How to include a custom filter in a Keras based CNN?

Question

I am working on a fuzzy convolution filter for CNNs. I have the function ready - it takes in the 2D input matrix and the 2D kernel/weight matrix. The function outputs the convolved feature or the activation map.

Now, I want to use Keras to build the rest of the CNN that will have the standard 2D convolution filters too.

Is there any way I can insert my custom filter into the Keras model in such a way that the kernel matrix is updated by the built in libraries of the Keras backend? Alternatively is there any library that I can use to update the kernel with every iteration?

Answer 1

Suppose we want to apply a 3x3 custom filter onto an 6x6 image.

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Necessary import

import keras.backend as K
import numpy as np
from keras import Input, layers
from keras.models import Model

Definition of the custom filter

# custom filter
def my_filter(shape, dtype=None):

    f = np.array([
            [[[1]], [[0]], [[-1]]],
            [[[1]], [[0]], [[-1]]],
            [[[1]], [[0]], [[-1]]]
        ])
    assert f.shape == shape
    return K.variable(f, dtype='float32')

Dummy example input image (it is 1 channel image. So dimension will be 6x6x1 . Here, pixel values are random integer. Generally pixel values should be 0 to 255 or 0.0 to 1.0.)

input_mat = np.array([
    [ [4], [9], [2], [5], [8], [3] ],
    [ [3], [6], [2], [4], [0], [3] ],
    [ [2], [4], [5], [4], [5], [2] ],
    [ [5], [6], [5], [4], [7], [8] ],
    [ [5], [7], [7], [9], [2], [1] ],
    [ [5], [8], [5], [3], [8], [4] ]
])

# we need to give the batch size. 
# here we will just add a dimension at the beginning which makes batch size=1
input_mat = input_mat.reshape((1, 6, 6, 1))

Dummy conv model where we will use our custom filter

def build_model():
    input_tensor = Input(shape=(6,6,1))

    x = layers.Conv2D(filters=1, 
                      kernel_size = 3,
                      kernel_initializer=my_filter,
                      strides=2, 
                      padding='valid') (input_tensor)

    model = Model(inputs=input_tensor, outputs=x)
    return model

Testing

model = build_model()
out = model.predict(input_mat)
print(out)

Output

[[[[ 0.]
   [-4.]]

  [[-5.]
   [ 3.]]]]

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