Changhyung Lee

The Hybrid Screen—Improving the Breed

The hybrid screen is a halftoning method that generates stochastic dispersed dot textures in highlights and periodic clustered dot textures in midtones. Each tone level is sequentially designed from highlight to midtone by applying an iterative halftoning algorithm such as direct binary search (DBS). By allowing random seeding followed by swap-only DBS in a predefined core region within each microcell, each level can be designed while satisfying the stacking constraint and guaranteeing a smooth transition between levels. This paper describes a complete design process for the hybrid screen, introduces a number of enhancements to the original hybrid screen, and evaluates their impact on print quality. These enhancements include a multilevel screen design based on either extending a bilevel screen or directly generating a multilevel screen on a high resolution grid, and extending the hybrid screen design procedure to color by jointly optimizing the color screens using color DBS. For the multilevel screen, we show that the best choice for the core size critically depends on the bit depth and screen frequency. For the color hybrid screen, we demonstrate a significant improvement in the highlights over halftones generated by independently designed screens.

An algorithm for automatic skin smoothing in digital portraits

We describe an automatic method for beautifying digital portraits by smoothing the skin of the face. The method builds on existing face detection and face feature alignment technology to automatically segment the face and neck areas to be smoothed. A smoothing filter is then applied to these areas. The resulting portraits are enhanced in a subtle and natural fashion.

The hybrid screen: improving the breed

The hybrid screen is a halftoning method that generates stochastic dispersed dot textures in highlights and periodic clustered dot textures in midtones. Each tone level is sequentially designed from highlight to midtone by applying an iterative halftoning algorithm such as direct binary search (DBS). By allowing random seeding and swap-only DBS in a predefined core region within each microcell, we design each level while satisfying the stacking constraint and guaranteeing a smooth transition between different levels. This paper focuses on a number of enhancements to the original hybrid screen and their impacts on print quality. These include analytical determination of the human visual system filter in the spatial domain for DBS, multilevel screen design either by extending a bilevel screen or by directly generating a multilevel screen on the high resolution grid. Our results show that the multilevel screen design method has a direct impact on hybrid screen design parameters such as the optimal core size. We also extend the whole design process to color by jointly optimizing the color screens using color DBS. Our results demonstrate a significant improvement in the highlights over halftones generated by independently designed screen.