I have some problems in understanding a passage of the JPEG compression algorithm:
Consider an $8\times8$ matrix $M$ that in our case is a "piece'' of a channel (for example the red channel $R$) of the image. The entries of $M$ are integers in the range $[-128, 127]$ that represents the hues of red. At this point we apply the $2D$ DCT (II) transformation (discrete cosine transformation) in order to obtain a ''matrix of frequencies''.
I don't understand what phisically represent these frequencies! For example if I have a set of points $(x_1,\ldots,x_n)$ and then I calculate the DFT on these points, I obtain the frequencies of the fundamental armonics which constitute the trigonometric interpolant.
In the case of an image, what is the meaning of the matrix of frequencies coming from the DCT?
Many thanks.