cv2.imshow() does not work inside a custom function

Question

OpenCV imshow function seems to work outside a function but not inside that function (and I don't think the answer is waitKey()). Why is this happening and how can I solve it?

I am learning 2D Gabor wavelets. For experimenting and observing I first created a kernel visualizer. That is a nice, happy javascript code that showed me what kernel I am creating. I would really appreciate if you take a look on it: http://alpersunter.github.io/Gabor2D/

This obviously does not give me how the resulting image looks like after applying the kernel. So I installed OpenCV (4.1.0) and wrote a program on python (3.6.7 installed on Ubuntu 18.04) that supposedly should act in a similar but more impressive way to my kernel visualizer. It should not only calculate the kernel in real time but also apply the filter to original image and its gray-scale counterpart and lastly show them.

I am using cv2.createTrackbar() to create sliders and after any parameter changes, trackbar calls a "myValueChanged(newVal)" function. That function than calculates new kernel with updated parameter and finally it redraws new image (which is the return value of the function cv2.filter2D(src, -1, kernel)).

Now, everything works if I call imshow() inside the valueChanged() function. However, if I encapsulate kernel calculation, kernel application and cv2.imshow(result) inside another function that I called "redraw()", imshow does not want to work.

Here is a sample of my code:

import cv2
import numpy as np

messi_0_window = "Messi0" # original image
cv2.namedWindow(messi_0_window, cv2.WINDOW_NORMAL)
messi_grey_window = "Messigray" # grayscale of origial
cv2.namedWindow(messi_grey_window, cv2.WINDOW_NORMAL)
messi_1_window = "Messi1" # this will be the result of kernel convolution of rgb messi
cv2.namedWindow(messi_1_window, cv2.WINDOW_NORMAL)
messi_2_window = "Messi2" # and this will be the kernel conv. of grayscale messi
cv2.namedWindow(messi_2_window, cv2.WINDOW_NORMAL)
messi_0 = cv2.imread("messi.jpg")
messi_grey = cv2.cvtColor(messi_0, cv2.COLOR_BGR2GRAY)

kernelSize = (21, 21)
sigma = 10
theta = 0
psi = 0
lambd = 10

def thetaChanged(degrees):
    theta = (degrees/180)*np.pi
    print("I have got a call")
    kernel = cv2.getGaborKernel(kernelSize, sigma, theta, lambd, 1, psi)
    messi_1 = cv2.filter2D(messi_0, -1, kernel)
    messi_2 = cv2.filter2D(messi_grey, -1, kernel)
    cv2.imshow(messi_1_window, messi_1)
    cv2.imshow(messi_2_window, messi_2)

def psiChanged(degrees):
    psi = (degrees/180)*np.pi
    redraw()
    cv2.waitKey(1) # removing this line doesn't help

def redraw():
    print("I have got a call") # This is printed always when I move the slider, independent of whether new image is drawn or not

    kernel = cv2.getGaborKernel(kernelSize, sigma, theta, lambd, 1, psi)
    messi_1 = cv2.filter2D(messi_0, -1, kernel)
    messi_2 = cv2.filter2D(messi_grey, -1, kernel)
    cv2.imshow(messi_1_window, messi_1)
    cv2.imshow(messi_2_window, messi_2)


cv2.createTrackbar("Theta", messi_0_window, 0, 180, thetaChanged)
cv2.createTrackbar("Psi", messi_0_window, 0, 180, psiChanged)

cv2.imshow(messi_0_window, messi_0)
cv2.imshow(messi_grey_window, messi_grey)

cv2.waitKey()
print("Key is now pressed!")

When I move the theta slider, new image gets drawn on screen. Everything works well. If, however I move the psi slider, it only draws for once and then stops drawing new image. Interestingly, if I move the theta again and show another image, psi slider works only for one more time and stops again. Then you slide theta, and psi becomes live again but for only one image.

I don't know how to fix this or if this is a question related to opencv. Maybe it is because my python is bad. Anyways I hope you can teach me something new. Anything new taught to me is really appreciated.

Answer 1

The error in your code, psi is considered as a local variable inside the psiChanged method. All you need to do is explicitly state that psi is a global variable in that method. A bit of restructuring your code layout will definitely help. Here is the final working script:

import cv2
import numpy as np

messi_0_window = "Messi0" # original image
cv2.namedWindow(messi_0_window, cv2.WINDOW_NORMAL)
messi_grey_window = "Messigray" # grayscale of origial
cv2.namedWindow(messi_grey_window, cv2.WINDOW_NORMAL)
messi_1_window = "Messi1" # this will be the result of kernel convolution of rgb messi
cv2.namedWindow(messi_1_window, cv2.WINDOW_NORMAL)
messi_2_window = "Messi2" # and this will be the kernel conv. of grayscale messi
cv2.namedWindow(messi_2_window, cv2.WINDOW_NORMAL)
messi_0 = cv2.imread("messi.jpg")
messi_grey = cv2.cvtColor(messi_0, cv2.COLOR_BGR2GRAY)

kernelSize = (21, 21)
sigma = 10
theta = 0
psi = 0
lambd = 10

def thetaChanged(degrees):
    global theta
    theta = (degrees/180)*np.pi # Changes the global `theta`
    print("Theta changed - {}".format(theta))
    redraw() # Removed the duplicate code and replaced it with `redraw()` method

def psiChanged(degrees):
    global psi
    psi = (degrees/180)*np.pi # Changes the global `psi`
    print("Psi changed - {}".format(psi))
    redraw()

def redraw():
    global kernelSize, sigma, theta, lambd, psi
    print("I have got a call - {}".format(psi)) # This is printed always when I move the slider, independent of whether new image is drawn or not

    kernel = cv2.getGaborKernel(kernelSize, sigma, theta, lambd, 1, psi)
    messi_1 = cv2.filter2D(messi_0, -1, kernel)
    messi_2 = cv2.filter2D(messi_grey, -1, kernel)
    cv2.imshow(messi_1_window, messi_1)
    cv2.imshow(messi_2_window, messi_2)


cv2.createTrackbar("Theta", messi_0_window, 0, 180, thetaChanged)
cv2.createTrackbar("Psi", messi_0_window, 0, 180, psiChanged)

cv2.imshow(messi_0_window, messi_0)
cv2.imshow(messi_grey_window, messi_grey)

cv2.waitKey()
print("Key is now pressed!")