1. A computer-implemented method for classifying breast density from mammographic imagery, the method comprising: pre-processing one or more digital mammogram images of a patient with a processing circuitry to remove noise and enhance contrast; segmenting pectoral muscle from the one or more pre-processed digital mammogram images to produce one or more extracted region of interest images and saving the one or more extracted region of interest images in a computer memory; grouping the one or more extracted region of interest images to produce one large mammogram image; decomposing a m-by-n mammogram image matrix A associated with the large mammogram image with the processing circuitry by non-negative matrix factorization to extract and retain a plurality of non-negative matrix factors; and classifying the plurality of non-negative matrix factors with a kernel matrix-based support vector machine according to breast density to two or more breast density categories; wherein the non-negative matrix factorization is according to one of the following equations: 
 A m×n ≈W m×r P r×n (Eq. 2); or 
 A m×n =W m×r P r×n +E m×n (Eq. 3); where columns of W are NMF base images, rows of P are encoding coefficients of the NMF base images, E is factorization error, r is rank and such that (m+n)r<m×n.