A number of image quality metrics depend on the structure of the printed halftone dot. The dot structure depends in a complex way on the individual processes in a xerographic marking engine. This paper describes a model which reduces this complexity into a set of key parameters. The parameters are derived from microscopic optical measurements of the dot structure in the halftoned print on paper. The halftone printer model is written specifically for a single color separation, fast calculation, and physically based parameters. Given a fill pattern and a threshold value, it calculates the local image density as a function of position in a halftone supercell. The model has been applied to the appearance of partial pixels and to high frequency banding caused by irregularities in the raster optical scanner (ROS) beam spacing.