How to recognize a marker in an image without machine learning?

Question

.NET

The task is to find the coordinates (in pixels) of the marker (black circle on a light background) located on the image.
I would not want to use machine learning methods and sculpt bicycles for such a small task. Interested in algorithms, or ready-made solutions for similar tasks applicable to Net.

Answer 1

Calculate for each pixel a function equal, for example, to
a) the distance to the nearest bright pixel
or
b) the weighted (with decreasing distance) sum of the illumination of the pixels in some neighborhood around it.
The pixel with the lowest value in the picture is most likely the center of the black circle.
If, apart from the circle marker, there are large dark areas (larger than the marker) - and the marker size is fixed, come up with a function that penalizes for light pixels inside a circle with a radius equal to the marker and for dark pixels outside this circle.
Basically, be aware/clarify your assumptions and just design a penalty function. If it turns out to be efficient, but slow, the second question will arise - its optimization (possibly with some loss in recognition quality).