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图像阴影长期以来对于计算机视觉算法具有破坏性作用。它们以地表特征的形式出现，而事实上它们是由光线和物体互动引起的。这可能对景物理解、目标分割、追踪和识别造成困难。由于这种图像分析当中不受欢迎的效果，过去几十年人们在消除阴影领域投入了大量精力，涵盖了很多具体运用，比如交通监控，人脸识别等等。尽管有如此广泛大量的研究，要完全自动化、实现通用的移除阴影系统仍然很困难，不同的阴影有不同的物理性质。

在外景中，有两种主要的光源：直射阳光，属于点光源；漫射天光，属于区域光源。当来自光源的光线被部分或者全部遮挡时，就会出现阴影。阴影也可以被分为两大类：自身投影和投射阴影。自身投影是物体没有被直射光线照明的部分；投射阴影是物体背部投射的暗区。投射阴影又可以进一步被划分为本影和半影。本影就是直射光线完全被封锁部分投射的阴影，半影就是直射光线只被部分遮挡的区域投射的阴影。

Shadows in images have long been disruptive to computer vision algorithms. They appear as surface features, when in fact they are caused by the interaction between light and objects. This may lead to problems in scene understanding, object segmentation, tracking and recognition. Because of the undesirable effects of shadows on image analysis, much attention was paid to the area of shadow removal over the past decades and covered many specific applications such as traffic surveillance, face recognition, image segmentation and so on. In spite of these extensive studies, however, it is difficult to develop a fully automatic, general-purpose shadow removal system in that shadows and shadings vary from scene to scene and different kinds of shadows may have different physical properties.

In outdoor scenes, there are mainly two light sources: direct sunlight, which can be regarded as a point light source; diffuse skylight, which can be regarded as an area light source. Shadows will occur when direct light from a light source is partially or totally occluded. Shadow can be divided into two types: self shadow and cast shadow [20]. The self shadow is the part of an object that is not illuminated by direct light; the cast shadow is the dark area projected by an object on the background. Cast shadow can be further divided into umbra and penumbra region. The umbra region is the part of a cast shadow where direct light is completely blocked; the penumbra region is the part of a cast shadow where direct light is only partially blocked. As shown in Fig. 1.1, the illumination on nonshadow region is daylight (direct sunlight and diffused skylight); that on penumbra is skylight and part of sunlight; that on umbra is only skylight. Since skylight is a component of daylight, pixel intensity in shadow must be lower than that in nonshadow background.

Fig 1 Shadow will occur when direct light is occluded

As detecting shadows from image sequences has made great progress, detecting them from a single image remains a difficult problem. In contrast to multi images, shadow detection in still image is more difficult due to less information available. In spite of different extensive studies, many proposed approaches are either designed for specific applications or need some assumptions about the environments. In addition, the majority of the proposed methods focus on detecting moving shadows in image sequences. Even if some methods can work on single stil本论文由英语论文网提供整理，提供论文代写，英语论文代写，代写论文，代写英语论文，代写留学生论文，代写英文论文，留学生论文代写相关核心关键词搜索。