Shadow-invariant classification for scenes illuminated by daylight.
Shadow-invariant classification for scenes illuminated by daylight.
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DOI：
10.1364/josaa.17.001952
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发表时间：
2000-11
期刊：
影响因子：
--
通讯作者：
John A. Marchant;C. Onyango
中科院分区：
文献类型：
--
作者：
John A. Marchant;C. Onyango
提出了一种基于物理的日光照明场景阴影补偿方法。如果日光由黑体定律的简化形式表示，并且相机滤镜具有无限窄的带宽，那么随着黑体温度的变化，红/蓝（rm）和绿色/蓝（gm）比率之间的关系是一个简单的幂律，其中指数与表面反射率无关。当CIE日光模型被用来代替黑体和有限带宽的相机被假定时，它示出的幂律仍然保持与指数略有变化。这意味着图像可以被转换成rm/gmA的图，然后被阈值化以产生与阴影无关的分类。指数A可以从CIE日光模型和相机滤光器特性预先计算。结果显示为四个室外图像，包含阳光和阴影部分与植被和背景。结果表明，对于给定的分量，无论其是否处于阴影中，变换图像中像素的灰度分布都是非常相似的。转换导致双峰直方图，从阈值可以很容易地选择，以提供良好的分类。
A physics-based method for shadow compensation in scenes illuminated by daylight is proposed. If the daylight is represented by a simplified form of the blackbody law and the camera filters are of infinitely narrow bandwidth, the relationship between red/blue (rm) and green/blue (gm) ratios as the blackbody's temperature changes is a simple power law where the exponent is independent of the surface reflectivity. When the CIE daylight model is used instead of the blackbody and finite bandwidths for the camera are assumed, it is shown that the power law still holds with a slight change to the exponent. This means that images can be transformed into a map of rm/gmA and then thresholded to yield a shadow-independent classification. Exponent A can be precalculated from the CIE daylight model and the camera filter characteristics. Results are shown for four outdoor images that contain sunny and shadowed parts with vegetation and background. It is shown that the gray-level distributions of the pixels in the transformed images are quite similar for a given component whether or not it is in shadow. The transformation leads to bimodal histograms from which thresholds can easily be selected to give good classifications.