一种基于视点分段式体像素的具有100°正面观看视角的桌面式光场显示系统 下载: 918次封面文章
Tabletop three-dimensional (3D) display can provide natural and realistic terrain scenes for viewers, which has a great application potential in many fields. With the development of advanced real-time terrain rending, it becomes feasible to acquire high definition topography based on massive terrain data. However, traditional 3D displays are hard to reach the harsh requirements of tabletop 3D display. For example, the auto-stereoscopic display based on a lenticular lens array or parallax barrier suffers from a small viewing zone, viewing jumps, and perspective errors due to the lack of vertical parallax. Light field display is considered to be one of the most potential methods to realize ideal realistic 3D display. Through simulating the light rays emitted from each point of the spatial object, light field display can reconstruct the light distributions of the displayed scenes. Former efforts of our group have been devoted to achieving reconstructed high definition light field scenes in recent years. Detailed terrain data is showcased with a horizontal parallax viewing angle of more than 100°. However, if we apply this device to tabletop 3D light field display, the plane of the liquid crystal display(LCD)screen must be set horizontal, thus there are two serious flaws. First, the viewing range of this 3D light field display is only designed for viewing from right above the screen, therefore not suitable for tabletop viewing. Second, a wrong perspective relation occurs due to the lack of vertical parallax. We hope that our strategy and work can be helpful for the design of tabletop light field display with large frontal viewing angles and for the correction of multiple viewer perspective errors.
First, the light field display unit, which consists of collimated light and a linear lens array (LLA) that reversely sticks on the LCD panel with a resolution of 7680 pixel×4320 pixel, is designed to construct a high resolution 3D scene in a large horizontal viewing angle. Then, the direction turning films (DTF) can deflect the viewing range from the top to the front, and the holographic functional screen (HFS) is applied to distribute the thin collimated light beams in a specifically arranged geometry with a specifically spreading function. Therefore a vertical viewing angle is formed and the reconstruction of the discrete sampling light field scenes is realized. After that, the views-segmented image coding scheme is calculated to deduce the mapping relationship between sub-pixels on the LCD and the proposed views-segmented voxel. These voxels divide the whole frontal viewing range into a number of independent viewing zones based on the spatial coordinates of each viewer. In addition, instead of acquisited camera arrays according to horizontal parallax, each part of the acquisition camera array moves synchronously with the viewer in different viewing zones to realize the adaptive perspective correction of multiple viewers in real time rendering.
In our experiment, the proposed light field display unit successfully generates realistic, high definition 3D images within a 100° horizontal viewing angle. The light intensity measurement experiment in two different viewing directions shows that a frontal 90° vertical viewing angle, which is more suitable for tabletop light field display, is well covered based on the light-beam modulation of the HFS (Fig. 2) and the deflection effect of the proposed DTF (Fig. 7). With the appliance of the views-segmented image coding method, the sub-pixel arrangement in each image unit is settled and the views-segmented voxel is generated to provide each viewing zone with different 3D scenes and parallax relationships (Fig. 5). In the experiment of perspective correction, when multiple viewers in different positions (located by the depth camera with facial recognition) are watching the 3D terrain scenes of the tabletop light-field display device at the same time, all corrected perspective relations in different viewing zones are clearly observed (Fig. 11).
Tabletop 3D light-field display with full parallax based on direction-turning films and views-segmented voxels is demonstrated. In this system, the collimated backlight and the lenticular lens array reversely attached to an LCD panel with a resolution of 7680 pixel×4320 pixel are designed to achieve a 100° horizontal viewing angle, and the direction turning films and the holographic functional screen are applied to reconstruct 3D light-field images within a 90° frontal viewing angle. Views-segmented voxels are first proposed to separate the frontal viewing range into some disparate viewing zones according to the spatial coordinates of different viewers, then the views-segmented image coding method and the mapping function between the views-segmented voxels and sub-pixels on LCD are also deduced. Finally, after the real-time adaptive perspective correction in each viewing zone, a full parallax, high-definition tabletop 3D scene is presented to multiple viewers.
