| Best way to move a window [message #29532] |
Wed, 27 February 2002 18:03  |
Ned Horning
Messages: 6
Registered: March 2001
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Junior Member |
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I am writing a progarm that move a 3 x 3 window through an image. From
what I can tell IDL isn't well suited for the way I have programmed it
but I can't figure out how to improve it. I expect I'm missing a
fundamental concept.
The way I currently have it writen is to move the window, one pixel at
a time and process it like this:
FOR y=1, num_tile_lines - 2 DO BEGIN
out_tile[y*num_tile_samples] = 0
FOR x=1, num_tile_samples - 2 DO BEGIN
IF (((tile_data[x,y] gt 179.99999) AND (tile_data[x,y] lt
180.0001)) OR $
((tile_data[x+1,y] gt 179.99999) AND (tile_data[x+1,y] lt
180.0001)) OR $
((tile_data[x-1,y] gt 179.99999) AND (tile_data[x-1,y] lt
180.0001)) OR $
((tile_data[x,y+1] gt 179.99999) AND (tile_data[x,y+1] lt
180.0001)) OR $
((tile_data[x,y-1] gt 179.99999) AND (tile_data[x,y-1] lt
180.0001)) OR $
((tile_data[x+1,y+1] gt 179.99999) AND (tile_data[x+1,y+1] lt
180.0001)) OR $
((tile_data[x-1,y-1] gt 179.99999) AND (tile_data[x-1,y-1] lt
180.0001)) OR $
((tile_data[x-1,y+1] gt 179.99999) AND (tile_data[x-1,y+1] lt
180.0001)) OR $
((tile_data[x+1,y-1] gt 179.99999) AND (tile_data[x+1,y-1] lt
180.0001))) THEN $
out_tile[y*num_tile_samples+x] = 0 ELSE BEGIN
Is there a faster way to do this?
Thanks in advance for any advice.
Ned
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| Re: Best way to move a window [message #29641 is a reply to message #29532] |
Fri, 01 March 2002 06:16  |
Ned Horning
Messages: 6
Registered: March 2001
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Junior Member |
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Jaco,
Thanks for the advice. It looks like I need to get more creative with
CONVOL.
I am trying to detect ridges in a digital elevation model. I'm making
the assumption that the sides of a ridge are sloping away from the
ridgeline at roughly a 90 degree angle (i.e., if the ridge is
north-south the west side of the ridge should have a west aspect) and
the two sides of the ridge are roughly opposed (within a tolerance of
t and s).
The reason I do the 180 check is because flat areas are assigned an
aspect of 180 degrees and that really messes things up.
I also need to figure out how to calculate the mean of a moving window
and work with a circular window but first I need to get this working
faster.
Thanks again for your help.
Ned
On Fri, 1 Mar 2002 13:49:33 +0100, "Jaco van Gorkom"
<j.c.van.gorkom@fz-juelich.de> wrote:
> "Ned Horning" <nedh@lightlink.com> wrote in message
> news:uvis7ug8kfc4qtlnllpqj3223m19bo3pd3@4ax.com...
>> Thanks Marc, this is pretty slick and it helps a lot. Unfortunately I
>> just realized that I didn't post the second half of the window
>> routine. After doing the "abs(tile_data - 180) lt 0.0001)" test I
>> continue with:
>>
>> s= 45
>> t=45
>> IF (((ABS(tile_data[x+1,y+1] - 45) le s) AND (ABS(tile_data[x-1,y-1] -
>> tile_data(x+1,y+1) -180) le t)) OR $
>> ((ABS(tile_data[x+1,y] - 90) le s) AND (ABS(tile_data[x-1,y] -
>> tile_data(x+1,y) -180) le t)) OR $
>> ((ABS(tile_data[x+1,y+1] - 135) le s) AND (ABS(tile_data[x-1,y+1] -
>> tile_data(x+1,y-1) -180) le t)) OR $
>> ((ABS(tile_data[x,y-1] - 180) le s) AND (ABS(ABS(tile_data[x,y+1] -
>> tile_data(x,y-1)) -180) le t))) THEN $
>> out_tile[y*num_tile_samples+x] = 1 ELSE $
>> out_tile[y*num_tile_samples+x] = 0
>>
>> What this does is compares opposing pixels (top and bottom, left and
>> right, upper left lowerright, upper right and lower left) and I'm
>> trying to see how to use the where/convolution logic for this and not
>> having much luck.
