Video pre-filtering in phantom raster generating apparatus

摘要

Orthogonal Cartesian coordinates descriptive of the raster scanning of a television display screen are generated at a programmable skew angle to the Cartesian coordinates of an image stored in memory. Each of these generated coordinates has a number of more significant bits used to generate the column and row addresses for that memory, four spatially adjacent storage location of which are read from for developing each sample of video signal describing a rotated television image. Each of these generated coordinates has a number of less significant bits affording resolution higher than that with which the memory can be addressed, which are used for calculation of each video sample by two-dimensional interpolation between the read-outs from the four locations in memory. To avoid aliasing as the image is rotated, the digital video to be stored in memory is processed using two-dimensional spatial-frequency low-pass filters to reduce resolution in directions other than horizontal and vertical. Nonseparable two-dimensional filters with circular-or elliptical-cylindrical pass band shapes do this with minimal loss of image resolution.