Machine Vision In The Graphic Arts: Digital Registration Of Four Color Halftone Separations

Opti-Copy, Inc. has recently developed the RegiStar? digital registration machine, a machine that quickly and accurately pin registers four color halftone separations through the use of digital imaging coupled with high speed computation. In the machine, the computed translational misregistration at two points on the separations is the input to the machine movement computations that accomplish film registration. The heart of the digital registration machine is the computation of translational misregistration between the operator's first reference film and a given film to be registered to the reference film, at each of the tow points that locate halftone picture detail within a small area. This paper describes the image processing algorithms required to compute the translational movement of one small halftone picture into registration with the corresponding reference halftone picture. Basically, the highly resolved halftone pictures are converted to interpolatively resampled continuous tone pictures that are subsequently enhanced and then registered by a hierarchical resolution correlation method. The heart of the digital registration machine is the computation of translational misregistration between the operator's first reference film and a given film to be registered to the reference film, at each of the tow points that locate halftone picture detail within a small area. This paper describes the image processing algorithms required to compute the translational movement of one small halftone picture into registration with the corresponding reference halftone picture. Basically, the highly resolved halftone pictures are converted to interpolatively resampled continuous tone pictures that are subsequently enhanced and then registered by a hierarchical resolution correlation method.