基于四叉树法和波前法的曲面网格剖分算法

提出了一种基于四叉树法和波前法的有限元网格生成算法。该算法首先将裁剪曲线边界离散成多边形,然后使用四叉树法对待划分裁剪参数曲面按细分要求进行递归分解,最终生成规则的栅格点,最后利用波前法的原理生成三角形网格,通过合并处理生成三角形与四边形混合网格。     

Improved quadtree image segmentation approach to region information

Images are full of information and most often, little information is desired for subsequent processing. Hence, region of interest has key importance in image processing. Quadtree image segmentation has been widely used in many image processing applications to locate the region of interest for further processing. There are also variable block-size image coding techniques to effectively reduce the number of transmitted parts. This paper presents quadtree partition technique as a pre-processing step in image processing to determine what part should be more heterogeneous than the others. It also introduces an idea to solve the problem of squared images. Finally, proposed approach is implemented and analysed. The simulation of the Matlab code of the quadtree is represented by all algorithms and the figures. Thus, achieved results are promising in the state of the art.     

G-quadtree: A hierarchical representation of gray-scale digital images

We proposeG-quadtree as a hierarchical representation method for gray-scale digital images. G-quadtree is an extended quadtree each leaf of which holds a multi level-value. An algorithm constructing a G-quadtree from the array representation of a gray-scale image is described, implemented and tested. The algorithm is established in such a way that the conventional binary array-to-quadtree conversion algorithm is applied to each bit of array elements repeatedly in descending order of significance. Space efficiency analysis reveals that G-quadtree representation is sufficient in a particular application to a color coding of macroautoradiography of rat brains.     

New progressive image transmission based on quadtree and shading approach with resolution control

Employing the quadtree and Gouraud shading approach, this paper first presents a modified image compression method called the QSC. Then, based on the proposed compression method, a new progressive image transmission (PIT) scheme is presented. In the proposed PIT scheme, the encoding algorithm partitions all the quadtree nodes into several disjoint subsets such that the resolution of nodes in each subset must be in the specified interval. The decoding algorithm reconstructs the partitioned quadtree from coarse to the required quality stage by stage. Experimental results reveal that under the similar peak signal to noise ratio (PSNR) and bits per pixel (bpp), the proposed PIT scheme has a better feature-preserving capability when compared to the reduced-difference pyramid PIT scheme [Optic. Eng. 28 (7) (1989) 708] and the progressive JPEG [JPEB: Still Image Data Compression Standard, New York, 1993].     

Fast distance transformation on irregular two-dimensional grids

In this article, we propose a new fast algorithm to compute the squared Euclidean distance transform (E²DT) on every two-dimensional (2-D) irregular isothetic grids (regular square grids, quadtree based grids, etc.). Our new fast algorithm is an extension of the E²DT method proposed by Breu et al. [3]. It is based on the implicit order of the cells in the grid, and builds a partial Voronoi diagram of the centers of background cells thanks to a data structure of lists. We compare the execution time of our method with the ones of others approaches we developed in previous works. In those experiments, we consider various kinds of classical 2-D grids in imagery to show the interest of our methodology, and to point out its robustness. We also show that our method may be interesting regarding an application where we extract an adaptive medial axis construction based on a quadtree decomposition scheme.     

Quadtree-based classification in subband image coding

Classification of blocks of subband samples according to their energy and variable bit allocation within the subsequent classes has demonstrated considerable gains in coding efficiency. The gains due to classification increase as smaller blocks are used; however, this increases the overheads for transmitting the classification information. In this paper, a quadtree based method is proposed. This method allows for more efficient classification by using variable-sized blocks in order to maximize the classification gain, while maintaining a limit on the classification overheads. This method is applied for subband coding of images and the results indicate that it performs better than the other methods currently available in the literature.     

四叉树结构在复杂多目标图像分割中的应用

针对有噪声的复杂多目标图像,引入四叉树的数据结构,区域生长条件运用象素周围k×k邻域的特性进行判决.由于是两个邻域比较特性,需要用到统计学中的假设检验等方法,采用最优阈值作为区域增长的相似性检测准则.达到了以下目的:第一,通过剪切过程减少了许多计算冗余,分割速度明显快于传统的区域增长;第二,由于考虑了邻域性质,抗噪声能力和工作鲁棒性也有所增强.实验表明,把四叉树结构引入复杂多目标图像分割能取得较好的效果.     

非正方形四叉树变换系数能量分布模型及其扫描方式改进

传统的非正方形四叉树变换(NSQT)系数的扫描方式并不能很好地适应系数的能量分布,这将降低熵编码的编码效率。为了提高编码效率,该文提出非正方形四叉树变换系数能量分布模型,并改进了其系数扫描方式。首先推导了非正方形变换块的空间频率域位置分布,基于推导的分布,从理论上提出了其能量分布模型,并用实验进行了证实,发现能量从左上角到右下角逐渐递减,而左下角的能量与右上角的能量接近,根据该能量分布模型,重建了非正方形四叉树变换中量化系数的扫描模式,以适应熵编码,而熵编码中的系数扫描方式倾向于系数上的降序。实验结果显示,相比NSQT中已有的系数扫描方式,该文方法提出的扫描方式在编码计算复杂度和客观视频质量损失可忽略的前提下,平均下降了1.32%~3.74%的比特率,对于非正方形变换块,下降了9.05%~13.19%的比特率。     

Quadtree-based non-local Kuan’s filtering in video compression

Transform coding has been widely used in video coding standards, such as H.264 advanced video coding (H.264/AVC) and high efficiency video coding (HEVC). But the coded video sequences suffer from annoying coding artifacts, such as blocking and ringing artifacts. In this paper, we propose the quadtree-based non-local Kuan’s (QNLK) filter to suppress the quantization noise optimally and improve the objective and subjective quality of the reconstructed frame simultaneously. The proposed filter takes advantage of the non-local Kuan’s (NLK) filter to restore the quantized signal in transform domain. Restored coefficients are then projected onto designed quantization constraint sets (QCS). Quadtree-based signaling strategy is used at the end of QNLK for adaptive filtering on/off control. Experimental results of QNLK show that the proposed method achieves significant objective coding gain and visual quality improvement, compared with both H.264/AVC high profile and HEVC.     

Numerical simulation of wind-driven circulation and pollutant transport in Taihu Lake based on a quadtree grid

In this study,a two-dimensional flow-pollutant coupled model was developed based on a quadtree grid.This model was established to allow the accurate simulation of wind-driven flow in a large-scale shallow lake with irregular natural boundaries when focusing on important smallscale localized flow features.The quadtree grid was created by domain decomposition.The governing equations were solved using the finite volume method,and the normal fluxes of mass,momentum,and pollutants across the interface between cells were computed by means of a Godunov-type Osher scheme.The model was employed to simulate wind-driven flow in a circular basin with non-uniform depth.The computed values were in agreement with analytical data.The results indicate that the quadtree grid has fine local resolution and high efficiency,and is convenient for local refinement.It is clear that the quadtree grid model is effective when applied to complex flow domains.Finally,the model was used to calculate the flow field and concentration field of Taihu Lake,demonstrating its ability to predict the flow and concentration fields in an actual water area with complex geometry.