复杂表面的多投影系统的研究
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摘要
投影系统为人们提供一种更自然的显示过程,目前该技术已从单投影系统发展到多投影系统,从平面投影发展到复杂面投影。理论上,通过投影仪数量的增加,可以近乎无限的扩充投影面的尺寸和分辨率,从而来达到一种沉浸式的用户体验。在科学可视化,虚拟现实以及视感强化等诸多领略,多投影系统都有着广泛的应用。本文针对多投影系统在复杂表面进行投影的问题展开了研究。
就目前的多投影间光度校准技术而言,应用最广有两种:边缘融合和光度衰减矩阵(LAM)。对于边缘融合算法,无论投影面是固定或者不固定(如窗帘),其都可以很好的解决投影重叠区域无缝拼接。但是,由于其本身算法的局限性,它无法解决一些环境因素所造成的负面影响,如因投影仪之间亮度差别、或因屏幕不规则所造成的环境光折射。对于光度衰减矩阵,当投影面为固定的时候,它能很好地进行无缝拼接。但是对投影介质的固定度和几何校准的精确性都有很高的要求。所以,当遇到窗帘、幕布等具有轻微摆动的悬挂式投影面,或者因为几何校准算法所产生一些细微的误差时,该算法会产生“重叠区边缘光度不一致”的问题。简而言之,他们各自的优点却是对方的缺点。而这些缺点也不能靠简单地做两次不同校准而解决。
本文突破了传统投影仪应用对于投影表面的约束,改进了原有光度校准算法,能够解决LAM的缺点并且同时引进边缘融合的优点,使其能够投影在复杂的真实场景中,更接近增强现实的要求,具有积极的研究意义。
我们围绕复杂投影表面下多投影系统的光度校准问题,使其达到更好的视觉效果,主要做了两方面研究工作:
一方面,通过有限制地做局部光度衰减和加入区域羽化的方法,我们提出了具有融合效果的全局光度一致性矩阵(blending luminance uniformity matrix),简称BLUM。BLUM拥有边缘融合和LAM的各自优点,它成功解决了LAM的“重叠区边缘光度不一致”的问题,在复杂表面的多投影系统中达到全局光度一致性和融合边缘光度一致性效果的。同时通过融合技术消除了这些光度不一致。边缘光度不一致的主要原因是几何校准的误差,即便这些误差是合理的。而引起这些误差的是一些无可避免的环境因素,比如轻微摆动的悬挂式投影面。实验结果表明,所提的方法在重叠区域边缘的光度锐变可以通过BLUM很好地解决。
另一方面,LAM因其算法的局限性,为了确保全局光度一致,都是简单地采用最小流明值来计算各个像素点的衰减权值,使得光度校准后的显示亮度远远暗于原始图像。因此我们提出了一种寻找更合适的通用流明值算法,通过该值,我们投影效果的亮度要远远高于LAM的投影亮度,后者仅仅采用的是最小流明值。在我们的实验中,数据表明大约有占总量3%~7%的像素亮度值远远低于全部像素的中位值和平均值,而且这些像素几何全部分布在边缘,因此进行衰减权值计算时可以忽略它们,而对整体投影效果的影响几乎为零。这样,实验结果表明,所提的方法,通用流明值会有很大的提高,从而提高了整体亮度,光度校准的效果更好。
最后,本论文工作还在应用方面采用分布式的架构,满足了灵活性与扩展性的要求,同时兼顾设备成本、精度等因素,使得多投影系统实施更便捷、成本更低,更具普及性。
Recently, with the developing of reality augment technology, projecting system has been able to provide us with a natural process of display and its effect are become more and more fantastic. Today the projector-based display has evolved from single projectors to multi ones, and the display surface can be covered from a planar surface to a complicated one.Large-scale displays as multi-projecting systems are used in scientific visualization, virtual reality and other visually intensive applications. With the increasing of the number of the projector, we can get almost unlimited visual range, arbitrary display resolution, and immersed user experience. Hence, such techniques become a hot research topic in computer science worldwide at present.
The most popular methods of photometric calibration can be generalized into two types: edge blending and LAM (luminance attenuation maps). In this paper, we implement and compare the multi-projecting systems based both on blending and LAM. Both these two methods have their advantages and drawbacks. For the method of edge blending, it works fairly well whenever the display surface is fixed or unfixed, also it achieves a fantastic effect when the luminance parameters among the projectors are approximately same. However, it cannot resolve the negative effects caused by the luminance variance among the projectors. This limitation gives birth of the problem of“un-global luminance uniformity”. For the method of LAM, it can well solve the problem in blending. Nevertheless, the problem of“luminance inconsistency in the edge of overlapped region”occurs when the surface is unfixed, such as suspended-type projection screen, hung curtains and so on, which always swing slightly. In short, these two methods are mutual complementary.
