Source code for kornia.geometry.ransac

"""Module containing RANSAC modules."""
import math
from typing import Optional, Tuple

import torch
import torch.nn as nn

from kornia.geometry import (
    find_fundamental,
    find_homography_dlt,
    find_homography_dlt_iterated,
    symmetrical_epipolar_distance,
)
from kornia.geometry.homography import oneway_transfer_error, sample_is_valid_for_homography

__all__ = ["RANSAC"]


[docs]class RANSAC(nn.Module): """Module for robust geometry estimation with RANSAC. https://en.wikipedia.org/wiki/Random_sample_consensus Args: model_type: type of model to estimate, e.g. "homography" or "fundamental". inliers_threshold: threshold for the correspondence to be an inlier. batch_size: number of generated samples at once. max_iterations: maximum batches to generate. Actual number of models to try is ``batch_size * max_iterations``. confidence: desired confidence of the result, used for the early stopping. max_local_iterations: number of local optimization (polishing) iterations. """ supported_models = ['homography', 'fundamental'] def __init__(self, model_type: str = 'homography', inl_th: float = 2.0, batch_size: int = 2048, max_iter: int = 10, confidence: float = 0.99, max_lo_iters: int = 5): super().__init__() self.inl_th = inl_th self.max_iter = max_iter self.batch_size = batch_size self.model_type = model_type self.confidence = confidence self.max_lo_iters = max_lo_iters self.model_type = model_type if model_type == 'homography': self.error_fn = oneway_transfer_error # type: ignore self.minimal_solver = find_homography_dlt # type: ignore self.polisher_solver = find_homography_dlt_iterated # type: ignore self.minimal_sample_size = 4 elif model_type == 'fundamental': self.error_fn = symmetrical_epipolar_distance # type: ignore self.minimal_solver = find_fundamental # type: ignore self.minimal_sample_size = 8 # ToDo: implement 7pt solver instead of 8pt minimal_solver # https://github.com/opencv/opencv/blob/master/modules/calib3d/src/fundam.cpp#L498 self.polisher_solver = find_fundamental # type: ignore else: raise NotImplementedError(f"{model_type} is unknown. Try one of {self.supported_models}") def sample(self, sample_size: int, pop_size: int, batch_size: int, device: torch.device = torch.device('cpu')) -> torch.Tensor: """Minimal sampler, but unlike traditional RANSAC we sample in batches to get benefit of the parallel processing, esp. on GPU """ rand = torch.rand(batch_size, pop_size, device=device) _, out = rand.topk(k=sample_size, dim=1) return out @staticmethod def max_samples_by_conf(n_inl: int, num_tc: int, sample_size: int, conf: float) -> float: """Formula to update max_iter in order to stop iterations earlier https://en.wikipedia.org/wiki/Random_sample_consensus.""" if n_inl == num_tc: return 1.0 return math.log(1.0 - conf) / math.log(1. - math.pow(n_inl / num_tc, sample_size)) def estimate_model_from_minsample(self, kp1: torch.Tensor, kp2: torch.Tensor) -> torch.Tensor: batch_size, sample_size = kp1.shape[:2] H = self.minimal_solver(kp1, kp2, torch.ones(batch_size, sample_size, dtype=kp1.dtype, device=kp1.device)) return H def verify(self, kp1: torch.Tensor, kp2: torch.Tensor, models: torch.Tensor, inl_th: float) -> Tuple[torch.Tensor, torch.Tensor, float]: if len(kp1.shape) == 2: kp1 = kp1[None] if len(kp2.shape) == 2: kp2 = kp2[None] batch_size = models.shape[0] errors = self.error_fn(kp1.expand(batch_size, -1, 2), kp2.expand(batch_size, -1, 2), models) inl = (errors <= inl_th) models_score = inl.to(kp1).sum(dim=1) best_model_idx = models_score.argmax() best_model_score = models_score[best_model_idx].item() model_best = models[best_model_idx].clone() inliers_best = inl[best_model_idx] return model_best, inliers_best, best_model_score def remove_bad_samples(self, kp1: torch.Tensor, kp2: torch.Tensor) -> Tuple[torch.Tensor, torch.Tensor]: """""" # ToDo: add (model-specific) verification of the