Plot a filled black circle on a certain background in Matlab

Question

I have a 344*344 grey-level image with noise only. Now I want to plot a filled black circle in the middle of that gray image as background. How can I achieve that? And I hope I can also adjust the size (scale) of the filled black circle in the middle of the image. Thanks so much!!

Answer 1

Plotting Filled Circle On Greyscale Image

By calculating the distance from the midpoint of the image for each of the pixels a circle can be generated. By setting each of the pixels to an intensity of 0 that does not satisfy the radius threshold the impression of a circle can be achieved.

• Magnitude From Centre = (X - X_Midpoint)^2 + (Y - Y_Midpoint)^2

%Adjust for ellipse%
Scaling_Factor_1 = 3;
Scaling_Factor_2 = 1;


%Importing image%
Image = imread('Tiger_Drawing.jpg');
Image = rgb2gray(Image);

%The radius of the filled circle%
Radius = 100;

%Grabbing the dimensions of the image%
[Image_Height,Image_Width] = size(Image);

%Evaluating the midpoint of the image%
Image_Midpoint = [round(Image_Height/2), round(Image_Width/2)];

%Scanning through the pixels of the image%
for Row_Scanner = 1: +1: Image_Height
   for Column_Scanner = 1: +1: Image_Width 
      
      
      
%The pixel coordinate%
Pixel_Coordinate = [Row_Scanner Column_Scanner];
Magnitude_From_Centre = sqrt((abs(Pixel_Coordinate(1,1) - Image_Midpoint(1,1))^2)/Scaling_Factor_1 + abs(Pixel_Coordinate(1,2) - Image_Midpoint(1,2))^2/Scaling_Factor_2);



%If the magnitude from the centre is smaller than the set radius set
%intensity to 0%
if(Magnitude_From_Centre <= Radius) 
    Image(Row_Scanner,Column_Scanner) = 0; 
end 

   end 
end


imshow(Image);

Ran using MATLAB R2019b