import math import warnings import numpy as np from scipy import optimize def _check_data_dim(data, dim): if data.ndim != 2 or data.shape[1] != dim: raise ValueError('Input data must have shape (N, %d).' % dim) class BaseModel(object): def __init__(self): self.params = None @property def _params(self): warnings.warn('`_params` attribute is deprecated, ' 'use `params` instead.') return self.params class LineModel(BaseModel): """Total least squares estimator for 2D lines. Lines are parameterized using polar coordinates as functional model:: dist = x * cos(theta) + y * sin(theta) This parameterization is able to model vertical lines in contrast to the standard line model ``y = a*x + b``. This estimator minimizes the squared distances from all points to the line:: min{ sum((dist - x_i * cos(theta) + y_i * sin(theta))**2) } A minimum number of 2 points is required to solve for the parameters. Attributes ---------- params : tuple Line model parameters in the following order `dist`, `theta`. """ def estimate(self, data): """Estimate line model from data using total least squares. Parameters ---------- data : (N, 2) array N points with ``(x, y)`` coordinates, respectively. """ _check_data_dim(data, dim=2) X0 = data.mean(axis=0) if data.shape[0] == 2: # well determined theta = np.arctan2(data[1, 1] - data[0, 1], data[1, 0] - data[0, 0]) elif data.shape[0] > 2: # over-determined data = data - X0 # first principal component _, _, v = np.linalg.svd(data) theta = np.arctan2(v[0, 1], v[0, 0]) else: # under-determined raise ValueError('At least 2 input points needed.') # angle perpendicular to line angle theta = (theta + np.pi / 2) % np.pi # line always passes through mean dist = X0[0] * math.cos(theta) + X0[1] * math.sin(theta) self.params = (dist, theta) def residuals(self, data): """Determine residuals of data to model. For each point the shortest distance to the line is returned. Parameters ---------- data : (N, 2) array N points with ``(x, y)`` coordinates, respectively. Returns ------- residuals : (N, ) array Residual for each data point. """ _check_data_dim(data, dim=2) dist, theta = self.params x = data[:, 0] y = data[:, 1] return dist - (x * math.cos(theta) + y * math.sin(theta)) def predict_x(self, y, params=None): """Predict x-coordinates using the estimated model. Parameters ---------- y : array y-coordinates. params : (2, ) array, optional Optional custom parameter set. Returns ------- x : array Predicted x-coordinates. """ if params is None: params = self.params dist, theta = params return (dist - y * math.sin(theta)) / math.cos(theta) def predict_y(self, x, params=None): """Predict y-coordinates using the estimated model. Parameters ---------- x : array x-coordinates. params : (2, ) array, optional Optional custom parameter set. Returns ------- y : array Predicted y-coordinates. """ if params is None: params = self.params dist, theta = params return (dist - x * math.cos(theta)) / math.sin(theta) class CircleModel(BaseModel): """Total least squares estimator for 2D circles. The functional model of the circle is:: r**2 = (x - xc)**2 + (y - yc)**2 This estimator minimizes the squared distances from all points to the circle:: min{ sum((r - sqrt((x_i - xc)**2 + (y_i - yc)**2))**2) } A minimum number of 3 points is required to solve for the parameters. Attributes ---------- params : tuple Circle model parameters in the following order `xc`, `yc`, `r`. """ def estimate(self, data): """Estimate circle model from data using total least squares. Parameters ---------- data : (N, 2) array N points with ``(x, y)`` coordinates, respectively. """ _check_data_dim(data, dim=2) x = data[:, 0] y = data[:, 1] # pre-allocate jacobian for all iterations A = np.zeros((3, data.shape[0]), dtype=np.double) # same for all iterations: r A[2, :] = -1 def dist(xc, yc): return np.sqrt((x - xc)**2 + (y - yc)**2) def fun(params): xc, yc, r = params return dist(xc, yc) - r def Dfun(params): xc, yc, r = params d = dist(xc, yc) A[0, :] = -(x - xc) / d A[1, :] = -(y - yc) / d # same