One step forward, we have broken through the restriction of display surface in traditional projector application and provided a new method which combines the blending with luminance attenuation by slightly reducing the area of luminance attenuation in overlapped region, which efficiently solves the problem of“luminance inconsistency in the edge of overlapped region”in LAM and the problem of“un-global luminance uniformity”in blending. In addition, instead of finding the minimum common luminance value in the photometric calibration, we just find a more suitable common luminance value which can well solve the dim problem in camera-based photometric uniformity of prior approaches. Moreover, the process of photometric calibration on multi-projecting system will be no more complicated than before. The method is robust and accurate, and can be implemented with commercial off-the-shelf components.
This paper breaks through the restriction of display surface in traditional projector application and improve the algorithm of geometric registration and photometric calibration which ensure that our multi-projecting system can project seamless image on complicated surface. The distributed architecture has been involved to achieve flexibility and scalability. We also consider the device cost and precision. Moreover, he method is robust and accurate, and can be implemented with commercial off-the-shelf components.
就目前的多投影间光度校准技术而言,应用最广有两种:边缘融合和光度衰减矩阵(LAM)。对于边缘融合算法,无论投影面是固定或者不固定(如窗帘),其都可以很好的解决投影重叠区域无缝拼接。但是,由于其本身算法的局限性,它无法解决一些环境因素所造成的负面影响,如因投影仪之间亮度差别、或因屏幕不规则所造成的环境光折射。对于光度衰减矩阵,当投影面为固定的时候,它能很好地进行无缝拼接。但是对投影介质的固定度和几何校准的精确性都有很高的要求。所以,当遇到窗帘、幕布等具有轻微摆动的悬挂式投影面,或者因为几何校准算法所产生一些细微的误差时,该算法会产生“重叠区边缘光度不一致”的问题。简而言之,他们各自的优点却是对方的缺点。而这些缺点也不能靠简单地做两次不同校准而解决。
本文突破了传统投影仪应用对于投影表面的约束,改进了原有光度校准算法,能够解决LAM的缺点并且同时引进边缘融合的优点,使其能够投影在复杂的真实场景中,更接近增强现实的要求,具有积极的研究意义。
我们围绕复杂投影表面下多投影系统的光度校准问题,使其达到更好的视觉效果,主要做了两方面研究工作:
一方面,通过有限制地做局部光度衰减和加入区域羽化的方法,我们提出了具有融合效果的全局光度一致性矩阵(blending luminance uniformity matrix),简称BLUM。BLUM拥有边缘融合和LAM的各自优点,它成功解决了LAM的“重叠区边缘光度不一致”的问题,在复杂表面的多投影系统中达到全局光度一致性和融合边缘光度一致性效果的。同时通过融合技术消除了这些光度不一致。边缘光度不一致的主要原因是几何校准的误差,即便这些误差是合理的。而引起这些误差的是一些无可避免的环境因素,比如轻微摆动的悬挂式投影面。实验结果表明,所提的方法在重叠区域边缘的光度锐变可以通过BLUM很好地解决。
另一方面,LAM因其算法的局限性,为了确保全局光度一致,都是简单地采用最小流明值来计算各个像素点的衰减权值,使得光度校准后的显示亮度远远暗于原始图像。因此我们提出了一种寻找更合适的通用流明值算法,通过该值,我们投影效果的亮度要远远高于LAM的投影亮度,后者仅仅采用的是最小流明值。在我们的实验中,数据表明大约有占总量3%~7%的像素亮度值远远低于全部像素的中位值和平均值,而且这些像素几何全部分布在边缘,因此进行衰减权值计算时可以忽略它们,而对整体投影效果的影响几乎为零。这样,实验结果表明,所提的方法,通用流明值会有很大的提高,从而提高了整体亮度,光度校准的效果更好。
最后,本论文工作还在应用方面采用分布式的架构,满足了灵活性与扩展性的要求,同时兼顾设备成本、精度等因素,使得多投影系统实施更便捷、成本更低,更具普及性。
Recently, with the developing of reality augment technology, projecting system has been able to provide us with a natural process of display and its effect are become more and more fantastic. Today the projector-based display has evolved from single projectors to multi ones, and the display surface can be covered from a planar surface to a complicated one.Large-scale displays as multi-projecting systems are used in scientific visualization, virtual reality and other visually intensive applications. With the increasing of the number of the projector, we can get almost unlimited visual range, arbitrary display resolution, and immersed user experience. Hence, such techniques become a hot research topic in computer science worldwide at present.
The most popular methods of photometric calibration can be generalized into two types: edge blending and LAM (luminance attenuation maps). In this paper, we implement and compare the multi-projecting systems based both on blending and LAM. Both these two methods have their advantages and drawbacks. For the method of edge blending, it works fairly well whenever the display surface is fixed or unfixed, also it achieves a fantastic effect when the luminance parameters among the projectors are approximately same. However, it cannot resolve the negative effects caused by the luminance variance among the projectors. This limitation gives birth of the problem of“un-global luminance uniformity”. For the method of LAM, it can well solve the problem in blending. Nevertheless, the problem of“luminance inconsistency in the edge of overlapped region”occurs when the surface is unfixed, such as suspended-type projection screen, hung curtains and so on, which always swing slightly. In short, these two methods are mutual complementary.
