Method for identification of candidate points as possible characteristic points of a calibration pattern within an image of the calibration pattern

A method for identification of candidate points as possible characteristic points of a calibration pattern within an image of the calibration pattern, the method comprising the steps of:

• determining spots within a filtered image derived from the image of the calibration pattern, with a spot being defined as a coherent set of pixels of the filtered image having pixel values exceeding a threshold,
• for each determined spot calculating a central point of the determined spot, and
• identifying as candidate points all calculated central points,
• wherein the step of determining the spots comprises scanning the filtered image one pixel of the filtered image after an-other,
• wherein, if a scanned pixel has a pixel value exceeding the thresh-old, the scanning is paused and a spot contour tracing is executed starting with the last scanned pixel as current pixel and the second last pixel as previous pixel,
• the spot contour tracing comprising as a tracing step the steps of
• logging the current pixel as a contour pixel,
• for neighboring pixels adjacent to the current pixel, evaluating the condition, whether both the neighboring pixel has a pixel value exceeding the threshold and another neighboring pixel, which immediately precedes this neighboring pixel with respect to a sense of rotation around the current pixel, has a pixel value not exceeding the threshold,
• selecting from all neighboring pixels, for which the condition is fulfilled, the neighboring pixel farthest from the previous pixel with respect to said sense of rotation, and
• defining the current pixel as previous pixel and defining the selected neighboring pixel as current pixel;
• the spot contour tracing further comprising the steps of
• repeating the tracing step until the current pixel is again equal to the last scanned pixel,
• determining the spot as the set of pixels comprising the logged contour pixels and the pixels surrounded by the logged contour pixels.