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Classification of coral reef images from underwater video using neural networks

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Abstract

We use a feedforward backpropagation neural network to classify close-up images of coral reef components into three benthic categories: living coral, dead coral and sand. We have achieved a success rate of 86.5% (false positive = 6.7%) for test images that were not in the training set which is high considering that corals occur in an immense variety of appearance. Color and texture features derived from video stills of coral reef transects from the Great Barrier Reef were used as inputs to the network. We also developed a rule-based decision tree classifier according to how marine scientists classify corals from texture and color, and obtained a lower recognition rate of 79.7% for the same set of images.

©2005 Optical Society of America

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Figures (5)

Fig. 1.
Fig. 1. The LBP operator. A 3×3 pixel neighborhood (a) is binarized into (b) by thresholding neighboring pixels with the center pixel. Weights in powers of two as in (c) is multiplied to (b) to obtain (d). The LBP value is computed as the sum of neighboring pixel values in (d), e.g. 1+8+32+128 = 169. This value is assigned to the center pixel.
Fig. 2.
Fig. 2. Rotation invariant pixel neighborhood for LBP8riu2 in (a). Points g6, g4, g8 and g2 are obtained by bilinear interpolation. Binning is achieved through nine uniform patterns in (b) considered for the LBP8riu2 operator (black = 0, white = 1).
Fig. 3.
Fig. 3. Flowchart of two-step classification scheme for the coral reef images.
Fig. 4.
Fig. 4. (a) Image of a reef area from video. Shown are: L live coral, D dead coral and S sand/rock. (b) Examples of reef images with different textures. First row, images of regular texture; second row of irregular texture and third row of smooth texture. Images are of different sizes and cropped from image frames of a reef video.
Fig. 5.
Fig. 5. Confusion matrices of classification performance for the NN and two-step classifier. Each block contains the number of samples classified and beside it is its percentage over the total number of samples in the actual class. The diagonals shown as shaded blocks, indicates successful per class recognition rates. A perfect classifier is perceived as having 100% recognition rate in the diagonal elements. Columns in off-diagonal elements indicate misclassification (false-positive) rates. (a) and (b) reveal results for the NN classifier while (c) presents results for the two-step classifier.
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