A method of obtaining a spatial low pass filtered images from the video output of a video camera operable to produce successive video frames in the form of pixelated 2D sensed images. Each of the sensed images has a first predetermined number of rows of pixels and a second predetermined number of columns of pixels. The method may include scanning the sensor array row by row, and deriving the value of each pixel of said low spatial frequency image in such a way that said low spatial frequency value of a pixel is dependent on at least all earlier scanned pixels of the current row and all earlier scanned rows, but is not dependent on the pixels of rows located below a pixel row which is a third predetermined number of rows below the current row, said third predetermined number being significantly less than said first predetermined number.