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在实验部分,首先验证了自适应对焦窗口计算鬼成像目标深度估计算法的可行性,然后将该方法与传统方法相比[22],证明了该方法的优越性。
利用如图1(a)所示的成像系统进行了实验。定义
$z{\rm{ = 5}}$ cm至$z{\rm{ = 21}}$ cm这一段标定的深菲涅尔区为搜索区间。依据$\delta z{\rm{ = 2}}{\rm{.5}}$ mm将搜索区间划分为N个切片,每个切片位置被标记为$\left\{ {{z^{(k)}}} \right\}$ 。将USAF1 951分辨靶标中组别为‘-1’的数字目标‘3’与‘4’作为目标。将目标‘3’放置在
$z = {\rm{7}}$ cm处(成像区间的左端),目标‘4’放置在$z{\rm{ = 20}}$ cm处(成像区间的右端),计算给定深度位置$\left\{ {{z^{(k)}}} \right\}$ 的虚拟衍射光场图样,并将其与桶探测器的测量值关联,重构目标图像,采样数$M = 5\;000$ 。在自适应对焦窗口选择算法中,像素块的大小被设置为${\rm{5}} \times {\rm{5}}$ ,阈值$T$ 被设置为整个图像的平均灰度值。首先计算
$z = L{\rm{/4}}$ ,$z = {L / {\rm{2}}}$ ,$z = 3L/4$ 的DBC值,然后比较其大小。由表1可知,对于目标‘3’和‘4’分别选取成像区间的$\left( {{\rm{0 - }}{L / {\rm{2}}}} \right)$ 和$\left( {{L / {\rm{2}}}{\rm{ - }}L} \right)$ 区间作为算法的搜索区间。Distance(z)/cm 9 13 17 Object 3/$ \times {\rm{1}}{{\rm{0}}^{\rm{7}}}$ 4.827 3.768 3.346 Object 4/$ \times {\rm{1}}{{\rm{0}}^{\rm{7}}}$ 6.080 6.692 8.854 Table 1. Comparison of DBC value at z =L/4,z=L/2,z=3L/4
图5为自适应对焦窗口深度估计算法分别对目标‘3’和‘4’深度搜索过程。图5(a)与(c)中左侧纵坐标表示迭代的次数,横坐标表示每次迭代时VDP传播的距离。(图中只展示了选定搜索区间中的迭代过程,舍弃的一侧未画出)。图5(b)与(d)中展示了在搜索迭代过程中重构的图像以及相应的自适应对焦窗口。从图中可以看出,文中提出的算法对于目标‘3’和目标‘4’,在选定的搜索区间中经过5次迭代,重构了10张图像搜索到了目标的正确深度。图5证明了,文中方法在时间代价上与传统方法一致,可以有效地在
${\log _2}N$ 次迭代内搜索到目标所在轴向深度,并重构出清晰的目标图像,且误差不超过$\delta z$ 。Figure 5. Searching processes of object’3’and object’4’; Adaptive windows in each iterations. (a), (c) Searching processes of object’3’and object’4’; (b), (d) Adaptive windows in each iterations
图6(a)和(b)为目标‘3’和目标‘4’在传统深度估计算法和自适应对焦窗口算法下的DBC离散一阶导数对比图,可以看到:在采样数
$M = 5\;000$ 时,采用传统深度估计算法时,DBC离散一阶导数在中点处波动较大,无法正常工作导致系统无法准确估计对焦深度。Figure 6. The first derivative of DBC curves of object ‘3’and object’4’based on traditional fixed focus window and proposed adaptive windows with 5 000 samplings. (a) object’3’; (b) object’4’
采用自适应对焦窗口算法时,对于目标‘3’和目标‘4’,自适应窗口划分方法的
${w_d}$ 分别为6.25 cm和7.0 cm,相比较传统方法的4.75 cm和4.25 cm,文中算法可使工作距离稳定增长,且${w_d} > {L / {\rm{4}}}$ 。另外,搜索区间缩小为成像区间的一半,从搜索区间的中点处开始迭代,可准确估计目标深度并重构对焦图像,证明了自适应对焦窗口目标深度估计算法的有效性。为了证明文中算法的优越性,选定了目标‘3’和目标‘5’分别放置在
$z = {\rm{7}}$ cm和$z = {\rm{15\;cm}}$ 处,采样数$M{\rm{ = 2\;000}}$ ,其DBC离散一阶导数如图7所示,对焦位置的重构图像及其对焦窗口如图8所示。Figure 7. The first derivative of DBC curves of object ‘3’and object’5’ based on traditional fixed focus window and proposed adaptive windows with 2 000 samplings. (a) object’3’;(b) object’5’
Figure 8. Verify the proposed method with low samplings. (a), (c) The images of objects reconstructed at their axial positions (reference images); (b), (d) images of objects reconstructed using the adaptive focusing windows.
可以看出:随着采样数的降低,两种算法的工作距离都明显缩短,DBC离散一阶导数波动更加剧烈,传统目标深度估计算法不适用。但在自适应对焦窗口算法的搜索区间内,全局最值两侧区间仍然稳定单调。如图7(a)所示,对于目标‘3’,全局最值左侧的单调区间被成像区间边界截断,右侧的工作距离
${w_d} = 4.5$ cm,仍然满足${w_d} > {L / {\rm{4}}}$ ,使算法稳定工作。如图7(b)所示,对于目标‘5’,左右两侧的区间对称单调,与深菲涅尔区中正/负等距离焦时重构图像的模糊程度相似的理论一致,且在对焦位置的自适应窗口中的目标图像与传统固定对焦窗口在对焦位置得到的目标图像一致,如图8所示。这表明基于自适应对焦窗口的目标深度估计的算法在低采样时也可以准确完成对目标深度的搜索并得到对焦图像,这与算法设计思想吻合。
Depth estimation in computational ghost imaging system using auto-focusing method with adaptive focus window
doi: 10.3378/IRLA202049.0303020
- Received Date: 2019-11-03
- Rev Recd Date: 2019-12-18
- Available Online: 2020-03-20
- Publish Date: 2020-03-24
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Key words:
- ghost imaging /
- object depth estimation /
- adaptive focus window /
- deep Fresnel region /
- working distance
Abstract:
In a Computational Ghost Imaging (CGI) system, the axial depth of the target can be obtained by estimating the degree of blur of the reconstructed image. However, this method is easy to be affected by background noise and requires a long working distance for the image quality evaluation function, so this method needs more samplings and the practicability is reduced. To solve this problem, a target depth estimated algorithm with adapted focusing window was proposed. Firstly the local search interval was divided according to the global characteristics of the evaluation function, and then the actual axial depth of the target was searched iteratively in a given region. In iterations, the use of adaptive window decreased the area of background and contained the whole target. Experiments show that the proposed method greatly reduces the necessary working distance, increases the robustness of this method, reduces the effect of background noise on the evaluation function, and achieves the depth of target under low samplings. This work promotes the development of depth estimation method based on computational ghost imaging system.