“句芒号”激光光斑质心提取方法与稳定性分析
The first Terrestrial Ecosystem Carbon Monitoring (TECM-1) satellite—Gou Mang was successfully launched on August 4, 2022 in China. TECM-1 satellite is equipped with a multi-beam LiDAR which is mainly used to obtain the elevation of land and forest. The mission of measuring the elevation of a forest requires both high-ranging accuracy and horizontal positioning accuracy. The multi-beams LiDAR on the TECM-1 satellite is concurrently equipped with an optical axis monitoring camera to obtain the footprint images, including the ground images and spot images. The centroid of the laser spot in the spot image can indicate the laser pointing angle, helping reduce the horizontal positioning errors. However, the laser spot is inevitably influenced by the background features in the spot image, and traditional centroid extraction methods are difficult to achieve ideal accuracy. Many scholars have made improvements based on traditional methods, and most of them apply noise reduction processing to the spot image, such as grayscale threshold constraint and image filtering. However, due to the influence of photography conditions and complex terrain, it is challenging for these relatively simple noise reduction methods to separate the laser spot from background noise. The centroid extraction accuracy still faces challenges in complex background scenarios. We report a laser spot centroid extraction method against complex background noise, which can effectively remove background noise from the spot image. It achieves better adaptability and higher accuracy than previous methods. We hope that our research can provide a certain reference value for the future processing and application of laser data collected by the TECM-1 satellite.
A set of footprint images collected by the TECM-1 satellite consists of the ground image and spot image and the spot image can be considered to be formed by the laser spot and surrounding background features. The ground image and spot image are geometrically aligned and exhibit noticeable brightness differences due to varying exposure time (Fig. 2). Based on the characteristics of footprint images, the present study firstly crops spot area image pairs and non-spot area image pairs from footprint images. Then, the grayscale transformation coefficients are calculated by non-spot area image pairs, and the background grayscale values can be removed from the spot image. Subsequently, a distance constraint is adopted to limit the range of laser spots and a Gaussian filter is applied to smooth the spot image. Then an adaptive threshold is estimated by Otsu's method for binary segmentation. It generates a binary mask of the laser spot and with the mask processing, the noise of the spot image can be almost eliminated. Finally, the centroid coordinates of the laser spot are calculated by the grayscale centroid method. The proposed method is tested with simulated and actual spot images, and the accuracy of centroid extraction is analyzed along with the stability of the laser spot centroids.
For the generated 10215 simulated laser spots, our method exhibits significantly improved centroid extraction accuracy compared to the Gaussian fitting method, grayscale centroid method, and ellipse fitting method. It results in an average error of approximately 0.059 pixel, and a root mean square error of about 0.074 pixel, slightly higher than those of Ren's method. The maximum error is 0.482 pixel, better than Ren's 1.828 pixel (Table 2). Furthermore, the CE90 of the proposed method is approximately 0.11 pixel (Fig. 6). In addition, our method is approximately 60% faster than Ren's method and comparable to the ellipse fitting method in processing time (Table 3). When applied to 1000 sets of consecutive footprint images obtained within 25 s, our method suggests that the standard deviation of centroid coordinates is less than 0.05 pixel. The range is within 0.2 pixel in a single direction (Table 5), demonstrating strong stability (Fig. 7 and Fig. 8). We select over 40 scenes of footprint images at intervals of 5 d to analyze the stability of centroids, and the results show that the standard deviations of centroid coordinates distribute between 0.12 and 0.34 pixel. It corresponds to horizontal distances of about 1 to 2.7 m on the ground, signifying that the relative geometric relationship between the laser and optical axis monitoring camera is relatively stable (Table 6). Additionally, the stability of centroid coordinates varies from different laser beams in the monitoring range (Figs. 9-13). It can be found that regular monitoring for the stability of centroids is necessary to comprehend centroid variations.
