When Point To Point Is Not Enough

As all microscopists are taught, resolution can be measured by knowing the closest distance between two points that can be discriminated in the image. There are some occasions when we want to measure something in a digital image, and the nominal resolution isn't sufficient for the task. One of these, extracting a contour from the image, was summarized in a previous research report. The principles would be valid for any filled area, such as a particle or structure from an X-ray dot map. The ground rule was set that the outermost pixel would be selected at every point on the contour. It was clear from the geometry shown in Fig. 1 that only values of 0°, 45°, 90°, 135° and 180° could be measured. Because of the stair step defect, tracing the sequence of pixel locations only gave offsets to 8 pixels, one directly below, two on the corners below, three pixels in the same location above, and one pixel on either side of the subject pixel. Therefore it was impossible to measure the true curvature of the contour.