for all iterations, so not changed in each iteration #A[2, :] = -1 return A xc0 = x.mean() yc0 = y.mean() r0 = dist(xc0, yc0).mean() params0 = (xc0, yc0, r0) params, _ = optimize.leastsq(fun, params0, Dfun=Dfun, col_deriv=True) self.params = params def residuals(self, data): """Determine residuals of data to model. For each point the shortest distance to the circle is returned. Parameters ---------- data : (N, 2) array N points with ``(x, y)`` coordinates, respectively. Returns ------- residuals : (N, ) array Residual for each data point. """ _check_data_dim(data, dim=2) xc, yc, r = self.params x = data[:, 0] y = data[:, 1] return r - np.sqrt((x - xc)**2 + (y - yc)**2) def predict_xy(self, t, params=None): """Predict x- and y-coordinates using the estimated model. Parameters ---------- t : array Angles in circle in radians. Angles start to count from positive x-axis to positive y-axis in a right-handed system. params : (3, ) array, optional Optional custom parameter set. Returns ------- xy : (..., 2) array Predicted x- and y-coordinates. """ if params is None: params = self.params xc, yc, r = params x = xc + r * np.cos(t) y = yc + r * np.sin(t) return np.concatenate((x[..., None], y[..., None]), axis=t.ndim) class EllipseModel(BaseModel): """Total least squares estimator for 2D ellipses. The functional model of the ellipse is:: xt = xc + a*cos(theta)*cos(t) - b*sin(theta)*sin(t) yt = yc + a*sin(theta)*cos(t) + b*cos(theta)*sin(t) d = sqrt((x - xt)**2 + (y - yt)**2) where ``(xt, yt)`` is the closest point on the ellipse to ``(x, y)``. Thus d is the shortest distance from the point to the ellipse. This estimator minimizes the squared distances from all points to the ellipse:: min{ sum(d_i**2) } = min{ sum((x_i - xt)**2 + (y_i - yt)**2) } Thus you have ``2 * N`` equations (x_i, y_i) for ``N + 5`` unknowns (t_i, xc, yc, a, b, theta), which gives you an effective redundancy of ``N - 5``. The ``params`` attribute contains the parameters in the following order:: xc, yc, a, b, theta A minimum number of 5 points is required to solve for the parameters. Attributes ---------- params : tuple Ellipse model parameters in the following order `xc`, `yc`, `a`, `b`, `theta`. """ def estimate(self, data): """Estimate circle model from data using total least squares. Parameters ---------- data : (N, 2) array N points with ``(x, y)`` coordinates, respectively. """ _check_data_dim(data, dim=2) x = data[:, 0] y = data[:, 1] N = data.shape[0] # pre-allocate jacobian for all iterations A = np.zeros((N + 5, 2 * N), dtype=np.double) # same for all iterations: xc, yc A[0, :N] = -1 A[1, N:] = -1 diag_idxs = np.diag_indices(N) def fun(params): xyt = self.predict_xy(params[5:], params[:5]) fx = x - xyt[:, 0] fy = y - xyt[:, 1] return np.append(fx, fy) def Dfun(params): xc, yc, a, b, theta = params[:5] t = params[5:] ct = np.cos(t) st = np.sin(t) ctheta = math.cos(theta) stheta = math.sin(theta) # derivatives for fx, fy in the following order: # xc, yc, a, b, theta, t_i # fx A[2, :N] = - ctheta * ct A[3, :N] = stheta * st A[4, :N] = a * stheta * ct + b * ctheta * st A[5:, :N][diag_idxs] = a * ctheta * st + b * stheta * ct # fy A[2, N:] = - stheta * ct A[3, N:] = - ctheta * st A[4, N:] = - a * ctheta * ct + b * stheta * st A[5:, N:][diag_idxs] = a * stheta * st - b * ctheta * ct return A # initial guess of parameters using a circle model params0 = np.empty((N + 5, ), dtype=np.double) xc0 = x.mean() yc0 = y.mean() r0 = np.sqrt((x - xc0)**2 + (y - yc0)**2).mean() params0[:5] = (xc0, yc0, r0, 0, 0) params0[5:] = np.arctan2(y - yc0, x - xc0) params, _ = optimize.leastsq(fun, params0, Dfun=Dfun, col_deriv=True) self.params = params[:5] def residuals(self, data): """Determine residuals of data to model. For each point the shortest distance to the ellipse is returned. Parameters ---------- data : (N, 2) array N points with ``(x, y)`` coordinates, respectively. Returns ------- residuals : (N, ) array Residual for each data point. """ _check_data_dim(data, dim=2) xc, yc, a, b, theta = self.params ctheta = math.cos(theta) stheta = math.sin(theta) x = data[:, 0] y = data[:, 1] N = data.shape[0] def fun(t, xi, yi): ct = math.cos(t) st = math.sin(t) xt = xc + a * ctheta * ct - b * stheta * st yt = yc + a * stheta * ct + b * ctheta * st return (xi - xt)**2 + (yi - yt)**2 # def Dfun(t, xi, yi): # ct = math.cos(t) # st = math.sin(t) # xt = xc + a * ctheta * ct - b * stheta * st # yt = yc + a * stheta * ct + b * ctheta * st # dfx_t = - 2 * (xi - xt) * (- a * ctheta * st # - b * stheta * ct) # dfy_t = - 2 * (yi - yt) * (- a * stheta * st # + b * ctheta * ct) # return [dfx_t + dfy_t] residuals = np.empty((N, ), dtype=np.double) # initial guess for parameter t of closest point on ellipse t0 = np.arctan2(y - yc, x - xc) - theta # determine shortest distance to ellipse for each point for i in range(N): xi = x[i] yi = y[i] # faster without Dfun, because of the python overhead t, _ = optimize.leastsq(fun, t0[i], args=(xi, yi)) residuals[i] = np.sqrt(fun(t, xi, yi)) return residuals def predict_xy(self, t, params=None): """Predict x- and y-coordinates using the estimated model. Parameters ---------- t : array Angles in circle in radians. Angles start to count from positive x-axis to positive y-axis in a right-handed system. params : (5, ) array, optional Optional custom parameter set. Returns ------- xy : (..., 2) array Predicted x- and y-coordinates. """ if params is None: params = self.params xc, yc, a, b, theta = params ct = np.cos(t) st = np.sin(t) ctheta = math.cos(theta) stheta = math.sin(theta) x = xc + a * ctheta * ct - b * stheta * st y = yc + a * stheta * ct + b * ctheta * st return np.concatenate((x[..., None], y[..., None]), axis=t.ndim) def ransac(data, model_class, min_samples, residual_threshold, is_data_valid=None, is_model_valid=None, max_trials=100, stop_sample_num=np.inf, stop_residuals_sum=0): """Fit a model to data with the RANSAC (random sample consensus) algorithm. RANSAC is an iterative algorithm for the robust estimation of parameters from a subset of inliers from the complete data set. Each iteration performs the following tasks: 1. Select `min_samples` random samples from the original data and check whether the set of data is valid (see `is_data_valid`). 2. Estimate a model to the random subset (`model_cls.estimate(*data[random_subset]`) and check whether the estimated model is valid (see `is_model_valid`). 3. Classify all data as inliers or outliers by calculating the residuals to the estimated model (`model_cls.residuals(*data)`) - all data samples with residuals smaller than the `residual_threshold` are considered as inliers. 4. Save estimated model as best model if number of inlier samples is maximal. In case the current estimated model has the same number of inliers, it is only considered as the best model if it has less sum of residuals. These steps are performed either a maximum number of times or until one of the special stop criteria are met. The final model is estimated using all inlier samples of the previously determined best model. Parameters ---------- data : [list, tuple of] (N, D) array Data set to which the model is fitted, where N is the number of data points and D the dimensionality of the data. If the model class requires multiple input data arrays (e.g. source and destination coordinates of ``skimage.transform.AffineTransform``), they can be optionally passed as tuple or list. Note, that in this case the functions ``estimate(*data)``, ``residuals(*data)``, ``is_model_valid(model, *random_data)`` and ``is_data_valid(*random_data)`` must all take each data array as separate arguments. model_class : object Object with the following object methods: * ``estimate(*data)`` * ``residuals(*data)`` min_samples : int The minimum number of data points to fit a model to. residual_threshold : float Maximum distance for a data point to be classified as an inlier. is_data_valid : function, optional This function is called with the randomly selected data before the model is fitted to it: `is_data_valid(*random_data)`. is_model_valid : function, optional This function is called with the estimated model and the randomly selected data: `is_model_valid(model, *random_data)`, . max_trials : int, optional Maximum number of iterations for random sample selection. stop_sample_num : int, optional Stop iteration if at least this number of inliers are found. stop_residuals_sum : float, optional Stop iteration if sum of residuals is less equal than this threshold. Returns ------- model : object Best model with largest consensus set. inliers : (N, ) array Boolean mask of inliers classified as ``True``. References ---------- .. [1] "RANSAC", Wikipedia, http://en.wikipedia.org/wiki/RANSAC Examples -------- Generate ellipse data without tilt and add noise: >>> t = np.linspace(0, 2 * np.pi, 50) >>> a = 5 >>> b = 10 >>> xc = 20 >>> yc = 30 >>> x = xc + a * np.cos(t) >>> y = yc + b * np.sin(t) >>> data = np.column_stack([x, y]) >>> np.random.seed(seed=1234) >>> data += np.random.normal(size=data.shape) Add some faulty data: >>> data[0] = (100, 100) >>> data[1] = (110, 120) >>> data[2] = (120, 130) >>> data[3] = (140, 130) Estimate ellipse model using all available data: >>> model = EllipseModel() >>> model.estimate(data) >>> model.params # doctest: +SKIP array([ -3.30354146e+03, -2.87791160e+03, 5.59062118e+03, 7.84365066e+00, 7.19203152e-01]) Estimate ellipse model using RANSAC: >>> ransac_model, inliers = ransac(data, EllipseModel, 5, 3, max_trials=50) >>> ransac_model.params array([ 20.12762373, 29.73563063, 4.81499637, 10.4743584 , 0.05217117]) >>> inliers array([False, False, False, False, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True], dtype=bool) Robustly estimate geometric transformation: >>> from skimage.transform import SimilarityTransform >>> np.random.seed(0) >>> src = 100 * np.random.rand(50, 2) >>> model0 = SimilarityTransform(scale=0.5, rotation=1, ... translation=(10, 20)) >>> dst = model0(src) >>> dst[0] = (10000, 10000) >>> dst[1] = (-100, 100) >>> dst[2] = (50, 50) >>> model, inliers = ransac((src, dst), SimilarityTransform, 2, 10) >>> inliers array([False, False, False, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True], dtype=bool) """ best_model = None best_inlier_num = 0 best_inlier_residuals_sum = np.inf best_inliers = None if not isinstance(data, list) and not isinstance(data, tuple): data = [data] # make sure data is list and not tuple, so it can be modified below data = list(data) # number of samples N = data[0].shape[0] for _ in range(max_trials): # choose random sample set samples = [] random_idxs = np.random.randint(0, N, min_samples) for d in data: samples.append(d[random_idxs]) # check if random sample set is valid if is_data_valid is not None and not is_data_valid(*samples): continue # estimate model for current random sample set sample_model = model_class() sample_model.estimate(*samples) # check if estimated model is valid if is_model_valid is not None and not is_model_valid(sample_model, *samples): continue sample_model_residuals = np.abs(sample_model.residuals(*data)) # consensus set / inliers sample_model_inliers = sample_model_residuals < residual_threshold sample_model_residuals_sum = np.sum(sample_model_residuals**2) # choose as new best model if number of inliers is maximal sample_inlier_num = np.sum(sample_model_inliers) if ( # more inliers sample_inlier_num > best_inlier_num # same number of inliers but less "error" in terms of residuals or (sample_inlier_num == best_inlier_num and sample_model_residuals_sum < best_inlier_residuals_sum) ): best_model = sample_model best_inlier_num = sample_inlier_num best_inlier_residuals_sum = sample_model_residuals_sum best_inliers = sample_model_inliers if ( best_inlier_num >= stop_sample_num or best_inlier_residuals_sum <= stop_residuals_sum ): break # estimate final model using all inliers if best_inliers is not None: # select inliers for each data array for i in range(len(data)): data[i] = data[i][best_inliers] best_model.estimate(*data) return best_model, best_inliers