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机载SAR图像的动态金字塔实时显示技术

项海兵,刘劲松,吴涛,赵洪立,孙龙(中国电子科技集团公司第三十八研究所孔径阵列与空间探测安徽省重点实验室;河北师范大学)

摘 要
目的:随着机载SAR(Synthetic Aperture Radar,合成孔径雷达)图像分辨率越来越高,幅宽越来越大,传统雷达显控系统将整幅图像直接放在内存中,进行抽样显示。这种方法存在内存资源紧张、图像显示的等待时间过长等问题;以往图像实时显示研究主要分为链路设计型、显示引擎型和图像金字塔型三类。其中,图像金字塔型在大图像显示时,能够有效减少内存的使用,学界围绕金字塔文件构建,金字塔文件管理,金字塔文件显示开展了大量实证研究。然而,上述实践均要求以一幅完整的图像文件作为输入,不能满足实时显示的应用需求。在随收随显的过程中,金字塔文件存在大量缺失,一般只有0级分辨率的瓦片,当图像缩小显示时,需要将底层级的瓦片合成高层级数据进行显示,此类随收随显工作尚无研究涉猎。本文提出的动态金字塔实时显示技术,仅使用少量内存,就能在收到少量图像内容时,就可将图像展示给用户,并支持缩放至原始分辨率。方法:本文提出的实时显示技术包括动态金字塔构建技术和动态金字塔显示技术。动态金字塔构建技术包括:当接收到一个瓦片数据时,输出第0层级的金字塔瓦片;然后,需分6种情况生成更高层级瓦片,逐渐补全缺失层级瓦片: Case 1:如果当前瓦片的行号和列号都不是2的整数倍,则读取该瓦片在当前层左侧、上方和左上方的3个相邻瓦片,采用最近邻法,构建更高层瓦片。Case 2:如果当前瓦片位于当前层的最右侧,且行号不能被2整除,则读取该瓦片在当前层上一行对应位置的瓦片,采用最近邻法,建立更高一层瓦片。Case 3:如果当前瓦片位于当前层的最下方,且列号不能被2整除,则读取该瓦片在当前层左侧位置的瓦片,采用最近邻法,建立更高一层瓦片。Case 4:如果当前瓦片位于当前层右下角,且行号和列号均能被2整除,将该瓦片作为上一层对应位置瓦片的左上角,采用最近邻法,建立上一层对应位置瓦片。Case 5:如果原始图像的当前层缩略图的行数或列数不足瓦片边长时,则退出函数。Case 6:当前瓦片的行号或列号能被2整除,不做任何处理,退出函数。动态金字塔显示技术是指在瓦片数据不全的情况下,采用递归算法,读取较低层级瓦片,合成、显示当前显示层级图像的技术。获取单个瓦片的递归算法分别对以下5种情况进行处理:Case 1:如果所要获取的瓦片存在,且为最初想获取的层级,读取瓦片数据后,返回真;Case 2:如果所要获取的瓦片存在,且不是最初要获取的层级,根据瓦片所在位置,采用最近邻法进行重采样,返回真;Case 3:如果获取层级瓦片左上角所对应的第0级瓦片的行号大于最新瓦片的行号时,返回假。Case 4:如果所要获取层瓦片的层级小于0,返回假。Case 5:如果所要显示的瓦片不存在,且层级大于0,则调用本函数,分别获取该瓦片对应低一级的四个瓦片。这两种技术在两个独立的线程中运行,以硬盘文件(瓦片)作为接口,进行交互,协同工作。结果:在显示软件中,增加一个独立线程,分别读取64MB(8192X8192)、256MB(16384X16384)和1GB(32768X32768)测试图像文件,并定时将256X256的数据块传递给金字塔模块。传递数据的时间间隔从10ms变化到1ms。将该显示软件分别在固态硬盘和机械盘上进行运行,并统计显示第一块瓦片的时延和显示完整一帧图像的时延。分析结果表明,采用SAR图像实时显示技术后,显示第一块SAR图像瓦片的时延小于1秒,与传统显控系统对比,约能够减少一帧图像的传输时延。整帧图像显示的时延因存储介质的存取速率差异较大。固态硬盘的时延比较稳定,与图像大小有关,图像越大,时延越大,1GB图像的时延为10.33秒。机械盘的时延分两种情况:当发送时间间隔小于6毫秒时,受硬盘读写速度的限制,时延较为稳定,为整幅图像构建金字塔的时间;当发送时间间隔大于6毫秒时,显示一帧图像的时延才随发送速率的减小而增大,其时延大小与图像大小正相关。结论:此项技术能实时向用户呈现接收中的SAR图像,提高了机载SAR图像的显示时效性,降低了机载雷达显控终端的内存需求,改善了机载雷达显控终端的用户体验。
关键词
Real-time display technology of airborne SAR image in Pyramid format

