How can I measure an angle between center and pixel?

Hi, im doing a little detection project but Ive stumbled upon some issues I cant quite figure out.

My program detects wanted circle and marks the center but 1. the Y-axis values increase from top to bottom and 2. I tried measuring an angle between the X-axis and the found center using 'atan2d' but I can't make it work like I need to.

In the image I drew in blue lines how the angle is detected right now and in red and green how I need it to be detected from the center point of the screen.

I'd be thankfull for any advice on the matter.

Answer 1

[Image Analyst]

Find the centroid of the blob and the image, get delta y and delta x, and use arctand(). Something like

mask = bwareafilt(mask, 1); % Take largest blob only.
[rows, columns] = size(mask);
xci = columns/2; % Image centroid.
yci = rows/2;
props = regionprops(mask, 'Centroid');
xbi = props.Centroid(1); % Blob centroid.
ybi = props.Centroid(2);
angle = atan2d(ybi-yci, xbi-xci)

Adapt as needed.