One step forward, we have broken through the restriction of display surface in traditional projector application and provided a new method which combines the blending with luminance attenuation by slightly reducing the area of luminance attenuation in overlapped region, which efficiently solves the problem of“luminance inconsistency in the edge of overlapped region”in LAM and the problem of“un-global luminance uniformity”in blending. In addition, instead of finding the minimum common luminance value in the photometric calibration, we just find a more suitable common luminance value which can well solve the dim problem in camera-based photometric uniformity of prior approaches. Moreover, the process of photometric calibration on multi-projecting system will be no more complicated than before. The method is robust and accurate, and can be implemented with commercial off-the-shelf components.
This paper breaks through the restriction of display surface in traditional projector application and improve the algorithm of geometric registration and photometric calibration which ensure that our multi-projecting system can project seamless image on complicated surface. The distributed architecture has been involved to achieve flexibility and scalability. We also consider the device cost and precision. Moreover, he method is robust and accurate, and can be implemented with commercial off-the-shelf components.
引文
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[2] C. Cruz-Neira, D. Sandin, T. DeFanti. Surround-Screen Projection-Based Virtual Reality: The Design and Implementation of the CAVE. In Proceedings of SIGGRAPH 93, Computer Graphics Proceedings, Annual Conference Series. 1993, 135–142
[3] C. J. Chen and M. Johnson. Fundamentals of Scalable High Resolution Seamlessly Tiled Projection System. In Proceedings of SPIE Projection Displays VII, 4294:67–74, 2001.
[4] Oliver Bimber, Ramesh Raskar. Spatial Augmented Reality: Merging Real and Virtual Worlds. A.K.Peters, 2005
[5] Oliver Bimber, Ramesh Raskar, Masahiko Inami. Spatial Augmented Reality. Siggraph 2007 Course 17 Notes
[6] H. Chen, R. Sukthankar, and G. Wallace. Scalable Alignment of Large-Format Multi-Projector Displays Using Camera Homography Trees. In Proceeding of IEEE Visualization 2002, pages 339–346, 2002.
[7] Y. Chen, D. Clark, A. Finkelstein, T. Housel and K. Li. Automatic Alignment Of High-Resolution Multi-Projector Displays Using An Un-Calibrated Camera. In Proceeding of IEEE Visualization 2000, pages 125–130, 2000.
[8] D. Cotting, M. Naef, M. Gross and H. Fuchs. Embedding Imperceptible Patterns into Projected Images for Simultaneous Acquisition and Display. In Proceeding of International Symposium on Mixed and Augmented Reality (ISMAR 04), Nov 2-5, 2004.
[9] Ming Jin, Hui Zhang, Xubo Yang, Shuangjiu Xiao. A Real-Time ProCam System for Interaction with Chinese Ink-and-Wash Cartoons. IEEE CVPR Workshop on ProCams 2007
[10] D. Cotting, R. Ziegler, M. Gross and H. Fuchs. Adaptive Instant Displays: Continuously Calibrated Projections Using Per-Pixel Light Control. In Computer Graphics Forum Vol. 24, no. 3, pp. 705-714. 2005.
[11] K. Fujii, M, Grossberg and S. Nayar. A Projector-Camera System with Real-Time Photometric Adaptation for Dynamic Environments. IEEE Conference on Computer Vision and PatternRecognition (CVPR), Vol.1, pp.814-821, Jun, 2005
[12] G. Humphreys, M. Eldridge, G. Stoll, M. Everett and P. Hanrahan. WireGL: A Scalable Graphics System for Clusters. In Proceedings of SIGGRAPH 2001, August 2001.
[13] K. Li, H. Chen, Y. Chen, D.W. Clark, P. Cook, S. Damianakis, G. Essl, A. Finkelstein, T. Funkhouser, A. Klein, Z. Liu, E. Praun, R. Samanta, B. Shedd, J.P. Singh, G. Tzanetakis, and J. Zheng. Early Experiences and Challenges in Building and Using a Scalable Display Wall System. IEEE Computer Graphics and Applications, 20(4):671–680, 2000.
[14] K. Li and Y. Chen. Optical Blending for Multi-Projector Display Wall System. In Proceedings of the 12th Lasers and Electro-Optics Society 1999 Annual Meeting, 1999.
[15] A. Majumder. Properties of Color Variation across Multi-Projector Displays. In Proceedings of SID Eurodisplay, 2002.
[16] A. Majumder. A Practical Framework to Achieve Perceptually Seamless Multi-Projector Displays, PhD Thesis. Technical report, University of North Carolina at Chapel Hill, 2003.
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