We report a laser spot centroid extraction method against complex background noise based on the footprint images collected by the TECM-1 satellite. Our method uses grayscale matching to remove most of the background noise from the spot image, followed by distance constraint, Gaussian filtering, and Otsu's image segmentation to remove the residual noise. The method ultimately calculates the centroid coordinates of the laser spot by the grayscale centroid method. Assessed by simulated and actual spot images of various terrain types, the results demonstrate that our method exhibits robust adaptability to laser spots in complex background scenarios and displays strong stability. However, for ground images with a significant number of saturated grayscale values, the present grayscale matching method cannot remove the background noise completely and it will affect the final centroid extraction accuracy. Future research should explore optimizing the grayscale transformation model or introducing other constraints to address this issue. The results of the stability analysis reveal a relatively tight jitter scope of centroids during the monitoring range, indicating a relatively stable geometric relationship between the laser and optical axis monitoring camera. Our study will be employed to support laser positioning, with the goal of enhancing the horizontal positioning precision of laser data.
1 引言
2022年8月4日,我国成功发射了首颗陆地生态系统碳监测卫星——句芒号,其采用多波束激光雷达与相机结合的主被动一体化探测方式,同时具备全波形测距和高分辨率多光谱成像功能,主要用于树高测量、地表高程测量以及多光谱对地成像,进而反演出生物量和森林碳汇数据[1]。对于树高测量任务,多波束激光雷达需要同时兼顾激光测距精度和平面定位精度,以实现高精度、高准度测量。得益于现代激光器的快速发展,激光测距精度已达较高水平,句芒号多波束激光雷达的设计测距精度优于0.3 m[2];激光平面定位精度主要受激光指向精度影响,相对于测距精度还有很大的提升空间[3]。句芒号多波束激光雷达同步搭载了光轴监视相机,可获取地物影像和光斑影像,光斑影像中的光斑质心可表征5个激光波束的出射方向,而精确的质心坐标有助于减小激光指向误差引起的平面定位误差[4-7]。因此,准确提取激光光斑质心对于提高激光平面定位精度具有重要意义。
传统光斑质心提取方法有椭圆拟合法[8]、Hough变换法[9]、灰度重心法[10]和高斯拟合法[11]等,相关研究使用上述方法满足了具体需求[12-16]。然而,传统方法的抗干扰性较差,提取精度有限,为提高足印影像上激光光斑质心提取精度和抗干扰性,许多学者对传统方法进行了改进。袁小棋等[17]提出一种基于高斯阈值的椭圆拟合质心提取方法,获得优于0.085 pixel的仿真激光光斑影像提取精度,并使用该方法监视ICESat卫星对地激光测高系统(GLAS)的激光光斑质心。Yang等[18]利用阈值法去除GLAS激光光斑影像背景噪声,结合灰度重心法和椭圆拟合法提取光斑参数,结果表明,激光光斑质心的提取精度优于0.3 pixel。句芒号足印影像的光斑影像中光斑与地物背景叠加,激光光斑不可避免地受到地物背景的影响,从而降低了激光光斑质心的提取精度。针对此类光斑影像,任守峰等[3]通过设置灰度阈值和距离阈值双重约束,使用灰度重心法求解光斑质心,该方法对于高分七号仿真光斑影像的质心提取精度优于0.05 pixel。Yao等[19]通过统计高分七号光斑影像背景灰度均值去除背景噪声,并提出一种基于阈值约束的椭圆拟合光斑质心提取算法,在地物背景复杂情况下仍能保持0.08 pixel左右的质心提取精度。Ren等[20]通过统计高分七号地物影像与光斑影像的灰度差异,利用线性最小二乘法求解灰度转换系数,去除光斑影像地物背景噪声,并使用高斯拟合法提取光斑质心,提取精度优于0.7 pixel,其中绝大部分误差来源于图像配准。句芒号足印影像同时采集5个波束的激光光斑,影像幅宽大,地物背景条件更为复杂,上述方法虽能有效去除大部分地物背景,但在部分复杂场景下仍会残余部分噪声,影响质心提取精度。
