how do i select only a certain area of ​​an image and then use it to get the mean pixel value?

How can i select only the central red area of ​​the star and then get the mean pixel value of the red channel?

Answer 1

[KALYAN ACHARJYA]

This is all about how you can correctly segregate ROI, what the red fields refer to, as there is no clear boundary in the area of interest items, the easiest way-

red_channel=rgbImage(:,:,1);
bwimage=imbinarize(rgbImage(:,:,1),0.3);
% Get the largest blob only
bwimage=bwareafilt(bwimage,1);
% Get data for mean
sum_pix=sum(bwimage(:));
mask=uint8(cat(3,bwimage,bwimage,bwimage)); % Just for display purpose
% Mask operation on Red Channel
red_seg=red_channel(bwimage);
fprintf('The Mean is:  %f',sum(red_seg)/sum_pix);
imshow([rgbImage,mask.*rgbImage;])

[Giuseppe Ronci]

I'm new on matlab so sorry for my stupid question.

1)Where do I see in your code how you have delimited the ROI to be taken? For example, if I want to take an ROI in another position what can I do?

2)I don't see if there is a command that gives me the numerical result of the mean pixel value of the red channel

3)In your code where do I tell him which image to take to do the analysis? As I will have to do the same procedure for various images

4)I tried to run your code but it gave me as error "Unrecognized function or variable 'rgbImage' "

Thanks so much for your help and patience!

[KALYAN ACHARJYA]

At the initial section of the code, you need to load d image from current working directory-

rgbImage=imread('Image_file_name');

Basic details

https://in.mathworks.com/help/images/image-import-and-export.html

[Giuseppe Ronci]

Perfect now I was able to try the code with other images as well.

I wanted to ask you one last thing, how can I reduce the surface area to be taken as an ROI? on which command should I act and what should I change?

[KALYAN ACHARJYA]

bwimage=imbinarize(rgbImage(:,:,1),0.3);
%...................................^

Change this binarization threshold value (within 0 and 1) & see the result. You can also see other image segmentation approaches.

[Image Analyst]

@Giuseppe Ronci, my Image Segmentation Tutorial in my File Exchange might help you:

Image Analyst's File Exchange

[Giuseppe Ronci]

Thanks @Image Analyst!

@KALYAN ACHARJYA i have another question. Could you tell me how to change your code to give me also the standard deviation of the mean pixel value of the red channel?

if i tried to enter this command could it be ok? the command is: std (red_seg)

[Giuseppe Ronci]

@Image Analyst Can you help me? I tried to insert in various ways the command to do the standard deviation to add to the mean pixel value so as to have the error beyond the average but the code gives me an error. Do you know how I can change the code?