Find pixel coordinates value of a centroid ?

[EDIT: Fri May 13 20:58:34 UTC 2011 - Reformat - MKF]

I want to find the x n y coordinates and pixel value of the centroid. Here is the code I have:

I = imread('mobil.png');
%I2 = imtophat(I,strel('disk',15));
level = graythresh(I);
bw = im2bw(I,level);
I2 = 1 - bw;
I2 = bwareaopen(I2, 200);
I3 = imfill(I2, 'holes');
s  = regionprops(I3, 'centroid');
centroids = cat(1, s.Centroid);
imshow(I3)
hold on
plot(centroids(:,1), centroids(:,2), 'b*')
hold off
impixelinfo

The car is :

Image

please clarify your question.

Not sure how I messed this question (a year and a half ago), but I have a tutorial in my File Exchange where I find centroids: BlobsDemo

Answer 1

centroids = round(centroids);
cent_vals = Inotbw(sub2ind(size(Inotbw),centroids(:,2),centroids(:,1)))

In this context I think Sean means

cent_vals = I3(sub2ind(size(I3),centroids(:,2),centroids(:,2)))

I was assuming I3 was a label||binary image. Thus he'd have to use the original non-modified image to get the pixel value. It's hard to tell though...

Good point. Though if he wants the RGB values then the mechanism would need to be modified some in order to get all three planes.

i have try the code
but i still don't know how to use it..

i don't get any pixel value ?

Yes, something like this.

idxr = sub2ind(size(I3),centroids(:,2),centroids(:,1));
pixPerPlane = size(I3,1)*size(I3,2);
idxg = idxr+pixPerPlane;
idxb = idxg+pixPerPlane;
rgbCents = reshape(I3([idxr;idxg;idxb]),[],3);

What doesn't work for you Danny? Does it error out? Does it return incorrect numbers? Is there an m-lint error because of a parenthesis?

not return incorrect numbers..
but no numbers are shown

i'm using 2 version of matlab
to crosscheck the codes
but still no numbers are shown

Edit your question to contain EVERYTHING in your script. We can't diagnose it without seeing it...

146
181
96
170
162
144

the value above are the number that shown
but why the pixel value didn't match ?
here is the pixel value that i check with impixelinfo
(135,85)
(133,311)
(341,214)
(341,298)
(593,68)
(528,289)

Those look like pixel coordinates, not pixel values. What value is shown after the ")" when you position to those positions ?

i have revise the question
i mean the pixel coordinates
sorry for the missunderstanding..

Then that should just be

round(centroids)

to within the nearest pixel.

thx for the help

Answer 2

Is there a reason you are using the Pixel List instead of doing as Sean suggested to you before:

round the coordinates returned from regionprops and use those as indices to get the centroid.

because i can't get the coordinates ..
so i think i can get the coordinates use the pixelinfo

Answer 3

It works perfectly for me; I only had to modify the extraction to I instead of I3, as Walter and I had hypothesized about.

I = imread('ans513.png'); %your image
level = graythresh(I);
bw = im2bw(I,level);
I2 = 1 - bw;
I2 = bwareaopen(I2, 200);
I3 = imfill(I2, 'holes');
s  = regionprops(I3, 'centroid');
   centroids = cat(1, s.Centroid);
   imshow(I3)
   hold on
   plot(centroids(:,1), centroids(:,2), 'b*')
   hold off
centroids = round(centroids);
idxr = sub2ind(size(I3),centroids(:,2),centroids(:,1));
pixPerPlane = size(I3,1)*size(I3,2);
idxg = idxr+pixPerPlane;
idxb = idxg+pixPerPlane;
rgbCents = reshape(I([idxr;idxg;idxb]),[],3);

%{
rgbCents =
147  146  144
145  145  143
 66   62   63
 11   79  102  %%%This is the value of the pixel at the center of your car
 86   77   72
 83   70   64
%}

sean
11 79 102
is that come from the original image ?

if i point my mouse to the centroid of my car ( after segmentation )
in impixelinfo show (344,214) 1
which the 1 is represent the pixel
and 344,214 represent X and Y
can i get those x n y value

well it's a BINARY IMAGE!!!!!!!! That means only 1s and 0s and since your car is white it's a 1.

11 79 102 are the rgb intensity components of the original image.

imtool(I)
%hold your mouse near the centroid...

%or
imshow(I)
hold on
plot(centroids(:,1), centroids(:,2), 'b*')
hold off

Have a great weekend!
(hopefully away from the computer screen)

i'm sorry for the wrong question
what i search is the
pixel coordinates from my centroid

sorry for the wrong question

Danny, what is

round(centroids)

showing for you?