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I solved the problem, which is not related with the reprojectImageto3D --that works fine--, but with this piece of code I've wrote and that I used to reproject the points onto the original frames:

def proj_dist(P, dcoeffs, M):
import numpy as np
import cv2

K, R, t,_,_,_,_ = cv2.decomposeProjectionMatrix(P)
rotv, _ = cv2.Rodrigues(R)

# Projection. Returns a (N,2) shaped array
m,_ = cv2.projectPoints(M,rotv,t[0:-1],K,dcoeffs)
m = m.squeeze()

return m

I've wrote my own function for points projection:

def proj(P, M, hom=0):
# proj(): Esegue la proiezione prospettica dei punti 3D M secondo la MPP P,
# sul piano immagine 2D di una camera pinhole.

import numpy as np

n = M.shape[1]
M = np.concatenate((M, np.ones((1,n))))

# Proiezione
m = P @ M

m = m/m[2,:]

if hom !=1 :
    # Passo a cartesiane
    m = m[0:2,:]

return m

and the problem is solved! My function does not take in account for lens distortion. I'll further investigate the problem related with the projectPoints() OpenCV function.