background in hue-saturation color space (Figure 4-7). Gray level histograms for the

three classes in hue-luminance and red-green color plane are shown in Figure 4-8 and

4-9. Although it seemed there might be a distinction between the fruits and background in

red-green color plane, there were numerous false detections when different thresholds

were tested on the validation images. The main reason for so many false detections was

because of high contrast and brightness level in the image, which tended to make leaves

and background obj ects white in color and were classified as citrus class.

 The threshold in hue-saturation color plane was carefully chosen in a conservative

approach after many trials have been conducted over the calibration images. The

luminance component was added to the threshold to make it less dependent on the

brightness level of the image during binarization.

 The pixel distribution for various classes in the calibration images is shown in

Table 4-2. Although only 58% of citrus class was captured inside the threshold, the

binarization scheme was found to work very well with the validation images. The main

reason behind this threshold was that the threshold contained 0% of background and

0.03% of leaves. Since maj ority portion of an image consisted of leaves and background,

this binarization scheme performed well and there were some underestimation but very

small number of overestimation due to the conservative binarization scheme.

Table 4-2. Pixel distribution for citrus, leaf and background classes for 25 images in HSI
 color plane.
Pixel Category Citrus class Leaf class Background class
 Number Percentage Number Percentage Number Percentage
 of pixels of pixels of pixels
Inside threshold 15875 58.1% 68 0.03% 0 0%
Outside 11438 41.9% 23347 99.7% 8165 100%
threshold