How to get pixel value inside a circle

Hi.

I'm having problem of gaining a pixel values inside a circle.

I used drawcircle function to draw the circle.

Here's the image I want to show you.

As you see, there are three black cicles.

what i want to do is get a mean value inside the black circle.

Thanks.

Answer 1

[Matt J]

drawcircle() returns an object with a createMask method. Using the mask produced by createMask(), you can do,

mean(yourImage(mask))

[jaeyoung gwak]

Thank you so much.

I have a question for it though.

Image(mask) comes out like n x 1 uint8

But i don't know why it comes out in one row. If i use color image doesn't it should come out in three row?

This is mask row.

I think it's x,y coordinate. but when in Image(mask), this comes out.

I don't know how this values comes out.

I didn't use mean function but it comes out like this.

Thanks.

[Matt J]

But i don't know why it comes out in one row.

You mean one column, not one row. Indexing with a single logical index always produces a column vector as output.

If i use color image doesn't it should come out in three row?

If you want the mean of each color channel, you will have to do each channel separately:

R=Image(:,:,1); Rmean=mean(R(mask));
G=Image(:,:,2); Gmean=mean(G(mask));
B=Image(:,:,3); Bmean=mean(B(mask));

Or, you could process all channels at once by taking a slightly different approach,

RGBmean = sum(Image.*mask,[1,2])./nnz(mask)

[Image Analyst]

Or you could use the more convenient imsplit

[R, G, B] = imsplit(rgbImage);
Rmean=mean(R(mask));
Gmean=mean(G(mask));
Bmean=mean(B(mask));

[jaeyoung gwak]

Thanks!

My problem is solved

[Image Analyst]

You're welcome. By the way, did you see the full demo (below) I created for you?

Answer 2

[Image Analyst]

Try this. Adapt it to drawing multiple circles and showing them all should be no problem.

% Demo to show how drawcircle can be used to draw a circle on the image, and crop out that circular region to a new image and compute the mean RGB values and area.
clc;    % Clear the command window.
close all;  % Close all figures (except those of imtool.)
clear;  % Erase all existing variables. Or clearvars if you want.
workspace;  % Make sure the workspace panel is showing.
format long g;
format compact;
fontSize = 20;

%-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
% rgbImage = imread('peppers.png');
% rgbImage = imread('coloredchips.png');
rgbImage = imread('peacock.jpg');
hFig = figure;
imshow(rgbImage);
hFig.WindowState = 'maximized';

%-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
% Ask user to draw circle.
button = 'Redraw';
while contains(button, 'Redraw', 'IgnoreCase',true)
    uiwait(helpdlg('Draw a circle'));
    % User draws a circle.  It exits as soon as they lift the mouse.
    hCircle = drawcircle('Color', 'r')
    
    % Get the coordinates in the form [xLeft, yTop, width, height].
    xyCenter = hCircle.Center
    radius = hCircle.Radius
    % Get Mask image.
    mask = hCircle.createMask;
    % Delete the ROI object.
    delete(hCircle);
    % and replace it with a circle in the graphical overlay.
    hVC = viscircles(xyCenter, radius, 'Color', 'r', 'LineWidth', 2);
    % Ask user if the circle is acceptable.
    message = sprintf('Is this good?');
    button = questdlg(message, message, 'Accept', 'Redraw', 'Reject and Quit', 'Accept');
    if contains(button, 'Quit','IgnoreCase',true)
        delete(hVC); % Delete the circle from the overlay.
        roiPosition = [];
        break;
    elseif contains(button, 'Redraw','IgnoreCase',true)
        % OPTIONAL If you want to delete the prior one before drawing the next one.
        delete(hVC);
    elseif contains(button, 'Accept','IgnoreCase',true)
        break;
    end
end
% If you want to delete the circle from the overlay, do this:
delete(hVC); % Delete the circle from the overlay.

%-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
% OPTIONAL: Crop out the circle into a new image.
props = regionprops(mask, 'BoundingBox');
croppedImage = imcrop(rgbImage, props.BoundingBox);
% Display original image
subplot(2, 1, 1);
imshow(rgbImage);
axis('on', 'image')
title('Original Image', 'FontSize', fontSize)
% Display circle over original image.
hold on;
hVC = viscircles(xyCenter, radius, 'Color', 'r', 'LineWidth', 2);
% Display cropped image.
subplot(2, 1, 2);
imshow(croppedImage);
axis('on', 'image')
title('Cropped Image', 'FontSize', fontSize)
% Display circle over cropped image.
xyCenterCropped = [size(croppedImage, 2), size(croppedImage, 1)] / 2;
hold on;
hVC = viscircles(xyCenterCropped, radius, 'Color', 'r', 'LineWidth', 2);

%-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
% OPTIONAL: Get the area and the mean RGB values.
[R, G, B] = imsplit(rgbImage);
Rmean=mean(R(mask));
Gmean=mean(G(mask));
Bmean=mean(B(mask));
area = nnz(mask);
% Update title to show the area and RGB values.
caption = sprintf('Cropped Image.  Mean RGB values = [%.2f, %.2f, %.2f].  Area = %d pixels.',...
    Rmean, Gmean, Bmean, area)
title(caption, 'FontSize', fontSize);
% Close down figure.
% close(hFig);