针对句芒号足印影像复杂地物背景下光斑质心提取的噪声残留问题,本文提出一种针对复杂背景噪声的激光光斑质心提取方法。首先,使用灰度匹配方法去除大部分地物背景,结合距离约束、高斯滤波和大津阈值分割去除残余噪声后,利用灰度重心法计算光斑质心坐标。然后,使用仿真光斑影像以及短时间内的连续实测影像序列对所提方法进行测试,验证了该方法的适用性,同时对长时段的激光光斑质心稳定性进行分析,为后续句芒号激光雷达数据处理和应用提供参考。
2 激光光斑质心提取方法
2.1 句芒号光轴监视相机成像模式
句芒号多波束激光雷达配备一台面阵CMOS光轴监视相机,5个植被激光波束从激光器出射后先经过扩束镜头再射向地面,在扩束镜头边缘配有导光棱镜,可将部分激光导入光轴监视相机焦面,从而形成激光光斑[2]。光轴监视相机探测模式如
对于每次激光出光,光轴监视相机先采用短曝光时间对激光光斑成像得到光斑影像,再用长曝光时间对地物成像得到地物影像[2],影像大小均为2048 pixel×128 pixel,影像分辨率为8 m,地物影像与光斑影像在几何上完全对齐,一组足印影像数据如
图 2. 一组足印影像。(a)地物影像;(b)光斑影像
Fig. 2. A set of footprint images. (a) Ground image; (b) spot image
从足印影像可以看出,光斑影像为激光光斑与地物背景叠加的结果,5个激光光斑在影像上的亮度存在明显差别,部分光斑存在旁瓣。由于曝光时间短,光斑影像中地物背景亮度相较于地物影像更暗。
2.2 针对复杂背景噪声的光斑质心提取方法
在复杂地面场景下,激光光斑与背景难以直接分割。如
Ren等[20]提出一种经灰度匹配得到地物影像与光斑影像之间的灰度映射系数,从而去除地物背景的方法。该方法可有效去除光斑影像中绝大部分地物背景,然而,当背景地物过于复杂如地物影像信息熵低,甚至地物影像局部位置饱和时,仅使用灰度匹配方法难以完全去除地物背景。此外,光斑旁瓣以及颗粒噪声也会影响质心提取精度。为了在去除地物背景的基础上进一步滤除噪声影响,本文提出一种针对复杂背景噪声的光斑质心提取方法:首先,从光斑影像和对应的地物影像中截取光斑区域影像对和非光斑区域影像对,利用非光斑区域影像对求解灰度映射系数,从而计算出光斑区域影像上地物背景的灰度值,将其从光斑区域影像中去除;然后,使用高斯滤波和大津阈值分割方法去除光斑区域影像的残余噪声;最后,使用灰度重心法计算光斑质心坐标。本文方法的流程如
1)光斑影像地物背景灰度去除
由于相机曝光时间不一,地物影像与光斑影像存在灰度差异,灰度转换关系如
式中:
对非光斑区域地物影像和光斑影像的灰度值进行统计,可利用最小二乘法拟合出最优的灰度转换参数,得到地物影像与光斑影像的灰度映射关系。随后针对光斑区域,将地物影像灰度值代入灰度转换模型,求解光斑影像中的地物背景灰度值,进而从光斑影像中去除地物背景灰度。
式中:
2)光斑影像噪声去除
光斑影像减去地物背景灰度后,仍存在部分噪声,包括高斯噪声、椒盐噪声、光斑旁瓣、残余地物背景噪声。
针对以上残留噪声,采用距离阈值对光斑范围进行约束,如
式中:
使用高斯滤波方法进行平滑处理,并根据
式中:
采用大津法[21]估计自适应阈值对光斑影像进行分割,使光斑影像前景和背景的类内方差之和最小、类间方差最大,基于图像二值化分割结果得到光斑影像掩模
3)光斑质心提取
句芒号光斑影像中的激光光斑符合高斯分布,且形状规则,对于去噪处理后的光斑影像,使用传统质心提取方法即可获得较为理想的提取精度。因此,本文选用耗时少的灰度重心法计算光斑质心坐标,如
式中:
3 实验数据及结果分析
3.1 基于仿真光斑影像的提取精度分析
由于实测光斑影像中光斑质心真值未知,为准确评价质心提取精度,使用仿真光斑影像来验证所提方法的精度。
选取噪声较少的夜间实测光斑影像,使用高斯拟合法分别求解出各激光波束光斑的高斯参数,之后按
式中:
从实测影像数据中挑选出2043幅过境城区、湖泊和森林等多种类型区域的无云光斑影像作为地物背景底图。按
基于仿真数据集开展质心提取实验,使用灰度重心法[8](GCM)、椭圆拟合法[10](EFM)、高斯拟合法[11](GFM)、Ren方法[20](Ren CF)以及本文方法进行光斑质心提取。其中,灰度重心法、椭圆拟合法、高斯拟合法直接在原始光斑影像上进行质心提取,Ren方法使用灰度匹配去除地物噪声进而提取光斑质心。
复杂地物背景情况下灰度重心法、Ren方法以及本文方法的仿真光斑影像质心提取结果如
表 1. 复杂地物背景仿真光斑影像质心提取结果
Table 1. Centroid extraction results of simulated spot images with complex background
