An RGB channel operation for removal of the difference of atmospheric scattering and its application on total sky cloud detection
2016
The inhomogeneous sky background presents a great
challenge for accurate cloud recognition from the total-sky images. A
channel operation was introduced in this study to produce a new composite
channel in which the difference of atmospheric scattering has been removed
and a homogeneous sky background can be obtained. Following this, a new
cloud detection algorithm was proposed that combined the merits of the
differencing and threshold methods, named differencing and threshold
combination algorithm (DTCA). Firstly, the channel operation was applied
to transform 3-D RGB image to the new channel, then the circumsolar
saturated pixels and its circularity were used to judge whether the sun is
visible or not in the image. When the sun is obscured, a single threshold
can be used to identify cloud pixels. If the sun is visible in the image,
the true clear-sky background differencing algorithm is adopted to detect
clouds. The qualitative assessment for eight different total-sky images
shows the DTCA algorithm obtained satisfactory cloud identification
effectiveness for thin clouds and in the circumsolar and near-horizon
regions. Quantitative evaluation also shows that the DTCA algorithm achieved the
highest cloud recognition precision for five different types of clouds and
performed well under both visible sun and blocked sun conditions.
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