Discrepancy in command line utility (ird) and python function (similarity)

[cs16s027]

I am trying to register two images that have been spatially translated (no rotations). When I use the command line utility ird, I get the exact (x, y) coordinates. But when I use the python utility, I get erroneous results. This is not the case for all the images, but it does happen in a few cases. I am unable to attach the actual images for copyright reasons.

Using python

template = cv2.imread(template_image_path, 0)
print template.shape # 512x512
subject = cv2.imread(subject_image_path, 0)
print subject.shape # 512x512

from imreg_dft.imreg import similarity
similarity(template, subject)

Output

'tvec': array([-14.03552074, -0.08051801]), 'Dt': 0.25

Using ird command-line

ird template_image_path subject_image_path

Output

shift (x, y): 58.9625, -0.0204523 +-0.25

I am not able to figure out what is wrong here.