Source code for kornia.geometry.homography

import warnings

import torch
from typing import Tuple, Optional

import kornia
from kornia.geometry.epipolar import normalize_points

TupleTensor = Tuple[torch.Tensor, torch.Tensor]


[docs]def find_homography_dlt( points1: torch.Tensor, points2: torch.Tensor, weights: Optional[torch.Tensor] = None) -> torch.Tensor: r"""Computes the homography matrix using the DLT formulation. The linear system is solved by using the Weighted Least Squares Solution for the 4 Points algorithm. Args: points1 (torch.Tensor): A set of points in the first image with a tensor shape :math:`(B, N, 2)`. points2 (torch.Tensor): A set of points in the second image with a tensor shape :math:`(B, N, 2)`. weights (torch.Tensor, optional): Tensor containing the weights per point correspondence with a shape of :math:`(B, N)`. Defaults to all ones. Returns: torch.Tensor: the computed homography matrix with shape :math:`(B, 3, 3)`. """ assert points1.shape == points2.shape, points1.shape assert len(points1.shape) >= 1 and points1.shape[-1] == 2, points1.shape assert points1.shape[1] >= 4, points1.shape eps: float = 1e-8 points1_norm, transform1 = normalize_points(points1) points2_norm, transform2 = normalize_points(points2) x1, y1 = torch.chunk(points1_norm, dim=-1, chunks=2) # BxNx1 x2, y2 = torch.chunk(points2_norm, dim=-1, chunks=2) # BxNx1 ones, zeros = torch.ones_like(x1), torch.zeros_like(x1) # DIAPO 11: https://www.uio.no/studier/emner/matnat/its/nedlagte-emner/UNIK4690/v16/forelesninger/lecture_4_3-estimating-homographies-from-feature-correspondences.pdf # noqa: E501 ax = torch.cat([zeros, zeros, zeros, -x1, -y1, -ones, y2 * x1, y2 * y1, y2], dim=-1) ay = torch.cat([x1, y1, ones, zeros, zeros, zeros, -x2 * x1, -x2 * y1, -x2], dim=-1) A = torch.cat((ax, ay), dim=-1).reshape(ax.shape[0], -1, ax.shape[-1]) if weights is None: # All points are equally important A = A.transpose(-2, -1) @ A else: # We should use provided weights assert len(weights.shape) == 2 and weights.shape == points1.shape[:2], weights.shape w_diag = torch.diag_embed(weights.unsqueeze(dim=-1).repeat(1, 1, 2).reshape(weights.shape[0], -1)) A = A.transpose(-2, -1) @ w_diag @ A try: U, S, V = torch.svd(A) except: warnings.warn('SVD did not converge', RuntimeWarning) return torch.empty((points1_norm.size(0), 3, 3), device=points1.device) H = V[..., -1].view(-1, 3, 3) H = transform2.inverse() @ (H @ transform1) H_norm = H / (H[..., -1:, -1:] + eps) return H_norm
[docs]def find_homography_dlt_iterated(points1: torch.Tensor, points2: torch.Tensor, weights: torch.Tensor, soft_inl_th: float = 3.0, n_iter: int = 5) -> torch.Tensor: r"""Computes the homography matrix using the iteratively-reweighted least squares (IRWLS). The linear system is solved by using the Reweighted Least Squares Solution for the 4 Points algorithm. Args: points1 (torch.Tensor): A set of points in the first image with a tensor shape :math:`(B, N, 2)`. points2 (torch.Tensor): A set of points in the second image with a tensor shape :math:`(B, N, 2)`. weights (torch.Tensor): Tensor containing the weights per point correspondence with a shape of :math:`(B, N)`. Used for the first iteration of the IRWLS. soft_inl_th (float): Soft inlier threshold used for weight calculation. n_iter (int): number of iterations. Default is 5. Returns: torch.Tensor: the computed homography matrix with shape :math:`(B, 3, 3)`. """ '''Function, which finds homography via iteratively-reweighted least squares ToDo: add citation''' H: torch.Tensor = find_homography_dlt(points1, points2, weights) for i in range(n_iter - 1): pts1_in_2: torch.Tensor = kornia.transform_points(H, points1) error_squared: torch.Tensor = (pts1_in_2 - points2).pow(2).sum(dim=-1) weights_new: torch.Tensor = torch.exp(-error_squared / (2.0 * (soft_inl_th ** 2))) H = find_homography_dlt(points1, points2, weights_new) return H