单张图片树木L-system的智能提取算法

介绍了一种新颖的从单张树木图片中提取树木的L-system规则的算法,并将其应用于三维树木建模。用户首先在图片上勾画出树木的主要可见枝干和树冠轮廓,通过图像处理的方法识别出树木的可见枝干的二维骨架;然后依据树木枝干的分布规律对骨架进行三维重建,并抽取其L-system生长规则和几何参数。所得L-system规则在树冠轮廓的约束下,经过迭代生长可以重建树木的分支结构。实验证明,相比以往的规则提取方法,该方法在保持模型质量的前提下,成本更低,方法更加简便。     

Structure from silhouettes: a new paradigm for fast sketch-based design of trees

Modeling natural elements such as trees in a plausible way, while offering simple and rapid user control, is a challenge. This paper presents a method based on a new structure from silhouettes paradigm. We claim that sketching the silhouettes of foliage at multiple scales is quicker and more intuitive for a user than having to sketch each branch of a tree. This choice allows us to incorporate botanical knowledge, enabling us to infer branches that connect in a plausible way to their parent branch and have a correct distribution in 3D. We illustrate these ideas by presenting a seamless sketch-based interface, used for sketching foliage silhouettes from the scale of an entire tree to the scale of a leaf. Each sketch serves for inferring both the branches at that level and construction lines to serve as support for sub-silhouette refinement. When the user finally zooms out, the style inferred for the branching systems he has refined (in terms of branch density, angle, length distribution and shape) is duplicated to the unspecified branching systems at the same level. Meanwhile, knowledge from botany is again used for extending the branch distribution to 3D, resulting in a full, plausible 3D tree that fits the user-sketched contours. As our results show, this system can be of interest to both experts and novice users. While experts can fully specify all parts of a tree and over-sketch specific branches if required, any user can design a basic 3D tree in one or two minutes, as easily as sketching it with paper and pen.     

个性化编辑的轻量化3维树木模型构建

目的 3维树木几何结构和拓扑结构的复杂性,不仅使得真实感3维树木模型的构建过程十分复杂,而且构建的模型文件包含大量的几何数据。针对3维树木模型的构建过程复杂和模型数据量大的问题,提出一种支持骨架个性化编辑的轻量化3维树木模型构建方法。方法 该方法在提取树木模型骨架结构的基础上,通过交互方式对3维树木模型的骨架进行个性化编辑以生成3维树木模型的全新骨架结构,并采用枝干和树冠模型的简化方法实现轻量化3维树木模型的构建。结果 该方法不仅能快速创建轻量化的3维树木模型,减少3维树木可视化时的模型绘制时间;而且能通过骨架个性化编辑来设计树木的拓扑结构,有助于增加同一品种树木外部表现形态的多样性。结论 通过应用表明,本文轻量化3维树木模型构建方法不仅可以构建具有不同表现形态的3维树木模型,而且简化后的3维树木模型可以在无线网络、移动终端等资源有限情况下进行3维树木可视化。     

基于描绘单幅图像的三维树木交互建模系统

为了提高树木建模的真实感与可交互性,提出了一种基于单幅树木图像或直接手绘的交互式植物建模系统。系统根据植物形态学规律构建了标准三维树木的模板。通过描绘图像中树的主要枝干,结合叶序周规律,基于三维模板树将其从二维图像变换到三维模型,并通过弯曲、增加和删除枝干以及添加树叶等交互式操作生成和修改三维树木模型。实验结果表明,该系统能便捷快速地生成真实感较强的三维树木。     

手绘草图笔触识别与规整技术研究*

结合工业设计师个人的构思和工作习惯,提出一套针对产品概念设计在线草图笔触识别与规整方法。利用笔触的绘制速度和型值点密度来估计笔触中的角点数量,并对角点进行识别;然后以最小二乘法为基础,利用型值点反求控制点拟合曲线,将笔触以参数化表示,以及判定和处理多笔触的重叠问题;最后对草图拓扑结构的获取进行了描述。实验结果表明,该方法可以有效地解决草图识别和规整中遇到的问题,适用于对复杂草图的处理,增强在产品创新过程中对草图的理解。     

