draw circle on image

I want to draw a circle with centre at centre of given image. Image get displayed but circle is missing. Without using hold on/off, a separate figure window shows circle, but i want the circle on the image. Plz correct the following code.

imshow(PICpng);  
centx = x / 2;
centy = y / 2;
r = 10;
hold on;
theta = 0 : (2 * pi / 10000) : (2 * pi);
pline_x = r * cos(theta) + centx;
pline_y = r * sin(theta) + centy;
k = ishold;
plot(pline_x, pline_y, '*');
hold off;

[Jan]

I have formatted youz code. Please read the instructions and use the "{} Code" button, Thanks.

What are x and y? How large is your image?

[sangeeta]

Thankyou Jan for the correction.

x and y are length and width of image (500, 400) Image can be of varying sizes, the one I m using is of 5ookb.

[Spandan Tiwari]

FYI, there's a function in the Image Processing Toolbox called VISCIRCLES which can be used for drawing circles. The basic syntax takes the centers and radii of the circles and draws them.

[sangeeta]

ok, Thanks Spandan

[Image Analyst]

Spandan, does the help's "See also" connect to other toolboxes? If so, then the help for rectangle(), which people usually use to draw circles, should mention viscircle().

Answer 1

[Image Analyst]

It was working. Your x and y were probably messed up. Try this:

PICpng = imread('peppers.png');
[rows columns numberOfColorChannels] = size(PICpng)
x = columns/2
y = rows/2
imshow(PICpng);  
centx = x / 2;
centy = y / 2;
r = 60;
hold on;
theta = 0 : (2 * pi / 10000) : (2 * pi);
pline_x = r * cos(theta) + centx;
pline_y = r * sin(theta) + centy;
k = ishold;
plot(pline_x, pline_y, 'r-', 'LineWidth', 3);
hold off;

[sangeeta]

its still the same. Kindly find out the loophole.

[Jan]

Pleas explain, what is still the same. I do not get, what you are exactly asking for.

[Image Analyst]

It's DEFINITELY not the same. My code works. Your code doesn't. Did you copy and paste my code? Evidently not. So let's see your new code. Did you actually make any changes to it? Like specifying values for x and y?

[sangeeta]

Thanks alot. Its working. Removed comments, placed image in current directory, removed some other irrelevant code...

[Image Analyst]

So you did change/break my code. You mixed up x and y with rows and columns. x=columns, and y = rows. You have it the opposite way. Try

[y, x]=size(pngPIC);

[sangeeta]

Yes, I mixed up x and y. Thankyou.

[rubina naz]

thanks alot it is working...

[RAKESH KUCHANA]

Hello Image Analyst, I saw the code of drawing circle on image and it worked out. Can you explain how to identify whether the red circled area contains all white pixel area or not? Please provide code for it.

The sample images are provided for your reference.

[Walter Roberson]

https://www.mathworks.com/help/images/ref/images.roi.assistedfreehand.createmask.html

[Image Analyst]

Once you have a mask for the inside the red circle (call poly2mask() if you need to), you can do

pixelsInside = binaryImage(circleMask);
if all(pixelsInside)
    % All values in mask are true/white/1
else
    % At least one pixel is false/black/0.
end

Answer 2

[Muhammad Nauman Arshad]

ICpng = imread('peppers.png');
[rows columns numberOfColorChannels] = size(PICpng)
x = columns/2
y = rows/2
imshow(PICpng);  
centx = x / 2;
centy = y / 2;
r = 60;
hold on;
theta = 0 : (2 * pi / 10000) : (2 * pi);
pline_x = r * cos(theta) + centx;
pline_y = r * sin(theta) + centy;
k = ishold;
plot(pline_x, pline_y, 'r-', 'LineWidth', 3);
hold off;