Epipolar geometry pose estimation: Epipolar lines look good but wrong pose

Question

I am trying to use OpenCV to estimate one pose of a camera relative to another, using SIFT feature tracking, FLANN matching and subsequent calculations of the fundamental and essential matrix. After decomposing the essential matrix, I check for degenerate configurations and obtain the "right" R and t.

Problem is, they never seem to be right. I am including a couple of image pairs:

1. Image 2 taken with 45 degree rotation along the Y axis and same position w.r.t. Image 1.

Image pair

Result

2. Image 2 taken from approx. couple of meters away along the negative X direction, slight displacement in the negative Y direction. Approx. 45-60 degree rotation in camera pose along Y axis.

Image pair

Result

The translation vector in the second case, seems to be overestimating the movement in Y and underestimating the movement in X. The rotation matrices when converted to Euler angles give wrong results in both the cases. This happens with a lot of other datasets as well. I have tried switching the fundamental matrix computation technique between RANSAC, LMEDS etc., and am now doing it with RANSAC and a second computation using only the inliers with the 8 point method. Changing the feature detection method does not help either. The epipolar lines seem to be proper, and the fundamental matrix satisfies x'.F.x = 0

Am I missing something fundamentally wrong here? Given the program understands the epipolar geometry properly, what could possibly be happening that results in a completely wrong pose? I am doing the check to make sure points lie in front of both cameras. Any thoughts/suggestions would be very helpful. Thanks!

EDIT: Tried the same technique with two different calibrated cameras spaced apart; and computed essential matrix as K2'.F.K1, but still the translations and rotations are still way off.

Code for reference

import cv2
import numpy as np

from matplotlib import pyplot as plt

# K2 = np.float32([[1357.3, 0, 441.413], [0, 1355.9, 259.393], [0, 0, 1]]).reshape(3,3)
# K1 = np.float32([[1345.8, 0, 394.9141], [0, 1342.9, 291.6181], [0, 0, 1]]).reshape(3,3)

# K1_inv = np.linalg.inv(K1)
# K2_inv = np.linalg.inv(K2)

K = np.float32([3541.5, 0, 2088.8, 0, 3546.9, 1161.4, 0, 0, 1]).reshape(3,3)
K_inv = np.linalg.inv(K)

def in_front_of_both_cameras(first_points, second_points, rot, trans):
    # check if the point correspondences are in front of both images
    rot_inv = rot
    for first, second in zip(first_points, second_points):
        first_z = np.dot(rot[0, :] - second[0]*rot[2, :], trans) / np.dot(rot[0, :] - second[0]*rot[2, :], second)
        first_3d_point = np.array([first[0] * first_z, second[0] * first_z, first_z])
        second_3d_point = np.dot(rot.T, first_3d_point) - np.dot(rot.T, trans)

        if first_3d_point[2] < 0 or second_3d_point[2] < 0:
            return False

    return True

def drawlines(img1,img2,lines,pts1,pts2):
    ''' img1 - image on which we draw the epilines for the points in img1
        lines - corresponding epilines '''
    pts1 = np.int32(pts1)
    pts2 = np.int32(pts2)
    r,c = img1.shape
    img1 = cv2.cvtColor(img1,cv2.COLOR_GRAY2BGR)
    img2 = cv2.cvtColor(img2,cv2.COLOR_GRAY2BGR)
    for r,pt1,pt2 in zip(lines,pts1,pts2):
        color = tuple(np.random.randint(0,255,3).tolist())
        x0,y0 = map(int, [0, -r[2]/r[1] ])
        x1,y1 = map(int, [c, -(r[2]+r[0]*c)/r[1] ])
        cv2.line(img1, (x0,y0), (x1,y1), color,1)
        cv2.circle(img1,tuple(pt1), 10, color, -1)
        cv2.circle(img2,tuple(pt2), 10,color,-1)
    return img1,img2


img1 = cv2.imread('C:\\Users\\Sai\\Desktop\\room1.jpg', 0)  
img2 = cv2.imread('C:\\Users\\Sai\\Desktop\\room0.jpg', 0) 
img1 = cv2.resize(img1, (0,0), fx=0.5, fy=0.5)
img2 = cv2.resize(img2, (0,0), fx=0.5, fy=0.5)

sift = cv2.SIFT()

# find the keypoints and descriptors with SIFT
kp1, des1 = sift.detectAndCompute(img1,None)
kp2, des2 = sift.detectAndCompute(img2,None)

# FLANN parameters
FLANN_INDEX_KDTREE = 0
index_params = dict(algorithm = FLANN_INDEX_KDTREE, trees = 5)
search_params = dict(checks=50)   # or pass empty dictionary

flann = cv2.FlannBasedMatcher(index_params,search_params)

matches = flann.knnMatch(des1,des2,k=2)

good = []
pts1 = []
pts2 = []

