| Re: Cannot understand a part of the IDL routine!! pls help!! [message #71034 is a reply to message #70925] |
Tue, 25 May 2010 12:16   |
Brian Daniel
Messages: 80
Registered: July 2009
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Member |
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On May 25, 3:13 pm, Brian Daniel <Daniels...@yahoo.com> wrote:
> 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
Oh, and in the generalized case, x is a vector of size N consisting of
dimensional indices and dim is an N sized vector of the array
dimensions.
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