>
> Well, two options spring to mind:
> - use the SHIFT() function, e.g. for left-right:
> shiftleft = SHIFT(tile_data, -1, 0) & shiftright = SHIFT(tile_data, 1, 0)
> expression:
> ( ABS(shiftleft - 90) LE s ) AND ( ABS(shiftright - shiftleft - 180) LE
> t )
>
> - use CONVOL() with kernels like [-1,0,1] for left-right, [[-1],[0],[1]]
> for top-bottom, and 3x3 arrays for the diagonals. For left-right:
> expression:
> ( ABS(shiftleft - 90) LE s ) AND $
> ( ABS( CONVOL(tile_data, [-1,0,1]) - 180) LE t )
>
> Then it should be possible to OR the above expressions directly together
> and AND them into out_tile. If most of the elements get selected already
> in the first half (180. being in their neigbourhood), then a loop over the
> remaining elements might be a faster solution for this second half.
>
> Getting totally confused about what this routine is actually doing,
> Jaco
>
>
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| Re: Best way to move a window [message #29643 is a reply to message #29532] |
Fri, 01 March 2002 04:49 |
Jaco van Gorkom
Messages: 97
Registered: November 2000
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Member |
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"Ned Horning" <nedh@lightlink.com> wrote in message
news:uvis7ug8kfc4qtlnllpqj3223m19bo3pd3@4ax.com...
> Thanks Marc, this is pretty slick and it helps a lot. Unfortunately I
> just realized that I didn't post the second half of the window
> routine. After doing the "abs(tile_data - 180) lt 0.0001)" test I
> continue with:
>
> s= 45
> t=45
> IF (((ABS(tile_data[x+1,y+1] - 45) le s) AND (ABS(tile_data[x-1,y-1] -
> tile_data(x+1,y+1) -180) le t)) OR $
> ((ABS(tile_data[x+1,y] - 90) le s) AND (ABS(tile_data[x-1,y] -
> tile_data(x+1,y) -180) le t)) OR $
> ((ABS(tile_data[x+1,y+1] - 135) le s) AND (ABS(tile_data[x-1,y+1] -
> tile_data(x+1,y-1) -180) le t)) OR $
> ((ABS(tile_data[x,y-1] - 180) le s) AND (ABS(ABS(tile_data[x,y+1] -
> tile_data(x,y-1)) -180) le t))) THEN $
> out_tile[y*num_tile_samples+x] = 1 ELSE $
> out_tile[y*num_tile_samples+x] = 0
>
> What this does is compares opposing pixels (top and bottom, left and
> right, upper left lowerright, upper right and lower left) and I'm
> trying to see how to use the where/convolution logic for this and not
> having much luck.
Well, two options spring to mind:
- use the SHIFT() function, e.g. for left-right:
shiftleft = SHIFT(tile_data, -1, 0) & shiftright = SHIFT(tile_data, 1, 0)
expression:
( ABS(shiftleft - 90) LE s ) AND ( ABS(shiftright - shiftleft - 180) LE
t )
- use CONVOL() with kernels like [-1,0,1] for left-right, [[-1],[0],[1]]
for top-bottom, and 3x3 arrays for the diagonals. For left-right:
expression:
( ABS(shiftleft - 90) LE s ) AND $
( ABS( CONVOL(tile_data, [-1,0,1]) - 180) LE t )
Then it should be possible to OR the above expressions directly together
and AND them into out_tile. If most of the elements get selected already
in the first half (180. being in their neigbourhood), then a loop over the
remaining elements might be a faster solution for this second half.
Getting totally confused about what this routine is actually doing,
Jaco
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