1 引言
桌面三维显示可以为观众提供真实自然的三维场景,在众多领域具有很大的应用潜力。随着先进实时地形渲染技术的发展,基于海量地形数据获取高质量的地形模型成为了可能[1-3]。然而,传统三维显示很难满足桌面三维显示的苛刻要求。例如,基于柱透镜阵列或狭缝光栅的自由立体显示有着视角小、视区跳跃和辐辏调节矛盾等固有缺陷[4-5]。体三维显示虽然没有上述这些问题,可以显示真实的三维图像,但是,透明的体像素使其难以表现正确的三维遮挡关系[6-7]。全息三维显示可以再现被记录物光的振幅和相位,被看作是最有前景的显示技术之一,但可惜的是,由于高信息吞吐量的动态设备难以实现,实现全息三维显示的大视角、大尺寸和真彩色依然存在挑战[8-10]。显然,以上三维显示技术并不是桌面三维显示的最佳解决方案。
光场显示技术能够实现大视角、大尺寸、真彩色以及具有正确空间遮挡关系的动态三维影像,是目前最具有前景的三维显示技术之一[11-12]。光场显示是在空间中构建体像素,并以此来模拟真实三维物体上的发光物点。体像素发出的不同方向的光线携带有不同的视点信息,所有的体像素共同在空间中重构出物体的原始光场分布,所再现的3D图像具有完整的深度信息。由于光场显示的显示效果出众,国内外诸多科研团队都将其应用到了桌面三维显示系统中[13-17]。在先前的工作中,本团队已经实现了基于透镜阵列和全息功能屏的全视差360°桌面光场显示[18-19],但这些显示技术由于受到较小的观看视角以及低分辨率等因素的限制,应用场景有限,难以满足显示精细三维场景的需要。
本团队近几年的研究已经实现了大视角的高分辨率光场显示,能够在视角范围内实现清晰的水平视差光场信息重构[20]。但是,如果将仅有正上方水平视差的光场显示设备应用于桌面式光场沙盘显示中,会出现如下两个缺陷:其一是仅有正上方观看视角的光场显示系统并不满足桌面式显示中正面观看的要求,其二是由于缺少垂直视差,当观看者沿前后或垂直方向移动时,会出现透视关系错误的情况。本文提出了一种基于光学偏折膜和视点分段式体像素的全视差桌面式光场显示系统,光学偏折膜可以将观看视角从桌面正上方偏折至适合正面观看的角度范围,视点分段式体像素可以向不同方向提供具有正确透视关系的3D场景。在实验中,此系统可在100°水平视场角范围内同时为移动中的多名观看者提供具有正确透视关系和高分辨率的全视差3D图像,实现高质量的桌面式光场显示。
2 基本原理
2.1 系统结构
图 1. 基于视点分段式体像素和光学偏折膜的实验装置示意图
Fig. 1. Schematic of experimental setup based on views-segmented voxel and direction turning films
图 2. 所提桌面式光场显示系统中体像素的构建示意图。(a) 光场显示单元的光路示意图;(b)光学偏折膜的微结构示意图;(c)同一位置加入HFS前与加入HFS后的3D图像对比图
Fig. 2. Construction of voxel in proposed tabletop light-field display system. (a) Schematic of light ray of light field display unit; (b) schematic of microstructure of direction turning films; (c) comparison of 3D images at same position with and without HFS
2.2 视点分段式体像素与图像编码
当多人多角度观看时,由于缺少垂直视差,且观看者身高、位置不一,传统的桌面式水平视差光场显示系统无法为所有人同时提供具有正确透视关系的3D场景。本文提出了一种根据不同观看者位置对生成的体像素进行视点分段式图像编码,在观看范围内为每个观看者所在的区域单独划分独立的观看区域,每个区域提供不同的透视关系校正。如
出射光线的参数(x,y,z,θ,φ)与对应的柱镜坐标(x′,y′)的映射关系为
图 4. LCD上子像素与所设计的体像素之间的映射关系。(a)LCD上子像素的图像编码过程;(b)入射光线与LLA上覆盖子像素的几何关系;(c)定向准直背光经过LLA的光线分布
Fig. 4. Mapping relationship between sub-pixels on LCD and designed voxels . (a) Image coding process of sub-pixels on LCD; (b) geometric relationship between incident light and covered sub-pixels on LLA; (c) ray distribution of directional collimating backlight through LLA
得到LCD上的子像素与HFS上的体像素之间的函数映射关系后,计算了基于视点分段式编码方法的显示单元中的子像素排列,如
图 5. 基于视点分段式编码方法的显示单元中的子像素排列
Fig. 5. Sub-pixel arrangement in display unit based on views-segmented coding method
透视关系校正方法如
图 6. 基于不同观看者位置的透视关系校正示意图。(a)校正前;(b)校正后
Fig. 6. Schematic of perspective relation correction based on different viewing positions. (a) Before correction; (b)after correction
当不进行透视关系校正时,采集相机不跟随观看者移动,此时ΔCx=ΔCy=ΔCz=0,随着观看者远离透视的正确位置,误差δy和δz逐渐增大。进行透视关系校正后,虚拟采集相机将跟随观看者空间坐标的变化同步移动。将误差δy和δz控制在1 mm内,可在多人观看的桌面式3D场景的各个分段式编码区域中实现流畅、平滑的透视关系自适应实时校正。
3 实验验证
本文首先利用光学偏折膜分别在改进前和改进后的观看视区的正上方和正前方进行了光强对比实验。
图 7. 光强对比实验和结果。(a)光强测量的实验过程;(b)观看区域正上方的光强分布;(c)观看区域正前方的光强分布