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使用平均误差(mean)、均方根误差(RMSE)和最大误差(Max. error)对质心提取精度进行评价,它们的计算公式分别为
式中:
不同算法对仿真光斑影像质心的提取精度如
表 2. 不同算法的仿真光斑影像质心提取精度
Table 2. Centroid extraction accuracy of simulated spot images by different algorithms
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图 6. 不同算法的质心提取误差分布图。(a)GCM;(b)EFM;(c)GFM;(d)Ren CF;(e)本文方法
Fig. 6. Distribution of centroid extraction errors by different algorithms. (a) GCM; (b) EFM; (c) GFM; (d) Ren CF; (e) proposed method
从
对不同算法的运行时间进行了统计,结果如
表 3. 不同算法的运行时间
Table 3. Running time of different algorithms
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3.2 基于实测光斑影像的提取精度分析
利用句芒号实测的光斑影像对本文质心提取方法进行验证。选取道路、厂房、市区楼房、湖泊和森林5种复杂地物背景的光斑影像,使用本文方法进行质心提取处理,结果如
表 4. 复杂地物背景实测光斑影像质心提取结果
Table 4. Centroid extraction results of actual spot images with complex background
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由
为了进一步分析实测光斑影像质心提取精度,挑选出连续拍摄的无云序列影像提取光斑质心,使用极差和标准差对结果进行评价。激光器工作时的温度变化会导致激光指向漂移,激光光斑质心表现为有一定周期性和规律性的抖动,为削弱温度变化所产生的影响,使用短时间内连续拍摄的影像进行分析。
选取2022年10月6日连续25 s拍摄得到的1000组清晰无云的句芒号足印影像,使用本文方法进行光斑质心提取测试,分别统计5个激光波束的标准差(STD)和极差(range),结果如
表 5. 实测光斑影像质心提取精度
Table 5. Centroid extraction accuracy of actual spot images
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式中:
对于连续25 s拍摄得到的1000组足印影像,所有激光波束质心坐标的标准差均值为0.038 pixel,在单方向上所有激光波束质心坐标的极差均在0.2 pixel以内,无明显异常点。激光波束1和3的质心提取结果分别如
图 7. 激光波束1的质心提取结果。(a)x方向;(b)y方向
Fig. 7. Centroid extraction results of laser beam 1. (a) x direction; (b) y direction
图 8. 激光波束3的质心提取结果。(a)x方向;(b)y方向
Fig. 8. Centroid extraction results of laser beam 3. (a) x direction; (b) y direction
从
4 句芒号激光光斑质心稳定性分析
基于句芒号发射以来在轨运行期间获取的足印影像数据,提取激光光斑质心,绘制质心变化曲线,分析光斑质心稳定性。激光光斑质心在系统刚开机时存在一定时长的抖动,为避免长时序稳定性分析受到光斑质心抖动的影响,选取位于各轨数据后段的足印影像进行质心提取,开展光斑质心稳定性分析。选取2022年10月24日至2023年4月16日的足印影像数据,每间隔5 d选择一轨受云层干扰少的数据,共计36轨,使用本文方法对第5景产品中的足印影像提取光斑质心,并统计质心均值。图
图 9. 激光波束1的质心稳定性监测结果。(a)x方向;(b)y方向
Fig. 9. Centroid stability monitoring results of laser beam 1. (a) x direction; (b) y direction
图 10. 激光波束2的质心稳定性监测结果。(a)x方向;(b)y方向
Fig. 10. Centroid stability monitoring results of laser beam 2. (a) x direction; (b) y direction
图 11. 激光波束3的质心稳定性监测结果。(a)x方向;(b)y方向
Fig. 11. Centroid stability monitoring results of laser beam 3. (a) x direction; (b) y direction
表 6. 质心稳定性分析结果
Table 6. Centroid stability analysis results
