series 1: Gramian Matrix
Given a set V of m vectors, gram matrix G is simply the matrix of all possible inner products of V.
T denotes the transpose.
Gram matrix plays a key role in the task of style transfer in deep neural network.
A pre-trained model, usually vgg19 is introduced into the transfer process, the reason behind using pre-trained model is, we assume a model could perform versatile categories’ classification should be able to understand the content of image we give. While this assumption could be challenged as well.
And get back to gram, when compare pastiche image with the given image with a certain “style”, how should the “style” of image been represented im math?
As we know, gram matrix is result from a matrix multiplying its transpose, aka every row is multiplied with every column in the matrix, we could treat this process as a way of finding correlations - bigger value multiply with bigger value gets bigger, and smaller gets smaller.
A value in gram matrix close to zero, which denotes the 2 features in given layer of style image do not activate simultaneously, vice versa. Hence with this feature, a gram matrix could reveal the activation pattern of the style image.
In the neural style transfer paper 2015, Gatys et al found better results were made through making a combination of taking both shallow and deep layers of the pre-trained network with vgg.