Question

I’m trying to draw a circle on an image in MATLAB with given X,Y coordinates and radius. Here’s the chuck of code of method that draws multiple circles for me –

function circle( Xs, Ys, Rs, LineWidth, LineColor)
    radius = Rs;
    centerX = Xs;
    centerY = Ys;

    for i=1:length(centerX)
        rectangle('Position',[centerX(i), centerY(i), radius(i), radius(i)],...
        'Curvature',[1,1],...
        'LineWidth',LineWidth,...
        'LineStyle','-',...
        'EdgeColor',LineColor);
    end
end

But whenever I see the circles in an image, I see that the circles are a little bit misplaced from the given coordinates (for example, they moved a little bit right/down). How do I fix this problem?

Answer 1

What you are drawing is actually a rectangle. But you have a curvature defined, which makes it look like a circle. The circle is then defined by a bounding box with the coordinates of the rectangle. The Position of the rectangle is the upper left corner (or in a regular plot the lower left corner) and what you called the radius is actually the width and height of that bounding box.

This is what I mean:

>> figure, imshow(I)
>> rectangle('Position',[100,100,120,120],'Curvature',[1,1])
>> rectangle('Position',[100,100,120,120],'Curvature',[0,0],'EdgeColor','r')
>> axis on

This code will produce a circle and a rectangle both in the same position defined by the same rectangle coordinates in the upper left corner. The red one is the bounding box I am speaking of.

Edit: If you don’t want to use the rectangle function you could maybe do the following:

>> figure,imshow(I)
>> hold on
>> plot(centerX+radius*sin(0:0.1:2*pi),centerY+radius*cos(0:0.1:2*pi))