中國山水畫山石皴法合成技術之研究

Abstract

電腦圖學自三十多年以來，追求逼真一直是不變的目標。所以各專家學者不斷研究新的方法，以更接近真實與具物理特性的運作環境。而追求逼真的目標之餘，有些效果卻是連真實世界都無法呈現的。藝術化視覺資訊在日常生活中也佔極重要的地位，它與視覺資訊同為人類感官功能中所最常接觸的資料型態，而藝術化視覺資訊更能引發人類的內心的感覺。當真實視覺資訊技術已逐漸成熟之時，近年來已有許多科學家研究印象派畫家電腦自動產生技術，此即是電腦圖學的一分支領域所專注之處，有別於前者，此領域名稱取為NPR(Non-Photorealistic Rendering)。NPR的目標其中包括了模擬人類各種繪畫風格，每年電腦圖學相關的國際研討會討論此領域技術之計劃也逐年以驚人的速度激增，顯示此領域技術逐漸受人重視。

中國國畫具有悠久的歷史，我國傳統繪畫藝術之一，在世界美術領域中自成體系。而山水畫於東晉時期萌芽，經一千六百多年眾名家不斷傳承創新下，累積了無數的技術經驗。由於它的技法複雜、山川變化大、題材多，光是運筆用墨上，便需很多時間磨練。在中國山水畫中，山石場景為首要的描寫對象之一，峯巒山石聯綴而成的優美律動，構成生靈活潑、含蘊無窮之山水畫境。目前有許多NPR的研究論文都著重在西畫的模擬，譬如鉛筆、油畫或水彩等，這些研究已經為西部繪畫模擬出好結果。但這些方法不適合中國水墨畫，通常西方繪畫需要更多的精確度，但中國墨水繪畫是比較抽象的。另外，筆墨的模擬在中國水墨畫是主要的工作，包括水墨擴散，乾溼筆的順序和各種各樣的筆觸大小和技術細節等，和西方繪畫的屬性完全不同。

在本論文中，我們提出「中國山水畫山石皴法合成技術之研究」。主要貢獻如下:完成毛筆的模型，並控制墨的「水分」，讓其自然滲化、溶接，產生不同的墨色效果。自動產生皴法的紋理，使用控制線及起始位置與結束位置，依山石的紋理以各種線條，畫出石頭的質感或立體感。首先，我們將定義基本皴法的參數，包括用墨濃淡，墨的水分多少，以及交錯的形式與方向等等。這些基本參數構成我們皴法筆觸的原型。我們可以事先透過不同的參數組合，先完成一部份的皴法筆觸。我們得到向量化的資料之後，就可以為皴法筆觸定義出起點以及終點。定義的方式當然可以自動，或是透過使用者完成。有了筆觸的起點以及終點之後，自然筆劃的方向就大致確定。最後，起始點用淡墨，終點用濃墨，再自動皴法筆觸，加以適合地動態調整，而形成我們所需要的一筆皴法。同樣的原理將重複運用，直到整個水墨畫完成為止。這一套自動化的描繪過程首先擷取三維山石模型的幾何形狀資訊，建立各類資訊的索引圖，從中分析以獲致描繪形狀與表皮紋理（皴法）的參考依據，然後產生適當的勾勒筆觸，及變化豐富的皴擦渲染效果，自動畫出俱山水畫風格的山川石岩，而成一幅幅引人入勝的山水畫作與具有中國水墨畫風格的3D動畫。

Computer graphics-related research has focused on obtaining photorealistic images. However, photorealism is occasionally not the most effective means of visually expressing emotions. Accordingly, photographs can never entirely replace paintings. Non-photorealistic rendering (NPR) approaches have recently received renewed interest. Researchers have begun to study how a photograph or a photorealistic image may be made to look like a painting. In recently years, most research in NPR focused on Western painting. Lots of paintings algorithms have been proposed for convert a photorealistic image into an art form such as an oil painting or watercolor. Typically, such a painting style is created by applying masks or by placing user-defined patterns. These methods deliver good results for Western painting. But these approaches are not appropriate to Chinese ink painting. Generally, Western painting involves more precision but Chinese ink painting is more abstract. However, strokes in Chinese ink painting are in many aspects based on Chinese brushwork, including ink diffusion, pattern placing and describing details with various brush sizes and techniques.

Chinese ink painting stresses the notion of "implicit meaning" in which painters use a minimum amount of strokes to express their deepest feelings. Landscapes are one of the most important themes in Chinese painting. In the Tang dynasty, the range of subjects in painting expanded and landscape became established as a distinct category. Chinese landscape painting provided a more spontaneous style that captured images in abbreviated suggestive forms. Chinese landscape painting has been cultivated by masters through a long evolution into an exquisite art form.

To simulate the style of Chinese landscape painting is not trivial at all. It usually uses brushes and ink as mediums, values the expression of the artistic conception far beyond the precise appearance of the painted subjects. By means of the blending effects of brushes and ink, painters communicate their frame of mind to the viewers. This thesis proposes a set of methods to automatically create 3D animation of Chinese landscape painting. Rocks are the major painting objects in Chinese landscape painting. The given 3D models are drawn by outline rendering and texture generation, from information of the shape, shade and orientation of model’s surface. Many reference maps are constructed to analyze the information; create brush strokes for the outline and texture strokes. Finally, the target of the project will automatically create the still images and animations.

The main contribution of this investigation is the modeling and implementation of six major texture strokes for terrain surface using traditional brush techniques in Chinese landscape painting. The proposed rendering technique involves many fundamental parts. First, 3D terrain information is extracted to detect the edges of the silhouettes, and to generate streamlines and ridge meshes. The outline of a silhouette is then constructed and the streamlines of the texture strokes are generated. All control lines that involve the silhouette and streamlines then are projected onto a 2D viewing plane. Brush strokes then are applied to create an outline drawing and the texture of rock is captured using vertical or slanted strokes along the control lines with a rich ink tone specified by a luminance map. Finally, the ink diffusion on the rice paper is simulated.