Why illuminant direction is fundamental to texture analysis

The author shows that directed illumination used in the image acquisition process can act as a directional filter of three-dimensional texture. An image model of texture is presented which, given the illuminant vector, may be used to predict the directional characteristics of image texture. Simulations and the results of laboratory experiments are presented that confirm the predicted directional filtering effects. The image model is used to predict the output of a directional texture measure: Laws' L5E5 operator (K.I. Laws, 1980). Empirical results using four samples of isotropic texture confirm that the operator's output is significantly affected by changes in the angle of tilt of the illuminant. They also show that the model provides a good basis for predicting the behaviour of such operators. Finally the effect of changes in illuminant tilt on the distributions of the operator for two isotropic textures are presented. These results show that considerable misclassification would result in using an L5E5-based classifier if the illuminant tilt angle were changed between training and classification sessions.