exchange circle pixels in an image in matlab - arrays

Question

All I want is to add a circle into an image of a radius and centre of my choice, however I DO NOT WANT to use plot COMMAND, I want a new output array which has my chosen image and circle on it.
here is my function
function [ out_image ] = draw_circle( in_image,x0,y0,r,value)
where in_image is the input image
x0 y0 he centre of circle and r the radius
value the intensity of image
this is my attempt which does not work
ang=0:pi/100:2*pi
x=r*cos(ang)+x0;
y=r*sin(ang)+y0;
cir=[x,y];
[j,k]=size(cir);
for cir=1:j
for cir=1:k
if cir(i,j)==0
out_image(i,j)=in_image(i,j);
end
end
end
imshow(out_image)
end

Answer 1

I'm pretty sure there are better ways, but following your approach, you might use the following code:
out_image = in_image;
cir = round([r*cos(ang)+x0;r*sin(ang)+y0]);
for i = 1:size(cir,2)
out_image(cir(1,i),cir(2,i),:) = 255;
end
Your for loops did not work the way you wrote them. Also, you should use round to get indices, and not floating point values. However, the circle is quite small (only 1 pixel), which might not be enough for large pictures. See example below. Also, you might need a finer discritization for large pictures, i.e. a larger number of elements in ang.
In case you want a larger linewidth, you can use
lw = 3; % Enter linewidth here
for i = 1:size(cir,2)
out_image(cir(1,i)+(-lw:lw),cir(2,i)+(-lw:lw),:) = 255;
end
EDIT: In case you don't see it. The circle is close to the top left corner.
The complete code for the function is:
function [ out_image ] = draw_circle( in_image,x0,y0,r,value,lw)
out_image = in_image;
ang = 0:pi/100:2*pi;
cir = round([r*cos(ang)+x0;r*sin(ang)+y0]);
for i = 1:size(cir,2)
out_image(cir(1,i)+(-lw:lw),cir(2,i)+(-lw:lw),:) = value;
end
imshow(out_image)
You can call it from command line with
draw_circle(in_image,100,200,50,0,3);
Note, I took the unused value as grayscale value.