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I have been trying to develop a convolution neural network following some guides online. However, most guides I have encountered gloss over an important detail, which is how to programmatically represent the weights in a CNN.

As far as I understand, in a "regular" neural network, the weight of a connection is a numerical value, which is adjusted in order to reduce the error; then back-propagation is used to further update the weights, reducing thus the error, etc.

However, in a CNN, the input is an array of numbers (the image), and a subset of those (the filter) to calculate the mean error, by multiplying the filter pixels by the original pixels.

So, is there a weight neuron for each filter (kernel or feature map) of the image? Or is a single weight neuron represented by the sum of all the mean error's calculated from convolving the filter over the receptive field, such that you have one value, in the end, that is the total error for the entire image?

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    $\begingroup$ All of your confusions will be answered after watching the nice lecture: youtube.com/watch?v=AQirPKrAyDg $\endgroup$ Apr 14, 2017 at 0:50
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    $\begingroup$ this video really helped explain many things, but i struggled a bit with how he acquired his filters for training....like when you train the network initially are the filters just pre-classified images? or part's of an image? such as if i want to train a facial recognition cnn would the network be trained on pictures of entire faces as filters, or parts of a face? $\endgroup$
    – D3181
    Apr 15, 2017 at 19:55
  • $\begingroup$ on a 2d image, say 32 by 32 you prespecify a filter size 2 for example and compute the dot product/ convolution across all 2 by 2 squares in 32 by 32 image. $\endgroup$ Apr 15, 2017 at 20:28
  • $\begingroup$ ok so the process of training is choose an image, label it as whatever classification it is, choose filters for that labeled image (eye brows, mouth etc)..use a dot product between the filters and training image and use back propagation to reduce the squared mean error? $\endgroup$
    – D3181
    Apr 17, 2017 at 14:01
  • $\begingroup$ Seems you've confusion filter to be an actual filter! well ,it's not. Use kernel instead. Also you don't specify what convet to learn (eye brows etc.) you specify a kernel ( say 2 by 2) and slide it over all the places it can fit in your picture. Irony is abstractions (like detecting mouth etc.) are learned in different layers by themselves. $\endgroup$ Apr 17, 2017 at 16:32

3 Answers 3

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In convolutional layers the weights are represented as the multiplicative factor of the filters.


For example, if we have the input 2D matrix in green

enter image description here

with the convolution filter

enter image description here

Each matrix element in the convolution filter is the weights that are being trained. These weights will impact the extracted convolved features as

enter image description here

Based on the resulting features, we then get the predicted outputs and we can use backpropagation to train the weights in the convolution filter as you can see here.

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    $\begingroup$ so basically the filters are the weights? $\endgroup$
    – Khan
    May 10, 2020 at 12:59
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    $\begingroup$ @khan, yes, they are weights. Further, every output value is the linear combination of the weights multiplied by the values around the value (which is then passed through an activation function) $\endgroup$ Jul 9, 2020 at 19:34
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Building on @JahKnows theoretical answer, here is what the weights of Conv2D look like in action.

from keras import *
from keras.layers.convolutional import Conv2D

model = Sequential()
model.add(Conv2D(12, kernel_size=3, input_shape=(25, 25, 1)))
#just initialized, not fit to any data.
>>> weights[0].shape
'(3, 3, 1, 12)'

So a 3x3 matrix (9 arrays) where each array is a (1x12) vector

  • First 2 dimensions: looks like the kernel size; (3,3).
  • Last 2 dimensions: 1*12; where 12 is units and 1 is channels aka colors from the input_shape.

Plus 12 bias neurons:

  • It also looks like there is a separate weights[1] 12x1 for bias that would edge into each of the other 12x1s.
>>> weights[0]
array([[[[-0.22489263,  0.11462553,  0.1275196 ,  0.19356592,
          -0.06204098,  0.10875972, -0.09088454,  0.12002607,
           0.14580582, -0.10627564,  0.04845475,  0.16762014]],

        [[-0.00685272, -0.144605  ,  0.00162746, -0.17116429,
          -0.13180375, -0.13356137,  0.02543293,  0.09918924,
           0.19696428, -0.01112208, -0.17443556,  0.105253  ]],

        [[ 0.04283331,  0.1003729 , -0.21573427, -0.08311893,
          -0.0144719 ,  0.10843249, -0.1036434 ,  0.1704862 ,
           0.22398098, -0.2159951 ,  0.13356568, -0.13963732]]],


       [[[-0.00911894,  0.12489821, -0.1453647 ,  0.14670904,
           0.17318939, -0.16027464, -0.11050612, -0.19118567,
           0.06857748,  0.18323778, -0.22046578,  0.05927287]],

        [[-0.00602703, -0.18062721,  0.15344848, -0.15143515,
          -0.07210657,  0.177676  , -0.06143558, -0.17020151,
          -0.02092001,  0.19398673, -0.20247248,  0.17286496]],

        [[ 0.22057424,  0.10987107,  0.00975977,  0.00445287,
           0.09941946,  0.03192849, -0.19070472, -0.10779155,
           0.13622199, -0.11289301, -0.06379397,  0.06102996]]],


       [[[-0.11758636,  0.16921164, -0.151184  , -0.06386189,
           0.1991932 , -0.21000272, -0.12173925, -0.03071272,
           0.16692607, -0.12708151,  0.08756261,  0.178169  ]],

        [[-0.05779965, -0.10117687,  0.20407595, -0.21241538,
          -0.16404435, -0.0826612 ,  0.02122533,  0.1947081 ,
          -0.09203622,  0.08905725,  0.09665458, -0.06724563]],

        [[-0.22078277, -0.0093862 ,  0.02477093, -0.0090203 ,
           0.21535213, -0.16004324, -0.0708347 , -0.02972263,
           0.11906733,  0.05814315, -0.02641977, -0.09178646]]]],
      dtype=float32)



>>> weights[1]
array([0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0.], dtype=float32)
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The kernel or filter is just a matrix of values that you slide over the entire image and the values from sliding that kernel/filter are used to find an error rate and learn to detect features

The output features in a CNN are calculated by different kernels with different weights. In a convolutional layer, the weights are represented as the multiplicative factors of the filters or kernels.

Each kernel is a matrix of weights that is convolved with the input image or feature map to produce the output feature map. The number of kernels in a convolutional layer determines the number of output feature maps.

During the training process, the weights of the kernels are adjusted using backpropagation to minimize the loss between the predicted output and the actual. This allows the CNN to learn more effective feature representations for the given task. The kernels' initial values are often sampled from a distribution, such as normal or uniform distribution, or they can be initialized from the weights of another network (transfer learning).

As the input image or feature map passes through multiple convolutional layers, each layer's kernels capture different patterns and features, resulting in a hierarchy of feature representations. The final output features are calculated by applying different kernels with their learned weights to the input, effectively extracting meaningful features that can be used for the specific task the CNN is designed to perform.

https://medium.com/codex/kernels-filters-in-convolutional-neural-network-cnn-lets-talk-about-them-ee4e94f3319

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