Character animation creation using hand-drawn sketches

To create a character animation, a 3D character model is often needed. However, since humanlike characters are not rigid bodies, to deform the character model to fit each animation frame is tedious work. Therefore, we propose an easy-to-use method for creating a set of consistent 3D character models from some hand-drawn sketches while keeping the projected silhouettes and features of the created models consistent with the input sketches. Since the character models possess vertexwise correspondences, they can be used for frame-consistent texture mapping or for making character animations. In our system, the user only needs to annotate the correspondence of the features among the input-vector-based sketches; the remaining processes are all performed automatically.     

Feature based 3D garment design through 2D sketches

This paper presents a new approach for intuitively modeling a three-dimensional (3D) garment around a 3D human model by two-dimensional (2D) sketches input. Our approach is feature based—every human model has pre-defined features, and the constructed garments are related to the features on human models. Firstly, a feature template for creating a customized 3D garment is defined according to the features on a human model; secondly, the profiles of the 3D garment are specified through 2D sketches; finally, a smooth mesh surface interpolating the specified profiles is constructed by a modified variational subdivision scheme. The resulting mesh surface can be cut and flattened into 2D patterns to be manufactured. Our approach provides a 3D design tool to create garment patterns directly in the 3D space through 2D strokes, which is a characteristic not available in other computer aided garment design systems. The constructed garment patterns are related to the features on a human model, so the patterns can be regenerated automatically when creating the same style of garment for other human models. Our technique can greatly improve the efficiency and the quality of pattern making in the garment industry.     

Drawing for Illustration and Annotation in 3D

We present a system for sketching in 3D, which strives to preserve the degree of expression, imagination, and simplicity of use achieved by 2D drawing. Our system directly uses user-drawn strokes to infer the sketches representing the same scene from different viewpoints, rather than attempting to reconstruct a 3D model. This is achieved by interpreting strokes as indications of a local surface silhouette or contour. Strokes thus deform and disappear progressively as we move away from the original viewpoint. They may be occluded by objects indicated by other strokes, or, in contrast, be drawn above such objects. The user draws on a plane which can be positioned explicitly or relative to other objects or strokes in the sketch. Our system is interactive, since we use fast algorithms and graphics hardware for rendering. We present applications to education, design, architecture and fashion, where 3D sketches can be used alone or as an annotation of an existing 3D model.     

Path similarity skeleton graph matching

This paper presents a novel framework to for shape recognition based on object silhouettes. The main idea is to match skeleton graphs by comparing the shortest paths between skeleton endpoints. In contrast to typical tree or graph matching methods, we completely ignore the topological graph structure. Our approach is motivated by the fact that visually similar skeleton graphs may have completely different topological structures. The proposed comparison of shortest paths between endpoints of skeleton graphs yields correct matching results in such cases. The skeletons are pruned by contour partitioning with Discrete Curve Evolution, which implies that the endpoints of skeleton branches correspond to visual parts of the objects. The experimental results demonstrate that our method is able to produce correct results in the presence of articulations, stretching, and occlusion.     

Skeleton-enhanced line drawings for 3D models

We present a novel line drawing approach for 3D models by introducing their skeleton information into the rendering process. Based on the silhouettes of the input 3D models, we first extract feature lines in geometric regions by utilizing their curvature, torsion and view-dependent information. Then, the skeletons of the models are extracted by our newly developed skeleton extraction algorithm. After that, we draw the skeleton-guided lines from non-geometric regions through the skeleton information. These lines are combined with the feature lines to render the final line drawing result using the line optimization. Experimental results show that our algorithm can render line drawings more effectively with enhanced skeletons. The resulting artistic effects can capture the local geometries as well as the global skeletons of the input 3D models.     