XIANG Haibing,liu jinsong,wu tao,ZHAO Hongli,SUN Long(hebei normal university;Key Laboratory of Aperture Array and Space ApplicationNo. 38 Research Institute of CETC)

Abstract
Objective: With the increasing resolution of airborne SAR (Synthetic Aperture Radar) images, and the larger swath width, the conventional radar display and control system directly store the entire image in memory for sampling and displaying. The problems of conventional methods are resources intensive, time-consuming, and etc. Previously, the real-time image displaying techniques can be divided into three main types which are link design, display engine and image pyramid. Among them, the image pyramid method can effectively allocate and use memory when displaying large images. Scholars have conducted substantial empirical studies on pyramid document construction, file management, and file display. However, the above studies all require an entire image file as an input, which cannot meet the application requirements of real-time displaying. In the process of receiving and displaying, there are a lot of pyramid files missing. Generally, there are only 0-level resolution tiles. When the image is zoomed and displayed, the underlying-level tiles need to be combined with high-level data for display. There is no research involved in this field. The dynamic pyramid real-time display technique proposed in this paper occupies few memories to display images when receiving small amounts of images, and supports scaling to the original resolution. Methods: The real-time display techniques proposed in this study include dynamic pyramid construction technique and dynamic pyramid display technique. The processes of dynamic pyramid construction in detail are outputting the 0th-level pyramid tiles when receiving a tile data; then, there are 6 cases to generate higher-level tiles: Case 1: If Neither the row number nor the column number of the current tile is an integer multiple of 2, the 3 adjacent tiles on the left, top and top left of the current level are read, and the nearest neighbor method is used to build a higher level tile; Case 2: If the current tile is at the rightmost of the current Level, and the row number is not divisible by 2, then the tile of the corresponding position of the tile on the current level is read, and create a higher level tile. Case 3: If the current tile is at the bottom of the current level and the column number cannot be divisible by 2, then read the tile of the tile in the left position of the current level, and create a higher level tile. Case 4: If the current tile is located in the lower right corner of the current level, and both the row number and the column number are divisible by 2, the tile is used as the upper left corner to create a higher level tile. Case 5: Exit the function if the number of rows or columns of the current image''s current level is less than that of the tile. Case 6: The row number or column number of the current tile can be divisible by 2, without any processing, exit the function. The dynamic pyramid display technology refers to a technique that uses a recursive algorithm to read lower-level tiles and synthesizes and displays the current display hierarchy images when the tile data is incomplete. The recursive algorithm for obtaining a single tile handles the following five cases respectively: Case 1: If the tile to be acquired exists and is the level originally desired to be obtained, reading the tile data, returns true; Case 2: If desired The acquired tiles exist, and are not the originally desired to be obtained, According to the position of the tiles, the nearest neighbor method is used to re-sample and return true; Case 3: If the row number is greater than the row number of the received tile, return false. Case 4: If the level of the tile to be acquired is less than 0, return false. Case 5: If the tile to be displayed does not exist, and the level is greater than 0, this function is called to obtain four tiles corresponding to the lower level of the tile. These two technologies run in two separate threads, interacting with hard disk files (tiles) as interfaces and working together. Results: In the display mode of software, an independent thread was added to read testing images in different data blocks such as 64MB (8192*8192), 256MB (16384*16384) and 1GB (32768*32768),and to transmit the data block of 256*256 to the pyramid module. The time intervals for data transfer vary from 10ms to 1ms. The display software runs on the solid state hard (SSD) disk and the mechanical disk, respectively, and the time delay of displaying the first tile and a complete frame of the image is counted. The results show that using the SAR image real-time display technology, the time delay of displaying the first SAR image tile is less than one second. Compared with the traditional display control system, the transmission delay of one frame image can be reduced. The delay of an entire frame image display varies greatly due to the access rate of the storage media. The time delay of SSDs is relatively more stable, which is related to the size of the image. The larger the image, the greater the delay. The time delay of 1GB image is 10.33 seconds. The delay of the mechanical disk is divided into two cases: when the sending interval is less than 6 milliseconds, the time delay is relatively more stable due to the limitation of hard disk read/write speed, which is the time for constructing the pyramid for the entire image; when the sending interval is more than 6 milliseconds, The time delay for displaying a frame of image is increased accompanied by the decreasing of transmission rate, and the size of the delay is positively correlated with the size of the image. Conclusion: This technique can present the received SAR images to the user in real time, improve the display time effectiveness of the airborne SAR images, reduce the memory requirements of the airborne radar display and control terminals, and improve the users’ experience of the airborne radar display and control terminals.
Keywords
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