Systems and method of generating compound images having increased sharpness and reduced noise are described. In one aspect, a comparison image signal is determined. The comparison image signal includes pixels with values respectively indexing values of pixels of a first image signal in relation to values of spatially corresponding pixels of a second image signal. The comparison image signal is filtered with a spatial filter. A compound image signal is generated. The compound image signal has pixels with values composed of contributions from the pixel values of the first image signal and the pixel values of the second image signal. The contributions are functions of respective values of spatially corresponding pixels in the filtered comparison image signal in relation to values of noise variation in the filtered comparison image signal.