Thread Subject: a problem about Canny edge

Dear all, I am new to matlab and when I use canny detector to detect the edge of the Image. I find a interesting thing that if I use canny detector for the whole image and get the boundary, at the same time I cut some part of the Image and use canny to detect the part of the Image， I find that it is the same when I use the canny for the whole image and for the part. Could anyone tell me why it happens?

"dai " <279711585@qq.com> wrote in message <h41ql5$5hp$1@fred.mathworks.com>...
> Dear all, I am new to matlab and when I use canny detector to detect the edge of the Image. I find a interesting thing that if I use canny detector for the whole image and get the boundary, at the same time I cut some part of the Image and use canny to detect the part of the Image， I find that it is the same when I use the canny for the whole image and for the part. Could anyone tell me why it happens?

Sorry to say I cannot make any sense of what you are asking - it may be me being a bit thick this afternoon. Perhaps you could have another attempt at explaining exactly what you are trying to do, then I might be able to help.

Regards

Dave Robinson

"Dave Robinson" <dave.robinson@somewhere.biz> wrote in message <h41sgm$f65$1@fred.mathworks.com>...
> "dai " <279711585@qq.com> wrote in message <h41ql5$5hp$1@fred.mathworks.com>...
> > Dear all, I am new to matlab and when I use canny detector to detect the edge of the Image. I find a interesting thing that if I use canny detector for the whole image and get the boundary, at the same time I cut some part of the Image and use canny to detect the part of the Image， I find that it is the same when I use the canny for the whole image and for the part. Could anyone tell me why it happens?
>
> Sorry to say I cannot make any sense of what you are asking - it may be me being a bit thick this afternoon. Perhaps you could have another attempt at explaining exactly what you are trying to do, then I might be able to help.
>
> Regards
>
> Dave Robinson

My question is that the Canny edge detecter effecting the whole Image and some part of the image is the same.
For example:
>>load I
>>BW_I = edge(I,'canny');
>>J = I(50:100,50:100);
>>BW_J = edge(J,'canny');
Now it is shown that BW_J = BW_I(50:100,50:100).
I do not know why.

On Jul 20, 10:23 pm, "dai " <279711...@qq.com> wrote:
> "Dave Robinson" <dave.robin...@somewhere.biz> wrote in message <h41sgm$f6...@fred.mathworks.com>...
> > "dai " <279711...@qq.com> wrote in message <h41ql5$5h...@fred.mathworks.com>...
> > > Dear all, I am new to matlab and when I use canny detector to detect the edge of the Image. I find a interesting thing that if I use canny detector for the whole image and get the boundary, at the same time I cut some part of the Image and use canny to detect the part of the Image， I find that it is the same when I use the canny for the whole image and for the part. Could anyone tell me why it happens?
>
> > Sorry to say I cannot make any sense of what you are asking - it may be me being a bit thick this afternoon. Perhaps you could have another attempt at explaining exactly what you are trying to do, then I might be able to help.
>
> > Regards
>
> > Dave Robinson
>
> My question is that the Canny edge detecter effecting the whole Image and some part of the image is the same.
> For example:>>load I
> >>BW_I = edge(I,'canny');
> >>J = I(50:100,50:100);
> >>BW_J = edge(J,'canny');
>
> Now it is shown that BW_J = BW_I(50:100,50:100).
> I do not know why.

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OK so if you Canny the whole image and crop a portion of it, it looks
the same as if you cropped first and then Canny'ed it. Seems normal.
Why does this seem unusual to you? Why are you expecting anything
different? The edges present in that portion of the image will appear
in the output regardless if you crop it before or after edge
detection.
ImageAnalyst