The different with the image convolve with the conv2D of keras

Question

The goal is trying to using a customize weight on the first layer of model to full fill the function of highpass filter---make the first layer of Model is the same as the high pass filter to the image.

1.first, the similar solution will be: using a high pass filter in the image processing, and generate a new image, and use it in the model. ---this is have to use the image processing, which is cost of time.

2.I want to set the a layer of Conv2D , which is also able to high pass the image. with a custom filter( as a intializer). the basic is that the filter and the conv2D is both using convolution rules.

but the results are different from the first solution.

#The image processing code:
    kernel55 = np.array([[-1, 2, -2, 2, -1], 
                         [2, -6, 8, -6, 2], 
                         [-2, 8, -12, 8, -2], 
                         [2,-6, 8, -6, 2],
                         [-1, 2, -2, 2, -1]])/12
        # load the image, pre-process it, and store it in the data list
        image = cv2.imread('1.pgm',-1)
        image = ndimage.convolve(image, kernel55)
        print(image)

#the first layer of the Model:

    def kernel_init(shape):
        kernel = np.zeros(shape)
        kernel[:,:,0,0] = np.array([[-1, 2, -2, 2, -1], 
                             [2, -6, 8, -6, 2], 
                             [-2, 8, -12, 8, -2], 
                             [2,-6, 8, -6, 2],
                             [-1, 2, -2, 2, -1]])/12
        return kernel
    #Build Keras model
    model = Sequential()
    model.add(Conv2D(1, [5,5], kernel_initializer=kernel_init, 
                     input_shape=(256,256,1), padding="same",activation='relu'))
    model.build()

test_im=cv2.imread('1.pgm',-1)  # define a test image
test_im=np.expand_dims(np.expand_dims(np.array(test_im),2),0)
out = model.predict(test_im)

The problem is : using the image processing is able to produce a proper high passed image, but using the Conv2D is not the same result.

I am assuming two results should be the same or similar, the it turns out not...

Why, and it there any problem of my thoughts?

Answer 1

Apologies for the incomplete answer, but I've got something that partially works, and some explanation. Here's the code:

import cv2
import numpy as np
import scipy.ndimage as ndimage
from keras.models import Sequential
from keras.layers import Dense, Activation, Conv2D

#The image processing code:
#the first layer of the Model:

def kernel_init(shape):
    kernel = np.zeros(shape)
    kernel[:,:,0,0] = np.array([[-1, 2, -2, 2, -1],
                         [2, -6, 8, -6, 2],
                         [-2, 8, -12, 8, -2],
                         [2,-6, 8, -6, 2],
                         [-1, 2, -2, 2, -1]])
    #kernel = kernel/12
    #print("Here is the kernel")
    #print(kernel)
    #print("That was the kernel")
    return kernel

def main():
    print("starting")
    kernel55 = np.array([[-1, 2, -2, 2, -1],
                         [2, -6, 8, -6, 2],
                         [-2, 8, -12, 8, -2],
                         [2,-6, 8, -6, 2],
                         [-1, 2, -2, 2, -1]])
    # load the image, pre-process it, and store it in the data list
    image = cv2.imread('tiger.bmp',-1)
    gray = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)
    myimage = cv2.resize(gray,(256,256))
    myimage = myimage
    print("The image")
    #print(myimage)
    print("That was the image")
    segment = myimage[0:10, 0:10]
    print(segment)

    imgOut = ndimage.convolve(myimage, kernel55)
    #imgOut = imgOut/12
    print(imgOut.shape)
    cv2.imwrite('zzconv.png', imgOut)

    #print(imgOut)
    segment = imgOut[0:10, 0:10]
    print(segment)

    #Build Keras model
    print("And the Keras stuff")
    model = Sequential()
    model.add(Conv2D(1, [5,5], kernel_initializer=kernel_init, input_shape=(256,256,1), padding="same"))
    model.build()

    test_im=myimage
    test_im = test_im.reshape((1, 256, 256, 1))
    print(test_im.shape)
    imgOut2 = model.predict(test_im)
    imgOut2 = imgOut2.reshape(256, 256)
    print(imgOut2.shape)
    #imgOut2 = imgOut2 / 12
    imgOut2[imgOut2 < 0] += 256

    cv2.imwrite('zzconv2.png', imgOut2)

    #print(imgOut2)
    segment = imgOut2[0:10, 0:10]
    print(segment)

Here are the things to note:

• It's an image, pixels are bytes, anything bigger than a byte may be truncated and may be truncated incorrectly (note that I've had to remove your "/12" on the kernel. That's why I've added the "+=256" section.
• You can't assume that the "padded" areas will come out identical. I don't know what values keras and opencv use for padding, but it doesn't seem to be the same values. Your output images should only be identical from [3,3] (i.e. a border of 3 pixels on all sides might differ).
• Check your kernel before you use it. It was being rounded to -1 and 0 on my system. Presumably using integer arithmetic. Adding the line "kernel=kernel/12" gave more correct results for the kernel, but the rounding within the convolution function seemed to screw things up again, so I've left it without the "/12"
• The Relu was screwing things up, again because of the rounding (anything below zero that keras wasn't correctly truncating to unsigned byte was being filtered out by the activation function).