matlab_cp2tform.py

import numpy as np
from numpy.linalg import inv, norm, lstsq
from numpy.linalg import matrix_rank as rank


class MatlabCp2tormException(Exception):
    def __str__(self):
        return "In File {}:{}".format(
                __file__, super.__str__(self))

def tformfwd(trans, uv):
    """
    Function:
    ----------
        apply affine transform 'trans' to uv

    Parameters:
    ----------
        @trans: 3x3 np.array
            transform matrix
        @uv: Kx2 np.array
            each row is a pair of coordinates (x, y)

    Returns:
    ----------
        @xy: Kx2 np.array
            each row is a pair of transformed coordinates (x, y)
    """
    uv = np.hstack((
        uv, np.ones((uv.shape[0], 1))
    ))
    xy = np.dot(uv, trans)
    xy = xy[:, 0:-1]
    return xy


def tforminv(trans, uv):
    """
    Function:
    ----------
        apply the inverse of affine transform 'trans' to uv

    Parameters:
    ----------
        @trans: 3x3 np.array
            transform matrix
        @uv: Kx2 np.array
            each row is a pair of coordinates (x, y)

    Returns:
    ----------
        @xy: Kx2 np.array
            each row is a pair of inverse-transformed coordinates (x, y)
    """
    Tinv = inv(trans)
    xy = tformfwd(Tinv, uv)
    return xy


def findNonreflectiveSimilarity(uv, xy, options=None):

    options = {'K': 2}

    K = options['K']
    M = xy.shape[0]
    x = xy[:, 0].reshape((-1, 1))  # use reshape to keep a column vector
    y = xy[:, 1].reshape((-1, 1))  # use reshape to keep a column vector
    # print('--->x, y:\n', x, y

    tmp1 = np.hstack((x, y, np.ones((M, 1)), np.zeros((M, 1))))
    tmp2 = np.hstack((y, -x, np.zeros((M, 1)), np.ones((M, 1))))
    X = np.vstack((tmp1, tmp2))
    # print('--->X.shape: ', X.shape
    # print('X:\n', X

    u = uv[:, 0].reshape((-1, 1))  # use reshape to keep a column vector
    v = uv[:, 1].reshape((-1, 1))  # use reshape to keep a column vector
    U = np.vstack((u, v))
    # print('--->U.shape: ', U.shape
    # print('U:\n', U

    # We know that X * r = U
    if rank(X) >= 2 * K:
        r, _, _, _ = lstsq(X, U)
        r = np.squeeze(r)
    else:
        raise Exception("cp2tform: two Unique Points Req")

    # print('--->r:\n', r

    sc = r[0]
    ss = r[1]
    tx = r[2]
    ty = r[3]

    Tinv = np.array([
        [sc, -ss, 0],
        [ss,  sc, 0],
        [tx,  ty, 1]
    ])

    # print('--->Tinv:\n', Tinv

    T = inv(Tinv)
    # print('--->T:\n', T

    T[:, 2] = np.array([0, 0, 1])

    return T, Tinv


def findSimilarity(uv, xy, options=None):

    options = {'K': 2}

#    uv = np.array(uv)
#    xy = np.array(xy)

    # Solve for trans1
    trans1, trans1_inv = findNonreflectiveSimilarity(uv, xy, options)

    # Solve for trans2

    # manually reflect the xy data across the Y-axis
    xyR = xy
    xyR[:, 0] = -1 * xyR[:, 0]

    trans2r, trans2r_inv = findNonreflectiveSimilarity(uv, xyR, options)

    # manually reflect the tform to undo the reflection done on xyR
    TreflectY = np.array([
        [-1, 0, 0],
        [0, 1, 0],
        [0, 0, 1]
    ])

    trans2 = np.dot(trans2r, TreflectY)

    # Figure out if trans1 or trans2 is better
    xy1 = tformfwd(trans1, uv)
    norm1 = norm(xy1 - xy)

    xy2 = tformfwd(trans2, uv)
    norm2 = norm(xy2 - xy)

    if norm1 <= norm2:
        return trans1, trans1_inv
    else:
        trans2_inv = inv(trans2)
        return trans2, trans2_inv


def get_similarity_transform(src_pts, dst_pts, reflective = True):
    """
    Function:
    ----------
        Find Similarity Transform Matrix 'trans':
            u = src_pts[:, 0]
            v = src_pts[:, 1]
            x = dst_pts[:, 0]
            y = dst_pts[:, 1]
            [x, y, 1] = [u, v, 1] * trans

    Parameters:
    ----------
        @src_pts: Kx2 np.array
            source points, each row is a pair of coordinates (x, y)
        @dst_pts: Kx2 np.array
            destination points, each row is a pair of transformed
            coordinates (x, y)
        @reflective: True or False
            if True:
                use reflective similarity transform
            else:
                use non-reflective similarity transform

