Interactive Real-Time Color Pencil Drawing Of 3D Scenes

Non-photorealistic rendering(NPR) is an area of computer graphics that provides tools and algorithms to produce a wide variety of expressive styles. The first approach in the related researches involves taking an image as an input and applying some kinds of filtering techniques to produce an image of stylized appearance, such as watercolor, oil painting, and so on. The second approach provides real-time systems in 3D to abstract outlines and the tone of the scene, giving vivid visual effects. Color pencils are traditional painting tools which have the same characteristics of graphite pencils and can draw colorful strokes. The main work of this paper is applying color pencil style into 3D scenes according to skills used in hand-made color pencil drawing.This paper presents a real-time interactive system that produces color pencil style look from 3D scenes. The system is a painting tool for people who is not professional, and it is also a rapid prototyping tool to make animation. The system will run basic rendering algorithms in every frame first. If the user provides 3D models with materials and textures, the system will render the scene with lights, otherwise it will use a toon shader to render. The system saves color, depth and normal information of every frame before hatching, the color buffer is used for strokes, and the depth and normal buffer are used for outlines. The outlines are detected automatically.There are two kinds of outlines for user to choose, one style changes thickness uniformly with illumination, another makes more than one irregular lines and blends them together. There are two modes for the user to choose:observation mode and hatching mode. In the observation mode, the user can translate, rotate and scale the 3D scene or choose a single model to move, and the model will be highlighted. In the hatching mode, the user can arrange strokes along a straight line by two control points or a Bezier curve by three control points. Stroke positions on the screen will be projected into 3D space.The system uses two algorithms for projecting, one is based on dot-sprite, another puts strokes directly into 3D space, the two algorithms are suit for different situations and both of them can void the stroke-bend effect, this is the biggest contribution of this work.Strokes will jitter in alpha value, direction, position or length according to the user inputs. This effect which can be controlled by the user makes the result more consistent with the real hand-painted image. The system provides algorithms to make alpha ramp, user can hatch on layers and add these layers up by alpha blending. Paper textures can be made by the system. Compared to scanned real paper, the size and roughness of computer made paper textures can easily be controlled. When using the automatic detection scripts, the system will check the history workflows of hatching and make prediction of future strokes. This step will speed up the drawing process. When the camera or model moves, the strokes in 3D scenes will move correctly, the system will show the animation in real-time.