| Re: Calculate gradient [message #70082] |
Sat, 13 March 2010 00:35 |
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James[2]
Messages: 44
Registered: November 2009
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Member |
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You could use the SOBEL operator. This convolves the image with
kernels like so:
-1 0 1 1 2 1
-2 0 2 0 0 0
-1 0 1 -1 -2 -1
to get approximations of the x and y gradient respectively. IDL's
built-in SOBEL routine returns the magnitude, i.e. the sum of the
absolute values of each of these convolutions. It would be simple to
make the kernels yourself and use CONVOL if you want to keep the x and
y gradients separate.
I don't know much about contour plotting in IDL, but the CONTOUR
function is probably a good place to start...
On Mar 12, 1:41 pm, mslarkin <enhlw...@gmail.com> wrote:
> Hi IDL experts,
> I have a 2-D array (2708x4060), containing reflectance data. Is it
> possible to calculate the reflectance gradient (i.e. the rate of
> reflectance change over distance or array grid) throughout the array,
> and then draw lines of equal gradient (note that they’re NOT contour
> lines)?
> Thank you very much for your help!
> IDL beginner
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| Re: Calculate gradient [message #70084 is a reply to message #70082] |
Fri, 12 March 2010 14:40  |
R.G.Stockwell
Messages: 163
Registered: October 2004
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Senior Member |
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Perhaps it would be better to use Deriv() or something like that,
instead of differences.
cheers,
bob
>
> "Paolo" <pgrigis@gmail.com> wrote in message
> news:b7420af6-f104-4e00-968c-28afc378f8ac@g26g2000yqn.google groups.com...
> Hi,
>
> the simpler solution would be to use the "shift" function
> to calculate the dx and dy component of the gradient (whose
> norm is sqrt(dx^2+dy^2) - similarly to the one-dimensional
> case when you can compute the numerical derivative of x
> as (x-shift(x,-1))/dx.
>
> However you should be careful doing that with non-smooth
> datasets.
>
> Ciao,
> Paolo
>
>
>
> On Mar 12, 4:41 pm, mslarkin <enhlw...@gmail.com> wrote:
>> Hi IDL experts,
>> I have a 2-D array (2708x4060), containing reflectance data. Is it
>> possible to calculate the reflectance gradient (i.e. the rate of
>> reflectance change over distance or array grid) throughout the array,
>> and then draw lines of equal gradient (note that they�re NOT contour
>> lines)?
>> Thank you very much for your help!
>> IDL beginner
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| Re: Calculate gradient [message #70085 is a reply to message #70084] |
Fri, 12 March 2010 14:37 |
pgrigis
Messages: 436
Registered: September 2007
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Senior Member |
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Hi,
the simpler solution would be to use the "shift" function
to calculate the dx and dy component of the gradient (whose
norm is sqrt(dx^2+dy^2) - similarly to the one-dimensional
case when you can compute the numerical derivative of x
as (x-shift(x,-1))/dx.
However you should be careful doing that with non-smooth
datasets.
Ciao,
Paolo
On Mar 12, 4:41 pm, mslarkin <enhlw...@gmail.com> wrote:
> Hi IDL experts,
> I have a 2-D array (2708x4060), containing reflectance data. Is it
> possible to calculate the reflectance gradient (i.e. the rate of
> reflectance change over distance or array grid) throughout the array,
> and then draw lines of equal gradient (note that they’re NOT contour
> lines)?
> Thank you very much for your help!
> IDL beginner
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