How can I Get Laplacian pyramid using opencv

Question

​​I'm trying to get a layer of the Laplacian pyramid using the opencv functions: pyrUp and pyrDown.

In the documentation and in more detail in this book, I found that the i-th Laplacian layer should be obtained by the following expression:

Li = Gi - pyrDown(Gi+1)

where Gi is the i-th layer of the Gaussian pyramid.

What I've tried is:

def get_laplacian_pyramid_layer(img, n):
    gi = img
    for i in range(n):
        gi_prev = gi
        gi = cv2.pyrDown(gi_prev)
    pyrup = cv2.pyrUp(gi)
    return cv2.addWeighted(gi_prev, 1.5, pyrup, -0.5, 0)

But I get different sizes of the images involved in the substraction. I don't understand it because pyrUp is suppposed to invert the process of the Gaussian pyramid, i.e. pyrDown (with a lose of information of course but that should not affect the size, right?).

UPDATE

I refactored my code to:

def get_laplacian_pyramid_layer(img, n):
'''Returns the n-th layer of the laplacian pyramid'''  
    currImg, i = img, 0
    while i < n: # and currImg.size > max_level (83)
        down, up = new_empty_img(img.shape), new_empty_img(img.shape)
        down = cv2.pyrDown(img)
        up = cv2.pyrUp(down, dstsize=currImg.shape)
        lap = currImg - up
        currImg = down
        i += 1
    return lap

As you can see, I force the destination image to be of the same size of the source with the parameter dstsize of the pyrUp function.

However, this code also gives me an error when when executing the pyrUp function. The message of the error is:

OpenCV Error: Assertion failed (std::abs(dsize.width - ssize.width*2) == dsize.width % 2 && std::abs(dsize.height - ssize.height*2) == dsize.height % 2) in pyrUp_,

In debug mode I checked the expression of the assertion with:

up.shape[1]-down.shape[1]*2 == up.shape[1] %2 and up.shape[0]-down.shape[0]*2 == up.shape[0] %2

and it is satisfied.

So, I don't have a clue of what is happening.

Answer 1

As far as I can see you use pyrDown on your input image img in every iteration

down = cv2.pyrDown(img)

I suggest you change that line to

down = cv2.pyrDown(currImg)

so you actually compute the next pyramid layer.

The reason for the error is your down image. Its shape is width/2 x height/2 compared to the input image

down = cv2.pyrDown(img)

yet you try to store its pyrUp result (width * height) in a much smaller up image with its shape beeing smaller (width/2 x height/2) due to

up = cv2.pyrUp(down, dstsize=currImg.shape)
...
currImg = down