    Returns:
    ----------
       @trans: 3x3 np.array
            transform matrix from uv to xy
        trans_inv: 3x3 np.array
            inverse of trans, transform matrix from xy to uv
    """

    if reflective:
        trans, trans_inv = findSimilarity(src_pts, dst_pts)
    else:
        trans, trans_inv = findNonreflectiveSimilarity(src_pts, dst_pts)

    return trans, trans_inv


def cvt_tform_mat_for_cv2(trans):
    """
    Function:
    ----------
        Convert Transform Matrix 'trans' into 'cv2_trans' which could be
        directly used by cv2.warpAffine():
            u = src_pts[:, 0]
            v = src_pts[:, 1]
            x = dst_pts[:, 0]
            y = dst_pts[:, 1]
            [x, y].T = cv_trans * [u, v, 1].T

    Parameters:
    ----------
        @trans: 3x3 np.array
            transform matrix from uv to xy

    Returns:
    ----------
        @cv2_trans: 2x3 np.array
            transform matrix from src_pts to dst_pts, could be directly used
            for cv2.warpAffine()
    """
    cv2_trans = trans[:, 0:2].T

    return cv2_trans


def get_similarity_transform_for_cv2(src_pts, dst_pts, reflective = True):
    """
    Function:
    ----------
        Find Similarity Transform Matrix 'cv2_trans' which could be
        directly used by cv2.warpAffine():
            u = src_pts[:, 0]
            v = src_pts[:, 1]
            x = dst_pts[:, 0]
            y = dst_pts[:, 1]
            [x, y].T = cv_trans * [u, v, 1].T

    Parameters:
    ----------
        @src_pts: Kx2 np.array
            source points, each row is a pair of coordinates (x, y)
        @dst_pts: Kx2 np.array
            destination points, each row is a pair of transformed
            coordinates (x, y)
        reflective: True or False
            if True:
                use reflective similarity transform
            else:
                use non-reflective similarity transform

    Returns:
    ----------
        @cv2_trans: 2x3 np.array
            transform matrix from src_pts to dst_pts, could be directly used
            for cv2.warpAffine()
    """
    trans, trans_inv = get_similarity_transform(src_pts, dst_pts, reflective)
    cv2_trans = cvt_tform_mat_for_cv2(trans)

    return cv2_trans


if __name__ == '__main__':
    """
    u = [0, 6, -2]
    v = [0, 3, 5]
    x = [-1, 0, 4]
    y = [-1, -10, 4]

    # In Matlab, run:
    #
    #   uv = [u'; v'];
    #   xy = [x'; y'];
    #   tform_sim=cp2tform(uv,xy,'similarity');
    #
    #   trans = tform_sim.tdata.T
    #   ans =
    #       -0.0764   -1.6190         0
    #        1.6190   -0.0764         0
    #       -3.2156    0.0290    1.0000
    #   trans_inv = tform_sim.tdata.Tinv
    #    ans =
    #
    #       -0.0291    0.6163         0
    #       -0.6163   -0.0291         0
    #       -0.0756    1.9826    1.0000
    #    xy_m=tformfwd(tform_sim, u,v)
    #
    #    xy_m =
    #
    #       -3.2156    0.0290
    #        1.1833   -9.9143
    #        5.0323    2.8853
    #    uv_m=tforminv(tform_sim, x,y)
    #
    #    uv_m =
    #
    #        0.5698    1.3953
    #        6.0872    2.2733
    #       -2.6570    4.3314
    """
    u = [0, 6, -2]
    v = [0, 3, 5]
    x = [-1, 0, 4]
    y = [-1, -10, 4]

    uv = np.array((u, v)).T
    xy = np.array((x, y)).T

    print("\n--->uv:")
    print(uv)
    print("\n--->xy:")
    print(xy)

    trans, trans_inv = get_similarity_transform(uv, xy)

    print("\n--->trans matrix:")
    print(trans)

    print("\n--->trans_inv matrix:")
    print(trans_inv)

    print("\n---> apply transform to uv")
    print("\nxy_m = uv_augmented * trans")
    uv_aug = np.hstack((
        uv, np.ones((uv.shape[0], 1))
    ))
    xy_m = np.dot(uv_aug, trans)
    print(xy_m)

    print("\nxy_m = tformfwd(trans, uv)")
    xy_m = tformfwd(trans, uv)
    print(xy_m)

    print("\n---> apply inverse transform to xy")
    print("\nuv_m = xy_augmented * trans_inv")
    xy_aug = np.hstack((
        xy, np.ones((xy.shape[0], 1))
    ))
    uv_m = np.dot(xy_aug, trans_inv)
    print(uv_m)

    print("\nuv_m = tformfwd(trans_inv, xy)")
    uv_m = tformfwd(trans_inv, xy)
    print(uv_m)

    uv_m = tforminv(trans, xy)
    print("\nuv_m = tforminv(trans, xy)")
    print(uv_m)