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Why does the 3D dot appear behind the camera?

I wrote a simple script to project 3D points into image databases on the inside of the camera and extreme. But when I have a camera at the origin, directed down along the z axis, and 3D points further down on the z axis, it appears behind the camera, not in front of it. Here is my script, I tested it so many times.

import numpy as np

def project(point, P):
    Hp = P.dot(point)

    if Hp[-1] < 0:
        print 'Point is behind camera'

    Hp = Hp / Hp[-1]
    print Hp[0][0], 'x', Hp[1][0]
    return Hp[0][0], Hp[1][0]

if __name__ == '__main__':

    # Rc and C are the camera orientation and location in world coordinates
    # Camera posed at origin pointed down the negative z-axis
    Rc = np.eye(3)
    C  = np.array([0, 0, 0])

    # Camera extrinsics
    R = Rc.T
    t = -R.dot(C).reshape(3, 1)

    # The camera projection matrix is then:
    # P = K [ R | t] which projects 3D world points 
    # to 2D homogenous image coordinates.

    # Example intrinsics dont really matter ...

    K = np.array([
        [2000, 0,  2000],
        [0,  2000, 1500],
        [0,  0,  1],
    ])


    # Sample point in front of camera
    # i.e. further down the negative x-axis
    # should project into center of image
    point = np.array([[0, 0, -10, 1]]).T

    # Project point into the camera
    P = K.dot(np.hstack((R, t)))

    # But when projecting it appears to be behind the camera?
    project(point,P)

The only thing I can think of is that the identification rotation matrix does not match the camera pointing down the negative z axis with the vector up in the direction of the positive y axis. But I don’t see how it is not, for example, I built Rc from a function such as gluLookAt, and gave it a camera at the origin, directed down the negative z axis. I would get a unit matrix.

+6
2

, :

if Hp[-1] < 0:
    print 'Point is behind camera'

, Z , Z :

if Hp[-1] > 0:
    print 'Point is behind camera'

, , 3D- : , -Z, X , Y , Z.

+3
# Rc and C are the camera orientation and location in world coordinates
# Camera posed at origin pointed down the negative z-axis
Rc = np.eye(3)
C  = np.array([0, 0, 0])
# Camera extrinsics
R = Rc.T
t = -R.dot(C).reshape(3, 1)

. :

| 1, 0, 0| 
| 0, 1, 0|
| 0, 0, 1|

Z. , :

|1, 0,  0| 
|0, 1,  0|
|0, 0, -1|

, . :

http://www.andre-gaschler.com/rotationconverter/

: R.t() (), R - . . , - 0 .

https://www.quora.com/What-is-an-inverse-identity-matrix

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