We describe an automated computer algorithm for the classification of coral reef benthic organisms and substrates sampled using a typical photographic quadrat survey. The technique compares subsections of a quadrat sample image (blocks) to a library of identified species blocks and computes a distance or probability of identification in a multidimensional hypervolume of discrimination metrics. The discrimination metrics include texture (calculated from a radial sampling of a two-dimensional discrete cosine transform) and three channels of a normalized color space. A standard multivariate classification technique based on the Mahalanobis distance was unsuccessful in discriminating substrata because of the large morphological variation inherent in reef organisms. An alternative classification scheme based on an exhaustive search through an organism reference library yielded classification maps comparable to those obtained by manual analysis. (Marcos et al. 2005), in addition to color, have been used to identify corals, but their performance has been limited to discriminating general benthic groups (i.e., "algae," "dead coral," "branching corals," "live coral"). Johnson-Roberson et al. (2007) used both visual and acoustic data to classify reef images collected from an autonomous underwater vehicle. Gabor wavelet transformations were used as a texture descriptor and a SVM for semiautomatic classification, but limitations in training data sets for the SVM classification restricted the discrimination to four general coral classes.The partial success of the previous research is due in part to the extremely difficult imaging environment of the coral reef, which has large variations in overall illumination, an oftenvarying level of light absorption between clear and turbid water, and a complicated three-dimensional topography. The species to be identified have complex, plastic morphologies and exhibit subtly varying pigmentation that can be common across many different species groups.In this study, we explore the use of normalized color space and discrete cosine transforms as a textural descriptor in a statistical distance-based classification scheme to identify reef benthos from images collected during a photo-quadrat sampling survey typical of ecological studies. By combining a standardized method using a camera framer and high-resolution images with the automated image processing described here, it is much easier to analyze the large number of image frames required to adequately sample the distribution of reef organisms on a large scale.
ProcedureImage acquisition-All images were collected in Bonaire, Netherlands Antilles (12°12'25"N, 68°18'25"W) in January 2008, as part of the NOAA Ocean Explorations program, which included a photographic transect survey of the islands' reefs. Detailed descriptions of the leeward reefs of Bonaire can be found in Scatterday (1974) and the works of Kobluk andLysenko (1984) andvan Duyl (1985). To maximize the initial image quality, which would aid later manual or automatic proc...