samples, # E.g. constraints on not to be a degenerate sample if self.model_type == 'homography': mask = sample_is_valid_for_homography(kp1, kp2) return kp1[mask], kp2[mask] return kp1, kp2 def remove_bad_models(self, models: torch.Tensor) -> torch.Tensor: # ToDo: add more and better degenerate model rejection # For now it is simple and hardcoded main_diagonal = torch.diagonal(models, dim1=1, dim2=2) mask = main_diagonal.abs().min(dim=1)[0] > 1e-4 return models[mask] def polish_model(self, kp1: torch.Tensor, kp2: torch.Tensor, inliers: torch.Tensor) -> torch.Tensor: # TODO: Replace this with MAGSAC++ polisher kp1_inl = kp1[inliers][None] kp2_inl = kp2[inliers][None] num_inl = kp1_inl.size(1) model = self.polisher_solver(kp1_inl, kp2_inl, torch.ones(1, num_inl, dtype=kp1_inl.dtype, device=kp1_inl.device)) return model
[docs] def forward(self, kp1: torch.Tensor, kp2: torch.Tensor, weights: Optional[torch.Tensor] = None) -> Tuple[torch.Tensor, torch.Tensor]: r"""Main forward method to execute the RANSAC algorithm. Args: kp1 (torch.Tensor): source image keypoints :math:`(N, 2)`. kp2 (torch.Tensor): distance image keypoints :math:`(N, 2)`. weights (torch.Tensor): optional correspondences weights. Not used now Returns: - Estimated model, shape of :math:`(1, 3, 3)`. - The inlier/outlier mask, shape of :math:`(1, N)`, where N is number of input correspondences. """ if not isinstance(kp1, torch.Tensor): raise TypeError(f"Input kp1 is not torch.Tensor. Got {type(kp1)}") if not isinstance(kp2, torch.Tensor): raise TypeError(f"Input kp2 is not torch.Tensor. Got {type(kp2)}") if not len(kp1.shape) == 2: raise ValueError(f"Invalid kp1 shape, we expect Nx2 Got: {kp1.shape}") if not len(kp2.shape) == 2: raise ValueError(f"Invalid kp2 shape, we expect Nx2 Got: {kp2.shape}") if not (kp1.shape[0] == kp2.shape[0]) or (kp1.shape[0] < self.minimal_sample_size): raise ValueError(f"kp1 and kp2 should be \ equal shape at at least [{self.minimal_sample_size}, 2], \ got {kp1.shape}, {kp2.shape}") best_score_total: float = float(self.minimal_sample_size) num_tc: int = len(kp1) best_model_total = torch.zeros(3, 3, dtype=kp1.dtype, device=kp1.device) inliers_best_total: torch.Tensor = torch.zeros(num_tc, 1, device=kp1.device, dtype=torch.bool) for i in range(self.max_iter): # Sample minimal samples in batch to estimate models idxs = self.sample(self.minimal_sample_size, num_tc, self.batch_size, kp1.device) kp1_sampled = kp1[idxs] kp2_sampled = kp2[idxs] kp1_sampled, kp2_sampled = self.remove_bad_samples(kp1_sampled, kp2_sampled) if len(kp1_sampled) == 0: continue # Estimate models models = self.estimate_model_from_minsample(kp1_sampled, kp2_sampled) models = self.remove_bad_models(models) if (models is None) or (len(models) == 0): continue # Score the models and select the best one model, inliers, model_score = self.verify(kp1, kp2, models, self.inl_th) # Store far-the-best model and (optionally) do a local optimization if model_score > best_score_total: # Local optimization for lo_step in range(self.max_lo_iters): model_lo = self.polish_model(kp1, kp2, inliers) if (model_lo is None) or (len(model_lo) == 0): continue _, inliers_lo, score_lo = self.verify(kp1, kp2, model_lo, self.inl_th) # print (f"Orig score = {best_model_score}, LO score = {score_lo} TC={num_tc}") if score_lo > model_score: model = model_lo.clone()[0] inliers = inliers_lo.clone() model_score = score_lo else: break # Now storing the best model best_model_total = model.clone() inliers_best_total = inliers.clone() best_score_total = model_score # Should we already stop? new_max_iter = int(self.max_samples_by_conf(int(best_score_total), num_tc, self.minimal_sample_size, self.confidence)) # print (f"New max_iter = {new_max_iter}") # Stop estimation, if the model is very good if (i + 1) * self.batch_size >= new_max_iter: break # local optimization with all inliers for better precision return best_model_total, inliers_best_total