Why use CNN instead of normal Feed forward Network?

While we can use a simple Feed Forward Network, we use Convolutional Neural Network... What is the need for such a network?

I took a picture of me with my mobile camera which has 12 MP resolution ( Poco F1). I imported it as numpy array. Number of pixel in that image was 2756160. I have nearly 3 million pixel! If i use a Feed Forward Network it means i have 3 million features! I will require more number of data. A 10 million photos will satisfy my need. Also this would require a large memory and long time for training!

We can reduce the dimension? PCA? PCA of 3 million X 10 million will still be a great problem. Moreover we cannot get 10 million photos. We have to train better with small amount of data.

How else can we reduce the dimension? Feature extraction is a good option, we can have multiple filters for a image and use the results of filter as the feature. This significantly reduces our data. 128 filters (of size 3x3) applied to my image will reduce it into a 128 images of size 3x3. This is what a Convolutional Neural Network is! The problem gets reduced in dimension and difficulty...

The size of the image was found to be 2756160.

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