52f. See https://github.com/bkloppenborg/simtoi for information about this format",
    "area_height" : 256,
    "area_scale" : 0.050000000000000003,
    "area_width" : 256,
    "model_0" : 
    {
        "base_data" : 
        {
            "T_eff_pole" : [ 5000, 2000, 1000000, false, 100, 0 ],
            "g_pole" : [ 1, 0.01, 10000, false, 10, 2 ],
            "inclination" : [ 0, -180, 180, false, 1, 2 ],
            "n_side_power" : [ 4, 1, 10, false, 1, 0 ],
            "omega_rot" : [ 0.5, 0, 1, false, 0.10000000000000001, 2 ],
            "position_angle" : [ 0, 0, 360, false, 0.10000000000000001, 2 ],
            "r_pole" : [ 3, 2.5, 3.5, true, 1, 4 ],
            "von_zeipel_beta" : [ 0.5, 0.01, 1, false, 0.10000000000000001, 2 ],
            "z_axis_rotation" : [ 0, 0, 360, false, 1, 2 ],
            "z_axis_rotational_period" : [ 0, 0, 100, false, 1, 4 ]
        },
        "base_id" : "roche_rotator",
        "feature_0_data" : 
        {
            "delta_T" : [ -500, -2000, 0, true, 100, 0 ],
            "phi" : [ 0, 0, 180, true, 0.10000000000000001, 2 ],
            "radius" : [ 1, 0, 10, true, 1, 2 ],
            "theta" : [ 0, 0, 180, true, 0.10000000000000001, 2 ]
        },
        "feature_0_id" : "uniform_spot",
        "position_data" : 
        {
            "E" : [ 0, -1, 1, false, 0.10000000000000001, 2 ],
            "N" : [ 0, -1, 1, false, 0.10000000000000001, 2 ]
        },
        "position_id" : "xy",
        "shader_data" : 
        {
            "alpha" : [ 0.25, 0.10000000000000001, 0.5, true, 0.089999999999999997, 1 ]
        },
        "shader_id" : "ldl_power_law"
    }
}

[bkloppenborg]

The level of Healpix pixellation should be set high enough that the spots appear smooth, but not so high that it significantly degrades performance. Try increasing n_side_power to 5 or 6.