| Re: Cannot understand a part of the IDL routine!! pls help!! [message #71035 is a reply to message #70925] |
Tue, 25 May 2010 12:13   |
Brian Daniel
Messages: 80
Registered: July 2009
|
Member |
|
|
On May 25, 9:23 am, Jeremy Bailin <astroco...@gmail.com> wrote:
> On May 24, 1:26 pm, Brian Daniel <Daniels...@yahoo.com> wrote:
>
>
>
>
>
>> On 24 May, 08:16, Jeremy Bailin <astroco...@gmail.com> wrote:
>
>>> On May 23, 11:07 am, David Fanning <n...@dfanning.com> wrote:
>
>>>> bala murugan writes:
>>>> > The following is a part of the IDL routine for region grow. The
>>>> > following three lines of code is used to define the pixels that is the
>>>> > ROI pixels.
>>>> > x = FINDGEN(16*16) MOD 16 + 276
>>>> > y = LINDGEN(16*16) / 16 + 254
>>>> > roiPixels = x + y * imgDims[0]
>
>>>> > The question is how does it define the ROI pixels?
>>>> > I dont see how it does........ Somebody please help me by giving a
>>>> > simple and clear description.
>
>>>> What is happening here is the IDL is turning one-dimensional
>>>> image indices into two-dimensional image indices. Before
>>>> the advent of the function Array_Indices, we always had
>>>> to do this by hand. This code was obviously written in
>>>> those long-ago dark days.
>
>>>> Here is an article that explains this process in some
>>>> detail:
>
>>>> http://www.dfanning.com/tips/where_to_2d.html
>
>>>> Cheers,
>
>>>> David
>
>>>> --
>>>> David Fanning, Ph.D.
>>>> Fanning Software Consulting, Inc.
>>>> Coyote's Guide to IDL Programming:http://www.dfanning.com/
>>>> Sepore ma de ni thui. ("Perhaps thou speakest truth.")
>
>>> Incidentally, is there an in-built routine that I've missed that does
>>> the reverse mapping (multi-D to 1D)? I know I've written my own and I
>>> suspect others have too, but it seems like there ought to be a built-
>>> in version.
>
>>> -Jeremy.- Hide quoted text -
>
>>> - Show quoted text -
>
>> reform does the trick. For example:
>> image = indgen(20,30,3)
>> help, image
>> IMAGE INT = Array[20, 30, 3]
>> image_vector = reform(image,20*30*3)
>> help, image_vector
>> IMAGE_VECTOR INT = Array[1800]
>
>> -Brian
>
> Yes, you can of course use l/indgen to give you a multi-D array that
> you can use for the mapping (I'm not sure what the point of the reform
> is - I would just use image[4,3,0] to find out the 1D index of the
> point (4,3,0)). But it requires generating an auxiliary array with the
> required dimensions, which can be very wasteful of memory in the
> applications where I tend to use it!
>
> -Jeremy.
I was using indgen as an illustration. I thought you were looking for
a way to change a multi dimensional array into a vectorized array. I
see now you'd like the compliment to array_indices.pro. Say you have
an array of size x_dim, y_dim and z_dim. The index of a particular
point in the array (xi,yi,zi) is given as
index = xi + yi * x_dim + zi * x_dim * y_dim.
It can be generalized to N dimensions by
index = x[0] + x[1]*dim[0] +x[2]*dim[1]*dim[0] + ... +
x[N-1]*dim[N-2]*dim[N-3]*...*dim[0].
I haven't seen a library routine that does it, but I usually only work
in 2 or 3 dimensions and hardcode it in. Hope this helps.
-Brian
|
|
|
|
comp.lang.idl-pvwave archive
Members
Search
Help
Login
Home