# ratio test as per Lowe's paper
for i,(m,n) in enumerate(matches):
    if m.distance < 0.7*n.distance:
        good.append(m)
        pts2.append(kp2[m.trainIdx].pt)
        pts1.append(kp1[m.queryIdx].pt)

pts2 = np.float32(pts2)
pts1 = np.float32(pts1)
F, mask = cv2.findFundamentalMat(pts1,pts2,cv2.FM_RANSAC)

# Selecting only the inliers
pts1 = pts1[mask.ravel()==1]
pts2 = pts2[mask.ravel()==1]

F, mask = cv2.findFundamentalMat(pts1,pts2,cv2.FM_8POINT)

print "Fundamental matrix is"
print 
print F

pt1 = np.array([[pts1[0][0]], [pts1[0][1]], [1]])
pt2 = np.array([[pts2[0][0], pts2[0][1], 1]])

print "Fundamental matrix error check: %f"%np.dot(np.dot(pt2,F),pt1)
print " "


# drawing lines on left image
lines1 = cv2.computeCorrespondEpilines(pts2.reshape(-1,1,2), 2,F)
lines1 = lines1.reshape(-1,3)
img5,img6 = drawlines(img1,img2,lines1,pts1,pts2)

# drawing lines on right image
lines2 = cv2.computeCorrespondEpilines(pts1.reshape(-1,1,2), 1,F)
lines2 = lines2.reshape(-1,3)
img3,img4 = drawlines(img2,img1,lines2,pts2,pts1)

E = K.T.dot(F).dot(K)

print "The essential matrix is"
print E
print 

U, S, Vt = np.linalg.svd(E)
W = np.array([0.0, -1.0, 0.0, 1.0, 0.0, 0.0, 0.0, 0.0, 1.0]).reshape(3, 3)

first_inliers = []
second_inliers = []
for i in range(len(pts1)):
    # normalize and homogenize the image coordinates
    first_inliers.append(K_inv.dot([pts1[i][0], pts1[i][1], 1.0]))
    second_inliers.append(K_inv.dot([pts2[i][0], pts2[i][1], 1.0]))

# Determine the correct choice of second camera matrix
# only in one of the four configurations will all the points be in front of both cameras
# First choice: R = U * Wt * Vt, T = +u_3 (See Hartley Zisserman 9.19)

R = U.dot(W).dot(Vt)
T = U[:, 2]
if not in_front_of_both_cameras(first_inliers, second_inliers, R, T):

    # Second choice: R = U * W * Vt, T = -u_3
    T = - U[:, 2]
    if not in_front_of_both_cameras(first_inliers, second_inliers, R, T):

        # Third choice: R = U * Wt * Vt, T = u_3
        R = U.dot(W.T).dot(Vt)
        T = U[:, 2]

        if not in_front_of_both_cameras(first_inliers, second_inliers, R, T):

            # Fourth choice: R = U * Wt * Vt, T = -u_3
            T = - U[:, 2]

# Computing Euler angles

thetaX = np.arctan2(R[1][2], R[2][2])
c2 = np.sqrt((R[0][0]*R[0][0] + R[0][1]*R[0][1]))

thetaY = np.arctan2(-R[0][2], c2)

s1 = np.sin(thetaX)
c1 = np.cos(thetaX)

thetaZ = np.arctan2((s1*R[2][0] - c1*R[1][0]), (c1*R[1][1] - s1*R[2][1]))

print "Pitch: %f, Yaw: %f, Roll: %f"%(thetaX*180/3.1415, thetaY*180/3.1415, thetaZ*180/3.1415)

print "Rotation matrix:"
print R
print
print "Translation vector:"
print T

plt.subplot(121),plt.imshow(img5)
plt.subplot(122),plt.imshow(img3)
plt.show()

Answer 1

There are many things which can lead to inaccurate estimation of camera pose from point correspondences. Some factors you have to consider:-

(*) 8 point method minimizes algebraic error ( x'.F.x = 0). It is usually better to find a solution which minimizes a meaningful geometric error. For example, you can use re-projection error in your RANSAC implementation.

(*) The linear algorithm which solves for fundamental matrix from 8 points is sensitive to noise. Sub-pixel accurate point matching, proper data normalization and accurate camera calibration are all important for better results.

(*) Feature point localization and matching lead to noisy point matches, hence the solution you get by solving the algebraic equation x'Fx should really be used as an initial estimate and further steps such as parameter optimization need to be applied to refine the solution.

(*) Some two view camera configurations can lead to an ambiguous solution hence further methods (such as third view disambiguation) are needed for reliable results.