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表 7. 激光指向稳定性
Table 7. Stability of laser pointing
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图 12. 激光波束4的质心稳定性监测结果。(a)x方向;(b)y方向
Fig. 12. Centroid stability monitoring results of laser beam 4. (a) x direction; (b) y direction
图 13. 激光波束5的质心稳定性监测结果。(a)x方向;(b)y方向
Fig. 13. Centroid stability monitoring results of laser beam 5. (a) x direction; (b) y direction
统计各激光波束光斑质心坐标的标准差和极差,结果如
利用质心坐标变化量表征激光指向稳定性,结果如
鉴于不同激光波束质心坐标的变化趋势和幅度,有必要定期进行各激光波束质心稳定性监测,掌握质心变化规律,从而辅助激光定标,改善激光指向精度,最终提高激光平面定位精度。
5 结论
激光光斑质心对激光指向具有指示作用,根据句芒号足印影像特点,提出一种针对复杂背景噪声的激光光斑质心提取方法,先使用灰度匹配方法去除大部分地物背景,再经高斯滤波和大津阈值分割掩模操作去除噪声,最后使用灰度重心法计算光斑质心坐标。本文方法可有效排除光斑影像背景噪声的干扰,提高质心提取精度。利用仿真和实测影像数据对所提方法开展验证实验以及稳定性分析实验,得到以下结论:
1)使用灰度重心法、椭圆拟合法、高斯拟合法3种传统方法,以及Ren方法和本文方法对仿真光斑进行质心提取。实验结果表明,本文方法的提取精度明显优于传统方法,略优于Ren方法,且耗时与传统方法相当。本文方法的质心提取均方根误差约为0.074 pixel,最大误差为0.482 pixel,优于Ren方法的1.828 pixel,对于复杂地形混叠光斑有较好的适应性。
2)使用句芒号光轴监视相机连续25 s拍摄得到的1000组实测足印影像对本文方法进行测试。结果表明,所有波束质心坐标标准差均小于0.05 pixel,表明本文方法具有较好的稳定性。
3)使用本文方法对2022年10月至2023年4月的足印影像提取光斑质心,分析质心坐标变化情况。实验结果表明,所有波束质心坐标的标准差在0.34 pixel以内,对应的地面水平距离在2.7 m以内,激光器与光轴监视相机的相对几何关系较为稳定。
所提出的激光光斑质心提取方法对句芒号光斑影像的适应性好,提取精度较高,具有较好的稳定性,但对于地物影像存在大范围灰度值饱和的情况,使用当前的灰度匹配方法不能较好地去除地物背景噪声,影响质心提取精度,在后续研究中将针对此类情况对灰度转换模型进行优化或引入其他方法滤除上述情况下的残留背景噪声。同时,后续拟对更长时间跨度的激光光斑质心进行监测,分析激光光斑质心坐标变化趋势、日间与夜间光斑质心变化特点,并探索利用激光光斑质心坐标辅助激光定位的方法,提高激光数据平面定位精度。
[1] Li G Y, Gao X L, Hu F, et al. Overview of the terrestrial ecosystem carbon monitoring satellite laser altimeter[J]. The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, 2022, 43: 53-58.
[2] 孙立, 杨居奎, 王玉诏, 等. 主被动一体化多波束激光雷达设计与实现[J]. 航天返回与遥感, 2022, 43(6): 27-35.
Sun L, Yang J K, Wang Y Z, et al. Design and implementation of the active-passive multi-channel Li DAR[J]. Spacecraft Recovery & Remote Sensing, 2022, 43(6): 27-35.
[3] 任守峰, 唐新明, 祝小勇, 等. 一种高分七号卫星激光足印光斑质心的高精度提取算法[J]. 光学学报, 2021, 41(10): 1012001.
[4] 曹海翊, 张新伟, 赵晨光, 等. 高分七号卫星总体设计与技术创新[J]. 中国空间科学技术, 2020, 40(5): 1-9.
Cao H Y, Zhang X W, Zhao C G, et al. System design and key technolongies of the GF-7 satellite[J]. Chinese Space Science and Technology, 2020, 40(5): 1-9.
[5] 国爱燕, 戴君, 赵晨光, 等. 高分七号卫星激光测高仪总体设计与在轨验证[J]. 航天器工程, 2020, 29(3): 43-48.
Guo A Y, Dai J, Zhao C G, et al. Design and on-orbit validation of GF-7 satellite laser altimeter[J]. Spacecraft Engineering, 2020, 29(3): 43-48.
