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fix: correct MAGSAC++ IRLS gamma weight (B2 + B3) in sigmaConsensusPlusPlus #49

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fix: correct MAGSAC++ IRLS gamma weight (B2 + B3) in sigmaConsensusPlusPlus #49

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20 changes: 13 additions & 7 deletions src/pymagsac/include/magsac.h
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Original file line number Diff line number Diff line change
Expand Up @@ -773,10 +773,14 @@ bool MAGSAC<DatumType, ModelEstimator>::sigmaConsensusPlusPlus(
// Occupy the memory to avoid doing it inside the calculation possibly multiple times
sigma_weights.reserve(possible_inlier_number);

// The weight's noise scale is sigma_max = threshold / k (the paper convention,
// matching getModelQualityPlusPlus). The inlier-collection cutoff stays at
// current_maximum_sigma = maximum_threshold = k * sigma_max. (B3 fix.)
const double weight_sigma_max = current_maximum_sigma * threshold_to_sigma_multiplier;
// Calculate 2 * \sigma_{max}^2 a priori
const double squared_sigma_max_2 = current_maximum_sigma * current_maximum_sigma * 2.0;
const double squared_sigma_max_2 = weight_sigma_max * weight_sigma_max * 2.0;
// Divide C * 2^(DoF - 1) by \sigma_{max} a priori
const double one_over_sigma = C_times_two_ad_dof / current_maximum_sigma;
const double one_over_sigma = C_times_two_ad_dof / weight_sigma_max;
// Calculate the weight of a point with 0 residual (i.e., fitting perfectly) a priori
const double weight_zero = one_over_sigma * gamma_difference;

Expand Down Expand Up @@ -844,17 +848,19 @@ bool MAGSAC<DatumType, ModelEstimator>::sigmaConsensusPlusPlus(
{
// Calculate the squared residual
const double squared_residual = residual * residual;
// Get the position of the gamma value in the lookup table
size_t x = round(precision_of_stored_gammas * squared_residual / squared_sigma_max_2);
// Get the position of the gamma value in the lookup table.
// Use the fine table (step 1e-4) with its matching precision
// constant, so the index maps to the true argument. (B2 fix.)
size_t x = round(precision_of_stored_incomplete_gammas * squared_residual / squared_sigma_max_2);
// Put the index of the point into the vector of points used for the least squares fitting
sigma_inliers.emplace_back(idx);

// If the sought gamma value is not stored in the lookup, return the closest element
if (stored_gamma_number < x)
x = stored_gamma_number;
if (stored_incomplete_gamma_number < x)
x = stored_incomplete_gamma_number;

// Calculate the weight of the point
weight = one_over_sigma * (stored_gamma_values[x] - gamma_k);
weight = one_over_sigma * (stored_complete_gamma_values[x] - gamma_k);
}

// Store the weight of the point
Expand Down