Table 1.
Binarization methods implemented in currently available software for the processing of digital hemispherical photographs.
Figure 1.
Hemispherical photographs with high and low amounts of mixed pixels.
Histogram-exposed photographs of site V (A) with foliated vegetation and a low amount of mixed pixels and site VIII (B) with defoliated vegetation and a high amount of mixed pixels.
Figure 2.
(A) Pixel with a low gray value in a dark region of the sky; (B) blooming effect (overexposed vegetation pixel); (C) reflection at vegetation.
Figure 3.
The matrix is calculated for the = 24 reference pixels of each of
= 16 gray value-strata of a hemispherical photograph. Column and row sums show how many pixels were classified by a binarization algorithm (image classified data) and by an operator (reference data) into the categories vegetation (V) and sky (S). The four central cells show how many pixels were classified by an algorithm and the operator in agreement (
,
) and in disagreement (
,
). The image marginal proportion displays the overall fraction of pixels classified by the algorithm as vegetation (
) or sky (
) within the respective stratum. Adapted from Congalton and Green 2009.
Figure 4.
Accuracy assessment of seven binarization algorithms for hemispherical photographs
. Percentage correct and kappa values of binarizations obtained from different algorithms applied to hemispherical photographs. Photographs were taken at ten locations (indicated by latin numbers) and either histogram- or auto-exposed. Whiskers represent confidence intervals with a confidence level of 95%.
Figure 5.
Misestimation of gap fraction values by binarization algorithms.
For each algorithm, misestimation was quantified per photograph by dividing the estimated gap fraction of the algorithm () by the gap fraction estimated from manually classified reference pixels (
). Misestimations are displayed for histogram- and auto-exposed photographs. The horizontal line indicates the best possible classification.
Figure 6.
Overexposure of sky regions in hemispherical photographs.
Gray value histograms of the blue color plane of the photographs taken on site VI with auto-exposure (A) and histogram-exposure (B).