面向网络应用的三维树木模型简化方法研究

在网络环境下进行三维树木模型的绘制与传输,不仅对模型逼真度有着较高要求,还更需保证网络交互的实时性。由于现有的基于几何或图像的三维模型简化方法无法满足网络应用对三维树木的模型细节和网络传输速度的要求,因此提出了一种面向网络应用的三维树木模型简化方法。该方法对树木枝干部分的骨架节点进行提取,并采用非均匀的骨架简化策略对树木模型的枝干部分进行简化;此外,还利用纹理树冠方法对树冠部分进行简化,通过采用凸包纹理模板贴图重构树冠,使得重构后得到的模型与原始模型具有很好的相似度。实验和应用结果表明,面向网络应用的三维树木模型简化方法在保证视觉质量的基础上,能进一步减少模型文件的存储容量,从而提高树木模型的绘制效率及其在网络中的传输速率。     

Online Personalised Non‐photorealistic Rendering Technique for 3D Geometry from Incremental Sketching

This paper presents an online personalised non‐photorealistic rendering (NPR) technique for 3D models generated 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 addition, non‐geometric information such as surface textures can be added to the recognised object in different sketching modes. This will integrate sketch‐based incremental 3D modelling and NPR into conceptual design.     

Variational Tree Synthesis

Modelling trees according to desired shapes is important for many applications. Despite numerous methods having been proposed in tree modelling, it is still a non‐trivial task and challenging. In this paper, we present a new variational computing approach for generating realistic trees in specific shapes. Instead of directly modelling trees from symbolic rules, we formulate the tree modelling as an optimization process, in which a variational cost function is iteratively minimized. This cost function measures the difference between the guidance shape and the target tree crown. In addition, to faithfully capture the branch structure of trees, several botanical factors, including the minimum total branches volume and spatial branches patterns, are considered in the optimization to guide the tree modelling process. We demonstrate that our approach is applicable to generate trees with different shapes, from interactive design and complex polygonal meshes.     

Modeling Complex Unfoliaged Trees from a Sparse Set of Images

We present a novel image‐based technique for modeling complex unfoliaged trees. Existing tree modeling tools either require capturing a large number of views for dense 3D reconstruction or rely on user inputs and botanic rules to synthesize natural‐looking tree geometry. In this paper, we focus on faithfully recovering real instead of realistically‐looking tree geometry from a sparse set of images. Our solution directly integrates 2D/3D tree topology as shape priors into the modeling process. For each input view, we first estimate a 2D skeleton graph from its matte image and then find a 2D skeleton tree from the graph by imposing tree topology. We develop a simple but effective technique for computing the optimal 3D skeleton tree most consistent with the 2D skeletons. For each edge in the 3D skeleton tree, we further apply volumetric reconstruction to recover its corresponding curved branch. Finally, we use piecewise cylinders to approximate each branch from the volumetric results. We demonstrate our framework on a variety of trees to illustrate the robustness and usefulness of our technique.     

Efficient and Dynamic Simplification of Line Drawings

In this paper we present a pipeline for rendering dynamic 2D/3D line drawings efficiently. Our main goal is to create efficient static renditions and coherent animations of line drawings in a setting where lines can be added, deleted and arbitrarily transformed on‐the‐fly. Such a dynamic setting enables us to handle interactively sketched 2D line data, as well as arbitrarily transformed 3D line data in a unified manner. We evaluate the proximity of screen projected strokes to simplify them while preserving their continuity. We achieve this by using a special data structure that facilitates efficient proximity calculations in a dynamic setting. This on‐the‐fly proximity evaluation also facilitates generation of appropriate visibility cues to mitigate depth ambiguities and visual clutter for 3D line data. As we perform all these operations using only line data, we can create line drawings from 3D models without any surface information. We demonstrate the effectiveness and applicability of our approach by showing several examples with initial line representations obtained from a variety of sources: 2D and 3D hand‐drawn sketches and 3D salient geometry lines obtained from 3D surface representations.     

