Online Personalised Non-photorealistic Rendering Technique for 3D Geometry from Incremental Sketching Daychyi Ku 1 , Shengfeng Qin 1 , David K. Wright 1 and Cuixia Ma 2 1 School of Engineering and Design, Brunel University, Uxbridge, UB8 3PH, UK 2 Institute of Software, Chinese Academy of Sciences, Beijing, 100080, China ABSTRACT This paper presents an online personalised non-photorealistic rendering (NPR) technique for 3D models gener- ated from interactively sketched input. This technique has been integrated into a sketch-based modelling system. It lets users interact with computers by drawing naturally, without specifying the number, order, or direction of strokes. After sketches are interpreted as 3D objects, they can be rendered with personalised drawing styles so that the reconstructed 3D model can be presented in a sketchy style similar in appearance to what have been drawn for the 3D model. This technique captures the user’s drawing style without using template or prior knowledge of the sketching style. The personalised rendering style can be applied to both visible and initially invisible geometry. The rendering strokes are intelligently selected from the input sketches and mapped to edges of the 3D object. In addi- tion, non-geometric information such as surface textures can be added to the recognised object in different sketch- ing modes. This will integrate sketch-based incremental 3D modelling and NPR into conceptual design. Keywords: Non-photorealistic rendering, incremental sketching, 3D geometry modelling, conceptual design. ACM CCS: H.5.2 [Information Interface and Presentation] Interaction Styles, J.6 [Computer-Aided Engineering] Computer-aided design (CAD). 1. Introduction Sketching is inevitably an important support for the early design stage not only because sketching with a pen is still more natural than a CAD system with keyboard or mouse, but also that sketches are informal figures often created as a way of thinking about, or working through a problem [Dav07]. The most striking feature of hand-drawn sketches is their “incorrectness” such as wiggliness, overtracing and overshooting. People are more likely to discuss design variations with sketches. Schumann and others [SSRL96] show that hand-drawn images or sketches are more appro- priate for use in the early design phase and for discussion with customers. Users prefer freehand sketch style draw- ings for conceptual design rather than precise line drawings in the traditional CAD approach such as 2D drafting. Goel [Goe95] shows that the freehand sketches that are produced in the conceptual design phase are necessarily and generally vague and ambiguous. Lim [LLD01] argues that conceptual sketches include a variety of vague Infor- mation and further suggests that the vagueness in sketches needs to be preserved as long as possible during the early design process. The vagueness can exist in terms of texts, symbols as well as sketches. Most traditional CAD systems are not suitable in the conceptual stage since they require complete, concrete and precise design information input, which is only available at the end of the design process. Therefore, for conceptual design there is a need to develop appropriate techniques for modelling and rendering 3D objects from freehand sketches. They can allow users to interact with computers by freehand drawing, offering a freedom not available with traditional CAD systems. In addition, computerised infor- mation (processed models and drawings) can be visualised in a sketchy appearance similar to input sketches and car- ried forward in an incremental design process. One approach that allows the maintenance of the appear- ance of the processed model in the original sketch style drawn by designers is that of non-photorealistic rendering (NPR). In particular, studies such as [KMK*02][MSK02] have proposed and argued for the use of NPR on recon-  Corresponding author c  2008 The Author(s) Journal compilation c  2008 The Eurographics Association and Blackwell Publishing Ltd. Published by Blackwell Publishing, 9600 Garsington Road, Oxford OX4 2DQ, UK and 350 Main Street, Malden, MA 02148, USA. Pacific Graphics 2008 T. Igarashi, N. Max, and F. Sillion (Guest Editors) Volume 27 (2008), Number 7