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From: Nathan on 23 Mar 2010 20:41
On Mar 23, 5:37 pm, "James " <cche5...(a)uni.sydney.edu.au> wrote:
> ImageAnalyst <imageanal...(a)mailinator.com> wrote in message <afa2d5e2-7a85-4543-9734-f15acf97e...(a)j21g2000yqh.googlegroups.com>...
> > Why don't you just display the average of the two images?
>
> Sorry, i am kind of new to matlab so i don't really know the best way to overlay two images. What does taking the average of the two images do?

Why would ImageAnalyst suggest displaying the average of the two
images if it did not do what you were asking?...

The average of two images is essentially the two images overlaid
equally. (One image is not shown more than another image)
This is true (only?) if the two images have the same dimensions.

The average of two images can be found as such (assuming they have the
same dimensions)

avgimg = (img1 + img2)./2

I hope that helps

-Nathan
From: James on 23 Mar 2010 21:01
Nathan <ngreco32(a)gmail.com> wrote in message <90166712-6e95-4290-aa00-c8e23a74a917(a)g28g2000prb.googlegroups.com>...
> On Mar 23, 5:37 pm, "James " <cche5...(a)uni.sydney.edu.au> wrote:
> > ImageAnalyst <imageanal...(a)mailinator.com> wrote in message <afa2d5e2-7a85-4543-9734-f15acf97e...(a)j21g2000yqh.googlegroups.com>...
> > > Why don't you just display the average of the two images?
> >
> > Sorry, i am kind of new to matlab so i don't really know the best way to overlay two images. What does taking the average of the two images do?
>
> Why would ImageAnalyst suggest displaying the average of the two
> images if it did not do what you were asking?...
>
> The average of two images is essentially the two images overlaid
> equally. (One image is not shown more than another image)
> This is true (only?) if the two images have the same dimensions.
>
> The average of two images can be found as such (assuming they have the
> same dimensions)
>
> avgimg = (img1 + img2)./2
>
> I hope that helps
>
> -Nathan

Thank you very much, Nathan. But that's probably not what i am looking for.
The reason i wanted to overlay two images is because i want see if there is any difference between the two images, and if there is, i want to pinpoint exactly where it occurs and how much difference there is between the two images.
From: Nathan on 23 Mar 2010 21:05
On Mar 23, 6:01 pm, "James " <cche5...(a)uni.sydney.edu.au> wrote:
> Nathan <ngrec...(a)gmail.com> wrote in message <90166712-6e95-4290-aa00-c8e23a74a...(a)g28g2000prb.googlegroups.com>...
> > On Mar 23, 5:37 pm, "James " <cche5...(a)uni.sydney.edu.au> wrote:
> > > ImageAnalyst <imageanal...(a)mailinator.com> wrote in message <afa2d5e2-7a85-4543-9734-f15acf97e...(a)j21g2000yqh.googlegroups.com>...
> > > > Why don't you just display the average of the two images?
>
> > > Sorry, i am kind of new to matlab so i don't really know the best way to overlay two images. What does taking the average of the two images do?
>
> > Why would ImageAnalyst suggest displaying the average of the two
> > images if it did not do what you were asking?...
>
> > The average of two images is essentially the two images overlaid
> > equally. (One image is not shown more than another image)
> > This is true (only?) if the two images have the same dimensions.
>
> > The average of two images can be found as such (assuming they have the
> > same dimensions)
>
> > avgimg = (img1 + img2)./2
>
> > I hope that helps
>
> > -Nathan
>
> Thank you very much, Nathan. But that's probably not what i am looking for.
> The reason i wanted to overlay two images is because i want see if there is any difference between the two images, and if there is, i want to pinpoint exactly where it occurs and how much difference there is between the two images.  

I cannot see your images from this work computer, so I'll just ask
about them.

Are they pretty similar? What is the comparison between your two
images?
If they are almost exactly the same and all you want to do is find the
differences you can subtract the images from one another and view the
resulting image (it should give you a better idea of where the images
differ).

ImageAnalyst would probably know a more proper method for this,
however.

Good luck.

-Nathan
From: James on 23 Mar 2010 21:38
Nathan <ngreco32(a)gmail.com> wrote in message <58f6a602-b545-40f4-9fc3-a971c91c628c(a)y11g2000prf.googlegroups.com>...
> On Mar 23, 6:01 pm, "James " <cche5...(a)uni.sydney.edu.au> wrote:
> > Nathan <ngrec...(a)gmail.com> wrote in message <90166712-6e95-4290-aa00-c8e23a74a...(a)g28g2000prb.googlegroups.com>...
> > > On Mar 23, 5:37 pm, "James " <cche5...(a)uni.sydney.edu.au> wrote:
> > > > ImageAnalyst <imageanal...(a)mailinator.com> wrote in message <afa2d5e2-7a85-4543-9734-f15acf97e...(a)j21g2000yqh.googlegroups.com>...
> > > > > Why don't you just display the average of the two images?
> >
> > > > Sorry, i am kind of new to matlab so i don't really know the best way to overlay two images. What does taking the average of the two images do?
> >
> > > Why would ImageAnalyst suggest displaying the average of the two
> > > images if it did not do what you were asking?...
> >
> > > The average of two images is essentially the two images overlaid
> > > equally. (One image is not shown more than another image)
> > > This is true (only?) if the two images have the same dimensions.
> >
> > > The average of two images can be found as such (assuming they have the
> > > same dimensions)
> >
> > > avgimg = (img1 + img2)./2
> >
> > > I hope that helps
> >
> > > -Nathan
> >
> > Thank you very much, Nathan. But that's probably not what i am looking for.
> > The reason i wanted to overlay two images is because i want see if there is any difference between the two images, and if there is, i want to pinpoint exactly where it occurs and how much difference there is between the two images.  
>
> I cannot see your images from this work computer, so I'll just ask
> about them.
>
> Are they pretty similar? What is the comparison between your two
> images?
> If they are almost exactly the same and all you want to do is find the
> differences you can subtract the images from one another and view the
> resulting image (it should give you a better idea of where the images
> differ).
>
> ImageAnalyst would probably know a more proper method for this,
> however.
>
> Good luck.
>
> -Nathan

http://i42.tinypic.com/k9f6nr.jpg
http://i42.tinypic.com/kbowg6.jpg
These are CT images of the pelvis taken at different times. It is quite obvious there is a slight difference between them. It would be ideal to superimpose one image on top of the other such that i can easily see how much the pelvis has shrunk/stretched.
From: ImageAnalyst on 23 Mar 2010 22:21
You can average them if you want to see an average or blended
composite of both images together at the same time. You'll see double
images where they don't overlap well, and it will look like either
image where they're both the same. Or like Nathan said, you could
subtract them which will show up differences as extreme positive or
negative differences where they're different, and as zero (black)
where they overlap perfectly (or at least have the same gray level if
not overlapped). It depends on what kind of visual effect you want to
display.

There are other more sophisticated methods I'm sure - just check out
section 12 here for a list of hundreds of methods for registering
images:
http://iris.usc.edu/Vision-Notes/bibliography/contents.html
You may be especially interested in section 12.1.4.7 Fusion of Medical
Data
http://iris.usc.edu/Vision-Notes/bibliography/match-pl504.html#Fusion%20of%20Medical%20Data

With your images, the two images look totally different. I don't think
there's any way to meaningfully register them. I wouldn't call the
difference slight - it's substantial.
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