The Geometric Bitmap Image Analysis (GBIA) feature is a powerful tool allowing users to determine the quality of an image as it will appear through a particular lens. A very nice description of this tool may be found in the Knowledge Base article “How to Produce Photo-Realistic Output Images”. There may be times when it is necessary to determine how various parts of an image would appear through the lens if the lens were to be scanned incrementally over the image (i.e. if the full image can not be viewed through the lens). This situation arises for the case of a document scanner, when the scanner can only view a slice of the image at a time. For this application, it is necessary to model the image as it is reconstructed piece-wise by the scanner. Using the ZPL macro keywords IMAGEEXTRACT and IMAGECOMBINE, such a model can be developed in ZEMAX.