Thread Subject: regionprops AND x-y axis problem

hi all, i use 'regionprops' to get the region properties of an image
as following,

> > a = imread('test.jpg');
> > b = im2bw( a) ;
> > st = regionprops( b, 'PixelIdxList', 'PixelList');

the st should include all regions where the values are 1 in binary
image b. the PixelList is like:

>> st(3).PixelList
ans =
    16 123
    16 124
    16 125
    17 121
    17 122
    17 123
    17 124
    17 126
...

However, when I want to check the Value of b(16,123), b( 16,
124) . . . ,
MATLAB returned 0 instead of 1? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?
How did it happened? when i swap the indexes by b(123,16), it
returned 1.

thus i was totally confused about the indexes returned by regionprops
function. How do the x and y axises arranged in the image?

Can anybody help?

On Tue, 09 Feb 2010 05:51:54 -0500, Parker <xenoszh@gmail.com> wrote:

> hi all, i use 'regionprops' to get the region properties of an image
> as following,
>
>> > a = imread('test.jpg');
>> > b = im2bw( a) ;
>> > st = regionprops( b, 'PixelIdxList', 'PixelList');
>
> the st should include all regions where the values are 1 in binary
> image b. the PixelList is like:
>
>>> st(3).PixelList
> ans =
> 16 123
> 16 124
> 16 125
> 17 121
> 17 122
> 17 123
> 17 124
> 17 126
> ...
>
> However, when I want to check the Value of b(16,123), b( 16,
> 124) . . . ,
> MATLAB returned 0 instead of 1? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?
> How did it happened? when i swap the indexes by b(123,16), it
> returned 1.
>
> thus i was totally confused about the indexes returned by regionprops
> function. How do the x and y axises arranged in the image?
>
> Can anybody help?

Have a look at the doc for PixelList:
'PixelList' — p-by-Q matrix specifying the locations of pixels in the
region. Each row of the matrix has the form [x y z ...] and specifies the
coordinates of one pixel in the region.

They are 'arranged' the same way, but you index into a matrix using
(row,col). Rows increase along the y direction, hence in Cartesian
coordinates, (row,col) converts to (y,x).

On 2月9日, 下午1时40分, "Ashish Uthama" <first.l...@mathworks.com> wrote:
> On Tue, 09 Feb 2010 05:51:54 -0500, Parker <xeno...@gmail.com> wrote:
> > hi all, i use 'regionprops' to get the region properties of an image
> > as following,
>
> >> >  a =  imread('test.jpg');
> >> >  b = im2bw( a) ;
> >> >  st = regionprops( b, 'PixelIdxList', 'PixelList');
>
> > the st should include all regions where the values are 1 in binary
> > image b. the PixelList is like:
>
> >>> st(3).PixelList
> > ans =
> >     16   123
> >     16   124
> >     16   125
> >     17   121
> >     17   122
> >     17   123
> >     17   124
> >     17   126
> > ...
>
> > However,  when I want to check the Value of b(16,123),  b( 16,
> > 124)  . . . ,
> > MATLAB returned 0 instead of 1? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?
> > How did it happened?  when i swap the indexes by b(123,16), it
> > returned 1.
>
> > thus i was totally confused about the indexes returned by regionprops
> > function. How do the x and y axises arranged in the image?
>
> > Can anybody help?
>
> Have a look at the doc for PixelList:
> 'PixelList' — p-by-Q matrix specifying the locations of pixels in the  
> region. Each row of the matrix has the form [x y z ...] and specifies the  
> coordinates of one pixel in the region.
>
> They are 'arranged' the same way, but you index into a matrix using  
> (row,col). Rows increase along the y direction, hence in Cartesian  
> coordinates, (row,col) converts to (y,x).

many thx. I got it.