EDGE-DETECTION IN PETROGRAPHIC IMAGES

The automatic detection of mineral grain boundaries in images obtained from a polarized-light microscope requires special techniques. Observations in both plane- and cross-polarized light may be necessary and the section must be rotated relative to the plane of polarization of the microscope to see all the grain boundaries. In computer-based microscopy this can be accomplished by the sequential accumulation of individual images captured from one microscope field of view with different polarizer orientations. For real-time implementation the sequential images are segmented individually by applying Canny's algorithm. A separable Gaussian mask is used for smoothing and a 3 x 3 convolution mask is used to generate 1-pixel-wide boundaries, which are located at the zero-crossing of the second-order derivative of the intensity gradient. The boundaries are extracted and accumulated in a composite image. The resulting composite image is a synoptic grain-boundary image of the rock.