Compute Homography Direct - known camera location(s) - Image is... aliased?

Question

I'm following the code presented here:

Compute homography for a virtual camera with opencv

As a note, I made a tiny modification to the code: in the translation matrix, I'm left-multiplying the first 3 rows of the last column by -R to get the translation in the global frame. I also changed the translation matrix definition to use "-dist" because in the global frame, movement toward the camera would be in the negative z direction.

When I turn the X rotation to 0, I get a weird... aliased version of the loaded image that appears ABOVE the horizon line, where there should be nothing.

My question:

Why? Is this just a weird artifact of how the homography is calculated? How can I get rid of it? I know for x=0 (in the presented code) I can just ignore/erase anything above the horizon line, but my use case the x rotation might be -10 to 10 degrees or so - how can I calculate where the horizon line would be in those cases (so I can ignore image data above it) - or is there a mathematical solution the computing the homography that will bypass this problem all together?

Thanks!

EDIT: Adding in code/image in question:

import cv2
import numpy as np

rotXdeg = 90
rotYdeg = 90
rotZdeg = 90
f = 500
dist = 500

def onRotXChange(val):
    global rotXdeg
    rotXdeg = val
def onRotYChange(val):
    global rotYdeg
    rotYdeg = val
def onRotZChange(val):
    global rotZdeg
    rotZdeg = val
def onFchange(val):
    global f
    f=val
def onDistChange(val):
    global dist
    dist=val

if __name__ == '__main__':

    #Read input image, and create output image
    src = cv2.imread('/path/to/image.jpg')
    dst = np.ndarray(shape=src.shape,dtype=src.dtype)

    #Create user interface with trackbars that will allow to modify the parameters of the transformation
    wndname1 = "Source:"
    wndname2 = "WarpPerspective: "
    cv2.namedWindow(wndname1, 1)
    cv2.namedWindow(wndname2, 1)
    cv2.createTrackbar("Rotation X", wndname2, rotXdeg, 180, onRotXChange)
    cv2.createTrackbar("Rotation Y", wndname2, rotYdeg, 180, onRotYChange)
    cv2.createTrackbar("Rotation Z", wndname2, rotZdeg, 180, onRotZChange)
    cv2.createTrackbar("f", wndname2, f, 2000, onFchange)
    cv2.createTrackbar("Distance", wndname2, dist, 2000, onDistChange)

    #Show original image
    cv2.imshow(wndname1, src)

    h , w = src.shape[:2]

    while True:

        rotX = (rotXdeg - 90)*np.pi/180
        rotY = (rotYdeg - 90)*np.pi/180
        rotZ = (rotZdeg - 90)*np.pi/180

        #Projection 2D -> 3D matrix
        A1= np.matrix([[1, 0, -w/2],
                       [0, 1, -h/2],
                       [0, 0, 0   ],
                       [0, 0, 1   ]])

        # Rotation matrices around the X,Y,Z axis
        RX = np.matrix([[1,           0,            0, 0],
                        [0,np.cos(rotX),-np.sin(rotX), 0],
                        [0,np.sin(rotX),np.cos(rotX) , 0],
                        [0,           0,            0, 1]])

        RY = np.matrix([[ np.cos(rotY), 0, np.sin(rotY), 0],
                        [            0, 1,            0, 0],
                        [ -np.sin(rotY), 0, np.cos(rotY), 0],
                        [            0, 0,            0, 1]])

        RZ = np.matrix([[ np.cos(rotZ), -np.sin(rotZ), 0, 0],
                        [ np.sin(rotZ), np.cos(rotZ), 0, 0],
                        [            0,            0, 1, 0],
                        [            0,            0, 0, 1]])

        #Composed rotation matrix with (RX,RY,RZ)
        R = RX * RY * RZ

        #Translation matrix on the Z axis change dist will change the height
        T = np.matrix([[1,0,0,0],
                       [0,1,0,0],
                       [0,0,1,-dist],
                       [0,0,0,1]])

        extractT = T[:3, 3:4]
        solveT = -R[:3, :3]@extractT
        T[:3, 3:4] = solveT

        #Camera Intrisecs matrix 3D -> 2D
        A2= np.matrix([[f, 0, w/2,0],
                       [0, f, h/2,0],
                       [0, 0,   1,0]])

        # Final and overall transformation matrix
        H = A2 * (T * (R * A1))

        # Apply matrix transformation
        cv2.warpPerspective(src, H, (w, h), dst, cv2.INTER_CUBIC)

        #Show the image
        cv2.imshow(wndname2, dst)
        if (cv2.waitKey(1) == ord('q')):
            break

Image: