| Help requested in eradicating FOR loops [message #57539] |
Fri, 14 December 2007 08:06  |
dplatten
Messages: 32
Registered: December 2007
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Member |
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Dear IDL users,
I would really appreciate some help in removing a nested FOR loop.
FOR x=0, image_width-1 DO BEGIN
FOR y=0, image_height-1 DO BEGIN
; Write the current pixel value
results[(x*mtf_region_height)+y, 1] = image[x,y]
; Calculate the distance of the current pixel from the
line defined
by the
; two points (x1, y1) and (x2, y2).
results[(x*image_height)+y, 0] = ( (y1-y2)*x + (x2-
x1)*y + (x1*y2 -
x2*y1) ) / SQRT( (x2-x1)^2 + (y2-y1)^2 )
ENDFOR
ENDFOR
'image' is a 2d image array
I want to work out the distance of each pixel from a line that is
defined by the two points (x1, y1), (x2, y2) and store this distance,
together with the pixel value in a 'results' array. The 'results'
array has the dimensions [(image_height*image_width), 2].
The above bit of code works but is a bit slow.
Any advice would be much appreciated.
Thanks,
David
Northampton, UK
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| Re: Help requested in eradicating FOR loops [message #57646 is a reply to message #57539] |
Wed, 19 December 2007 19:02  |
Jean H.
Messages: 472
Registered: July 2006
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Senior Member |
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Kenneth Bowman wrote:
> In article <fkbg7s$vbl$1@news.ucalgary.ca>,
> Jean H <jghasban@DELTHIS.ucalgary.ANDTHIS.ca> wrote:
>
>>>> y_locations = fix(temp) / 4
>>> I have found that this works a bit better:
>>> y_locations = FLOOR(temp / 4)
>> It is exactly the same thing.... since fix() simply removes the
>> decimals (if used with no TYPE keyword), it acts like floor!
>>
>> Jean
>
> Be careful, FLOOR and FIX (or LONG) are not the same thing.
>
> IDL> print, floor(3.5)
> 3
> IDL> print, long(3.5)
> 3
> IDL> print, floor(-3.5)
> -4
> IDL> print, long(-3.5)
> -3
>
>
> Cheers, Ken Bowman
ooops... thanks for pointing that... thesis writing is bad for coding
details memory :-)
Jean
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| Re: Help requested in eradicating FOR loops [message #57648 is a reply to message #57539] |
Wed, 19 December 2007 13:45 |
Kenneth Bowman
Messages: 86
Registered: November 2006
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Member |
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In article <fkbg7s$vbl$1@news.ucalgary.ca>,
Jean H <jghasban@DELTHIS.ucalgary.ANDTHIS.ca> wrote:
>>> y_locations = fix(temp) / 4
>
>> I have found that this works a bit better:
>> y_locations = FLOOR(temp / 4)
>
> It is exactly the same thing.... since fix() simply removes the
> decimals (if used with no TYPE keyword), it acts like floor!
>
> Jean
Be careful, FLOOR and FIX (or LONG) are not the same thing.
IDL> print, floor(3.5)
3
IDL> print, long(3.5)
3
IDL> print, floor(-3.5)
-4
IDL> print, long(-3.5)
-3
Cheers, Ken Bowman
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| Re: Help requested in eradicating FOR loops [message #57678 is a reply to message #57539] |
Wed, 19 December 2007 08:18 |
Jean H.
Messages: 472
Registered: July 2006
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Senior Member |
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>> y_locations = fix(temp) / 4
> I have found that this works a bit better:
> y_locations = FLOOR(temp / 4)
It is exactly the same thing.... since fix() simply removes the
decimals (if used with no TYPE keyword), it acts like floor!
Jean
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| Re: Help requested in eradicating FOR loops [message #57687 is a reply to message #57539] |
Wed, 19 December 2007 07:57 |
dplatten
Messages: 32
Registered: December 2007
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Member |
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On Dec 18, 1:23 pm, Spon <christoph.b...@gmail.com> wrote:
> On Dec 18, 11:00 am, dplat...@gmail.com wrote:
>
>
>
>>> Off the top of my head:
>>> 1-reform the points to match the size of the image: x1 =
>>> reform(x1,image_height,image_width)
>>> 2-make in "index" array: idx = findgen(image_height,image_width)
>>> 3-make x-index and y-index arrays: x = idx mod image_height & y = idx/
>>> image_height (or maybe those should be image_width?)
>>> 4-do the calculation: distance = ( (y1-y2)*x + (x2-x1)*y + (x1*y2-
>>> x2*y1) ) / SQRT( (x2-x1)^2 + (y2-y1)^2 )
>>> 5-put together the results matrix: results =
>>> [[reform(image,image_height*image_width)],[distance]] (I'd have to
>>> check the brackets, I always do)
>
>> Thanks for the reply - as I understand it I need to make an array to
>> hold x pixel positions and one to hold y pixel positions. For a 4 x 6
>> array they would look like this:
>
>> x locations: [0,1,2,3, 0,1,2,3, 0,1,2,3, 0,1,2,3, 0,1,2,3, 0,1,2,3]
>> y locations: [0,0,0,0, 1,1,1,1, 2,2,2,2, 3,3,3,3, 4,4,4,4, 5,5,5,5]
>
>> I can make the x array by doing this:
>
>> test_image = findgen(4, 6) ; a dummy test "image"
>> temp = findgen(4 * 6)
>> x_locations = temp mod 4
>
>> but I am having problems creating the y locations array. Some help
>> would be appreciated.
>
>> Thanks, David
>
> y_locations = fix(temp) / 4
Thanks for the response. I initially was using
y_locations = REFORM( ROTATE( REFORM(temp MOD 6, 6, 4), 1), 4*6)
which does work, but your suggestion is much more elegant!
I have found that this works a bit better:
y_locations = FLOOR(temp / 4)
Many thanks for the friendly help.
David
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| Re: Help requested in eradicating FOR loops [message #57710 is a reply to message #57539] |
Tue, 18 December 2007 05:23 |
Spon
Messages: 178
Registered: September 2007
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Senior Member |
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On Dec 18, 11:00 am, dplat...@gmail.com wrote:
>> Off the top of my head:
>> 1-reform the points to match the size of the image: x1 =
>> reform(x1,image_height,image_width)
>> 2-make in "index" array: idx = findgen(image_height,image_width)
>> 3-make x-index and y-index arrays: x = idx mod image_height & y = idx/
>> image_height (or maybe those should be image_width?)
>> 4-do the calculation: distance = ( (y1-y2)*x + (x2-x1)*y + (x1*y2-
>> x2*y1) ) / SQRT( (x2-x1)^2 + (y2-y1)^2 )
>> 5-put together the results matrix: results =
>> [[reform(image,image_height*image_width)],[distance]] (I'd have to
>> check the brackets, I always do)
>
> Thanks for the reply - as I understand it I need to make an array to
> hold x pixel positions and one to hold y pixel positions. For a 4 x 6
> array they would look like this:
>
> x locations: [0,1,2,3, 0,1,2,3, 0,1,2,3, 0,1,2,3, 0,1,2,3, 0,1,2,3]
> y locations: [0,0,0,0, 1,1,1,1, 2,2,2,2, 3,3,3,3, 4,4,4,4, 5,5,5,5]
>
> I can make the x array by doing this:
>
> test_image = findgen(4, 6) ; a dummy test "image"
> temp = findgen(4 * 6)
> x_locations = temp mod 4
>
> but I am having problems creating the y locations array. Some help
> would be appreciated.
>
> Thanks, David
y_locations = fix(temp) / 4
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