SmartCanvas: Context‐inferred Interpretation of Sketches for Preparatory Design Studies

In early or preparatory design stages, an architect or designer sketches out rough ideas, not only about the object or structure being considered, but its relation to its spatial context. This is an iterative process, where the sketches are not only the primary means for testing and refining ideas, but also for communicating among a design team and to clients. Hence, sketching is the preferred media for artists and designers during the early stages of design, albeit with a major drawback: sketches are 2D and effects such as view perturbations or object movement are not supported, thereby inhibiting the design process. We present an interactive system that allows for the creation of a 3D abstraction of a designed space, built primarily by sketching in 2D within the context of an anchoring design or photograph. The system is progressive in the sense that the interpretations are refined as the user continues sketching. As a key technical enabler, we reformulate the sketch interpretation process as a selection optimization from a set of context‐generated canvas planes in order to retrieve a regular arrangement of planes. We demonstrate our system (available at http:/geometry.cs.ucl.ac.uk/projects/2016/smartcanvas/ ) with a wide range of sketches and design studies.     

Research on geometric features and point cloud properties for tree skeleton extraction

To solve skeleton extraction problems in the tree point cloud model, branch geometric features and local properties of point cloud are utilized to optimize tree skeleton extraction. First of all, according to the attribute information estimation and normal vector adjustment of point cloud neighbor domain, branch segmentation is made by estimated values and geometric features. Skeleton nodes are extracted in the branch subset in segmentations. Then, a graph is constructed based on skeleton node set and tree skeleton is reconstructed in this weighted directed graph. Finally, according to the tree growth characteristics, cubic Hermite curves are utilized to optimize the skeleton curve. This method is applied in the point cloud model of three-kind trees and it is compared with the skeleton extraction method based on voxel switch and point cloud contraction. The experiment results show that this method displays strong anti-interference and high-precision characteristics at branch bifurcation and crossed ending parts of fine tree branches. Thus, features of tree branches can be described more perfectly, obtaining the skeleton curve closer to the main axis.     

Physical sketching: Reconstruction and analysis of 3D objects from freehand sketches

When designing a 3D object, designers, engineers and teachers often begin investigating potential design tradeoffs by creating informal sketches. Ideally, these sketches-in combination with a variety of engineering analysis tools-would allow prediction of the object’s physical properties, especially those that affect the critical early design process. We introduce a pen-based system that reconstructs 3D spatial geometry from a single 2D freehand-sketch consisting of straight and curved lines in interactive time. Several optimization-based approaches to this problem have been proposed, but these generally have difficulty converging to an acceptable solution because the dimensionality of the search space is large. The primary contribution of this paper is a new reconstruction algorithm for orthographic projections of 3D wireframes. The algorithm reconstructs the depths of each vertex by exploiting geometric regularities among the graph lines in a reduced solution space, then optimizes a cost function over this space to recover the vertex depths. A second optimization algorithm is used to infer the 3D geometry of curved strokes once the vertex depths have been recovered. The proposed approach can recover the geometry of several objects with approximately 50 curved strokes in near interactive time. We also present an iterative, Tablet-PC-based design system that uses the proposed reconstruction algorithm to recover 3D objects from 2D orthographic sketches. The system allows the reconstructed objects to be subjected to two types of physical analysis, the results of which are superimposed directly on the sketch: a fast, kinematic simulation, and a complete finite-element-based static analysis. The object can quickly be modified in place using the pen-based interface according to the results of the analysis to allow for iterative design work. We demonstrate the system in action on a variety of early-stage design analyses.     

基于截线法的快速骨架提取算法

提出了一种快速的骨架提取算法.该方法首先在轮廓离散曲线演化的基础上,根据显著凸顶点的类型将轮廓多边形进行分块,得到一个主分支轮廓和多个水平分支轮廓;然后分别利用水平截线法和垂直截线法提取骨架的主分支和水平分支;最后将水平分支拼接在主分支上,得到完整的骨架.实验结果表明,该骨架提取算法可以得到连通的骨架,并在Kimia数据集上取得了较好的效果.此外,算法在自然图像上的效果也很好,尤其适用于视频中的行人骨架提取.与经典骨架提取算法相比,该算法的时间复杂度较低,可以满足实时处理的要求.