1. Apparatus for processing electrical signal data representing an image in a digital data processing device according to a Principal Component Transform for purposes of providing a revised representation of the image-representing electrical data signals for use in compressing the electrical data signals, or for extracting features of the electrical data signals, or for modifying the electrical data signals to pre present a rotation of the image, the apparatus comprising: hardware input means for providing signals representing an initial vector as a current estimate of a statistically most significant eigenvector of a covariance matrix representation of said signal data, said signal data being represented as vectors in a vector space; hardware means for storing and providing signals representing a next vector; an inner product hardware means for calculating an inner product of the current estimate and the next vector; hardware multiplication means for multiplying said next vector by a weight determined by said inner product; hardware adding means for adding the next vector multiplied by said weight to the current estimate for enhancing an adding result in a direction of the current estimate; and an output hardware memory for storing and providing the adding result as signals representing a produced approximation of an eigenvector, which is a transformation of the signal data, for further processing as a revised representation of the image-representing electrical data signals.