QGIS: Create point layer (centroids) with radius attribute from circle polygon layer

Question

I have polygon layer which consists of many circles with different radius (see below) and I want to convert it to centroids and write new attribute into attribute table of centroids with value of radius.

EDIT: I would rather prefer not using calculation from circle area as I would like to extend the model later to use it with irregular polygons. So I am more looking for some kind of function which could work also for other geometries (not only for circles).

So far I've tried v.distance and other build-in functions I've also tried Field Calculator (but it obviously doesn't work for two separated layers).

Answer 1

To calculate the distance from a centroid of any kind of (regular or irregular) polygon to the closest point on the outer boundaray of that polygon (this is what you want to get, right?), you can use this expression on the polygon layer:

length (
    make_line (
        centroid ($geometry), 
        closest_point (
            boundary ($geometry), 
            centroid ($geometry)
        )
    )
)

Screenshot: here, I used the expression from above as a label (and added a round() function to get rid of many decimals). The red line, created dynamically with geometry generator with an expression based on the one above, shows the line from the centroid to the closest point on the outer boundary of the polygon:

Edit: By the way, to have the answer to the question from the comments here: the following expression gives the length of the longest line inside the polygon, starting from the centroid:

make_line (
    centroid ($geometry),
    closest_point ( 
        boundary (
            minimal_circle( $geometry)
        ),
        $geometry
    )
)

See screenshot - the red line is created dynamically with this expression (minus the length part):