[6] 唐新明, 谢俊峰, 莫凡, 等. 高分七号卫星双波束激光测高仪在轨几何检校与试验验证[J]. 测绘学报, 2021, 50(3): 384-395.
Tang X M, Xie J F, Mo F, et al. GF-7 dual-beam laser altimeter on-orbit geometric calibration and test verification[J]. Acta Geodaetica et Cartographica Sinica, 2021, 50(3): 384-395.
[7] 王星星, 唐新明, 祝小勇, 等. 高分七号卫星足印光斑质心提取方法与稳定性监测[J]. 光学学报, 2021, 41(24): 2428001.
[8] Waerbeke L V, Mellier Y, Erben T, et al. Detection of correlated galaxy ellipticities on CFHT data: first evidence for gravitational lensing by large-scale structures[J]. Astronomy and Astrophysics, 2000, 358(1): 30-44.
[9] 杨耀权, 施仁, 于希宁, 等. 用Hough变换提高激光光斑中心定位精度的算法[J]. 光学学报, 1999, 19(12): 1655-1660.
[10] van Assen H C, Egmont-Petersen M, Reiber J H C. Accurate object localization in gray level images using the center of gravity measure: accuracy versus precision[J]. IEEE Transactions on Image Processing, 2002, 11(12): 1379-1384.
[11] 王丽丽, 胡中文, 季杭馨. 基于高斯拟合的激光光斑中心定位算法[J]. 应用光学, 2012, 33(5): 985-990.
[12] 蒋佳雯, 康杰虎, 吴斌. 激光光斑中心高精度定位补偿算法研究[J]. 激光与光电子学进展, 2021, 58(14): 1412002.
[13] 朱红, 谢俊峰, 吴向前, 等. 超分辨率重建在卫星平台颤振探测中的可行性[J]. 激光与光电子学进展, 2021, 58(3): 0314004.
[14] 么嘉棋, 李国元, 陈继溢, 等. 高分七号卫星激光测高仪光斑质心位置变化分析[J]. 红外与激光工程, 2021, 50(S2): 20210539.
[15] 刘慧洁, 买买提明·艾尼, 古丽巴哈尔·托乎提, 等. 一种合作编码标志的设计与识别定位[J]. 激光与光电子学进展, 2021, 58(12): 1210015.
[16] 伍煜, 龙小祥, 杨坚, 等. 高分七号卫星足印相机激光光斑中心定位方法研究[J]. 光学学报, 2021, 41(17): 1728001.
[17] 袁小棋, 李国元, 唐新明, 等. 星载激光光斑影像质心自动提取方法[J]. 测绘学报, 2018, 47(2): 135-141.
Yuan X Q, Li G Y, Tang X M, et al. Centroid automatic extraction of spaceborne laser spot image[J]. Acta Geodaetica et Cartographica Sinica, 2018, 47(2): 135-141.
[18] Yang X D, Li G Y, Yao J Q. Laser pointing and characterization parameter determination methods based on laser profile arrays of ICESat/GALS[J]. Optics Express, 2021, 29(7): 9861-9877.
[19] Yao J Q, Zhai H R, Wu S Q, et al. Evaluation of the emissions state of a satellite laser altimeter based on laser footprint imaging[J]. Remote Sensing, 2022, 14(4): 1025.
[20] Ren C F, Xie J F, Zhi X D, et al. Laser spot center location method for Chinese spaceborne GF-7 footprint camera[J]. Sensors, 2020, 20(8): 2319.
[21] 齐丽娜, 张博, 王战凯. 最大类间方差法在图像处理中的应用[J]. 无线电工程, 2006, 36(7): 25-26, 44.
Qi L N, Zhang B, Wang Z K. Application of the OTSU method in image processing[J]. Radio Engineering of China, 2006, 36(7): 25-26, 44.
Article Outline
万科, 黎荆梅, 韩启金, 李功伟, 王宁, 徐兆鹏, 赵航, 马灵玲. “句芒号”激光光斑质心提取方法与稳定性分析[J]. 光学学报, 2024, 44(6): 0628005. Ke Wan, Jingmei Li, Qijin Han, Gongwei Li, Ning Wang, Zhaopeng Xu, Hang Zhao, Lingling Ma. Laser Spot Centroid Extraction Method and Stability Analysis of "Gou Mang"[J]. Acta Optica Sinica, 2024, 44(6): 0628005.