# Negative values for y-coordinates after Triangulation

Hi,

I have been able to implement a basic version of triangulation ( using SIFT detector for stereo correspondence problem )
I have also noticed that the triangulation gives a poor result if the SIFT detector matches **non-corresponding** or incorrect feature points. However, as seen in images below, the SIFT detector seems to be doing a good job for the given frame.
However the triangulation doesn't seem to work out.

**Kindly see images below** for a given stereo camera setup-

1) Stereo Correspondence (for left and right camera images):

2) Triangulation (only shown in **left** camera image->):

**Clearly, the y-coordinate is negative** while values of x and z-coordinate for short distances seem "reasonable" (given that I have a rms re-projection error = 0.87 after calibration, which I'm aware needs to be improved).

**My Question**:

1) Is the camera pinhole model in openCV actually having the y-axis inverted ?

2) Is this a standard result of triangulation using openCV, ie having negative y-values ?

Any help would be greatly appreciated.

I was wondering the same thing but thanks for confirming that from the docs. If you can write this down as the answer instead of a comment, I will mark it as the answer.