Fig. 7. Experiment and results of light intensity comparison. (a) Experimental process of light intensity measurement; (b) light intensity distributions directly above viewing area; (c) light intensity distributions directly in front of viewing area
在传统的水平视差桌面式光场显示系统的3D信息采集过程中,采集相机只沿水平方向排成一列,经过编码后的再现3D场景缺少垂直视差信息,当观看者沿y轴方向前后移动或沿z轴方向垂直地面移动时,就会出现错误的透视关系,楼房“倾倒”、“压缩”,如
图 8. 透视关系校正前观看者观看到的3D场景。(a)观看者沿z轴垂直移动(距离为1 m);(b)观看者沿y轴前后移动(高度为1.8 m);(c)观看者沿x轴水平移动(距离为1 m,高度为1.8 m)
Fig. 8. Viewed 3D scenes by viewer before and after perspective correction. (a) Viewer moving vertically along z-axis (distance of 1 m); (b) viewer moving forward and back along y-axis (height of 1.8 m); (c) viewer moving horizontally along x-axis (distance of 1 m and height of 1.8 m)
本文在实验中设计了一套基于视点分段式体像素和光学定向偏折膜的32英寸桌面式全视差光场显示系统。该系统可以为多位观看者提供水平观看视角为100°的全视差、高分辨率的3D场景,观看者沿不同方向移动观看到的全视差光场重构图像(以城市建筑为例)如
图 9. 观看者沿不同方向移动观看到的全视差光场重构图像。(a)观看者沿z轴垂直移动(距离为1 m);(b)观看者沿y轴前后移动(高度为1.8 m);(c)观看者沿x轴水平移动(距离为1 m,高度为1.8 m)
Fig. 9. Reconstructed full parallax light field images viewed by viewer from different directions. (a) Viewer moving vertically along z-axis (distance of 1 m); (b) viewer moving forward and back along y-axis (height of 1.8 m); (c) viewer moving horizontally along x-axis (distance of 1 m and height of 1.8 m)
当处于不同位置的多个观看者(由深度相机定位)同时处在本文提出的光场显示设备的观看范围内时,在每个人所处的观看区域内都能看到具有正确透视关系的全视差光场信息。如
图 10. 多位移动中的观看者同时观看的示意图
Fig. 10. Schematic of multiple moving viewers watching simultaneously
图 11. 多位移动中的观看者同时观看到的全视差光场重构图像。(a)左侧观看者沿z轴方向垂直移动(距离为1 m);(b)中间观看者沿x轴方向水平移动(距离为1 m,高度为1.8 m);(c)右侧观看者沿y轴方向前后移动(高度为1.8 m)
Fig. 11. Reconstructed full parallax light field images viewed by multiple moving viewers simultaneously. (a) Left viewer moving vertically along z-axis (distance of 1 m); (b) middle viewer moving horizontally along x-axis (distance of 1 m and height of 1.8 m); (c) right viewer moving forward and back along y-axis (height of 1.8 m)
4 结论
提出了一种基于视点分段式体像素的具有大观看视角的全视差桌面式光场显示系统。为了实现该系统,设计了基于定向准直背光和反贴的柱透镜阵列的光学结构以增大观看视角,并利用光学偏折膜和全息功能屏在桌面式显示设备的正面观看范围内实现了体像素重构。设计了一种可将不同视点的光场内容分区域显示的分段式体像素,计算了LCD上子像素与观看者看到的不同体像素之间的函数映射关系,实时修正了观看者改变位置后看到的三维场景,并对缺失的垂直视差进行了透视关系校正。实验结果表明,光学偏折膜能够很好地将体像素发出的光线偏折至桌面式显示设备的最佳观看区域内,编码后的视点分段式体像素在保证高分辨率三维场景的精确光场重构的同时,为多位观看者提供了流畅平滑的全视差3D图像。
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董昊翔, 于迅博, 金秋, 桑新柱, 王葵如, 颜玢玢, 高鑫, 邢树军, 温旭东, 徐斌. 一种基于视点分段式体像素的具有100°正面观看视角的桌面式光场显示系统[J]. 中国激光, 2022, 49(4): 0409001. Haoxiang Dong, Xunbo Yu, Qiu Jin, Xinzhu Sang, Kuiru Wang, BinBin Yan, Xin Gao, Shujun Xing, Xudong Wen, Bin Xu. Tabletop 3D Light-Field Display with 100° Frontal Viewing Angle Based on Views-Segmented Voxels[J]. Chinese Journal of Lasers, 2022, 49(4): 0409001.