| Fast shear [message #29128] |
Tue, 05 February 2002 05:00  |
the_cacc
Messages: 104
Registered: October 2001
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Senior Member |
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Hi,
Anyone know a way of doing the following as fast as possible:
for i = 0, n-1 DO array[*,i] = SHIFT(array[*,i],i)
or, even better,
for i = 0, n-1 DO array[*,i] = SHIFT(array[*,i],delta[i])
where delta is an INTARR.
Ciao.
(NB. using IDL5.5 - when it works ;))
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| Re: Fast shear [message #29197 is a reply to message #29128] |
Thu, 07 February 2002 04:53  |
Martin Downing
Messages: 136
Registered: September 1998
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Senior Member |
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> Wayne Landsman <landsman@mpb.gsfc.nasa.gov> wrote in message
news:<3C60199D.E4810C70@mpb.gsfc.nasa.gov>...
>>
>> "Avoid the use of an asterisk on the left-hand side of an assignment."
>>
>> which in this case means to rewrite the assignment as
>>
>> for i = 0, n-1 DO array[0,i] = SHIFT(array[*,i],delta[i])
>>
>> For a fltarr(2048,2048) on my Solaris machine running V5.5, I find a
>> factor of 15 improvement in speed.
>>
>
> WOW! Wayne, that's magic. I get 10-15X speed-up. The world *has* to be
> told about this. One drawback - the code does not strictly make sense,
> hell who cares !?
>
I second that WOW! I had not realised that by specifying a single cell of
an image on the left hand side allows intelligent access to the elements
mapped by the dimensional shape of the right hand side. Offset assignments
in 2/3d images become a breeze.
e.g. to set image[100:109,100:129] = 255:
image[100,100] = replicate(255, 10, 30)
I had stupidly assumed that the assignment would follow C language rules and
fill in a raster ordering.
Thanks for the tip - I will be using this method from now on
Martin
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