As usual Henrik is far ahead.
Here is a first attempt to find a (simple) circular structure in an image.
First make the image. Define its limits and what else is needed
x1 = 3;
x2 = 9.8;
y1 = -.2;
y2 = 10;
rr = 1.35;
xc = 5;
yc = 4;
col = {.2, .7, .1};
Then show it
pl1 = Graphics[Disk[{xc, yc}, rr],
Background -> RGBColor[col],
PlotRange -> {{x1, x2}, {y1, y2}}]
Extract the informtion embedded in the image
dat = ImageData[pl1];
dim = Dimensions[dat]
Find the y-position of the centre of the disk, assuming the disk is black
d1 = Count[#, {0., 0., 0.}] & /@ dat
ymm = Flatten[Position[d1, Max[d1]]]
ym = Floor[Mean[ymm]]
yccalc = y2 + (ym - 1)/(dim[[1]] - 1) (y1 - y2)
and the same for x and the radius
posxblack = Flatten[Position[dat[[ym]], {0., 0., 0.}]]
dia1 = Last[posxblack] - First[posxblack]
xm = Floor[Mean[posxblack]]
xccalc = x1 + (xm - 1)/(dim[[2]] - 1) (x2 - x1)
radcalc = dia1 /dim[[2]] (x2 - x1)/2
Show what we have found in comparison to the original data
pl2 = Graphics[
{Blue, Disk[{xccalc, yccalc}, radcalc]},
PlotRange -> {{x1, x2}, {y1, y2}}]
Show[pl1, pl2]
This code should be checked with different settings for the parameters of the picture, which I haven't done yet.
And I am not sure how to extend this to find a Siemens star like that given in your very first pic above.
