Center for Perceptual Systems, University of Texas at Austin, Austin, TX 78712, USA.
Proc Natl Acad Sci U S A. 2011 Oct 4;108(40):16849-54. doi: 10.1073/pnas.1108491108. Epub 2011 Sep 19.
Defocus blur is nearly always present in natural images: Objects at only one distance can be perfectly focused. Images of objects at other distances are blurred by an amount depending on pupil diameter and lens properties. Despite the fact that defocus is of great behavioral, perceptual, and biological importance, it is unknown how biological systems estimate defocus. Given a set of natural scenes and the properties of the vision system, we show from first principles how to optimally estimate defocus at each location in any individual image. We show for the human visual system that high-precision, unbiased estimates are obtainable under natural viewing conditions for patches with detectable contrast. The high quality of the estimates is surprising given the heterogeneity of natural images. Additionally, we quantify the degree to which the sign ambiguity often attributed to defocus is resolved by monochromatic aberrations (other than defocus) and chromatic aberrations; chromatic aberrations fully resolve the sign ambiguity. Finally, we show that simple spatial and spatio-chromatic receptive fields extract the information optimally. The approach can be tailored to any environment-vision system pairing: natural or man-made, animal or machine. Thus, it provides a principled general framework for analyzing the psychophysics and neurophysiology of defocus estimation in species across the animal kingdom and for developing optimal image-based defocus and depth estimation algorithms for computational vision systems.
只有一个距离的物体可以被完美聚焦。而处于其他距离的物体的图像会因为瞳孔直径和镜头特性而产生一定程度的模糊。尽管离焦具有重要的行为、感知和生物学意义，但生物系统如何估计离焦仍然未知。在给定一组自然场景和视觉系统的特性的情况下，我们从第一性原理出发，展示了如何在任何单个图像中的每个位置上最优地估计离焦。我们证明了，在自然观察条件下，对于具有可检测对比度的斑块，人类视觉系统可以获得高精度、无偏的估计。考虑到自然图像的异质性，这些估计的高质量令人惊讶。此外，我们量化了单色像差（除离焦外）和色差可以在多大程度上解决离焦常被归因的符号模糊问题；色差可以完全解决符号模糊问题。最后，我们表明，简单的空间和空间色彩感受野可以最优地提取信息。该方法可以针对任何环境-视觉系统对进行定制：无论是自然的还是人为的，动物的还是机器的。因此，它为分析动物界中不同物种的离焦估计的心理物理学和神经生理学提供了一个有原则的通用框架，并为计算视觉系统中的基于图像的离焦和深度估计算法提供了最佳方法。
