The invention relates to an image processing apparatus for compressing or decompressing a segment of an image. The segment includes a plurality of pixels, each pixel includes a pixel value and a pixel position defined by a first coordinate system. The pixel values of the plurality of pixels form a pixel value vector. The apparatus includes processing circuitry configured to compress and/or decompress the segment. Compressing the segment includes computing a plurality of expansion coefficients by expanding the pixel value vector into a plurality of basis vectors that are discrete approximations of solutions of a boundary value problem of the Helmholtz equation on the segment of the image in a second coordinate system rotated relative to the first coordinate system. Decompressing the segment includes computing the pixel value vector by forming a linear combination of the basis vectors using the plurality of expansion coefficients.