The Image Prism: A Device for Rotating and Mirroring Bitmap Images

It is sometimes necessary to orthogonally reorient bitmap images. Traditional techniques for doing so are difficult to implement on classically organized bitmap memory systems because of their raster-scan ordering of images. In some cases, this can require several hundred times more processor activity, and many times more memory references, than are required to bitblt an image. This article describes a new bitmap graphic hardware component, the "image prism" which enables us to implement all orthogonal transformations with minimum processor and memory activity. A new algorithm, which is significantly faster than previously known techniques, is outlined. Along with the image prism, this algorithm performs all mirroring and 90-degree image rotations in time and memory activity equivalent to bitblt'ing an image. A VLSI implementation of the prism is also described.