Dave Higgins writes:
>
> Thanks for the advice, very much appreciated. I followed your general method as follows, but actually it's not the filter shape I want:
>
> FUNCTION CIRCLE, xcenter, ycenter, radius
> points = (2 * !PI / 99.0) * FINDGEN(100)
> x = xcenter + radius * COS(points )
> y = ycenter + radius * SIN(points )
> RETURN, TRANSPOSE([[x],[y]])
> END
>
> PRO apod_filter
> image = dist(512)
> WINDOW, 0, TITLE = 'Test data before filtering'
> SHADE_SURF, image
> s = Size(image, /Dimensions)
> hf = Hanning(s[0], s[1], ALPHA=0.5)
> maxRadius = Min(s)/2;
> WINDOW, 1, TITLE = 'temporary window', xsize=s[0], ysize=s[1]
> ; Use of data coords for "circle" ok since data extent may not be square
> POLYFILL, CIRCLE(s[0]/2-1, s[1]/2-1, 0.8*maxRadius), color=1
> circleMask = TVRD()
> WDELETE, 1
> indices = Where(circleMask EQ 1)
> hf[indices] = 1
> WINDOW, 2, TITLE = 'Filter to be applied'
> SHADE_SURF, hf
> WINDOW, 3, TITLE = 'Apodized image'
> SHADE_SURF, hf*image
> WDELETE, 0, 3
> END
>
> This produces a step down at the edge of the circle, to where the Hanning window was before the circleMask was applied. But this step-down causes Gibbs ringing in a FT of the data.
> I was aiming for a smooth "S" shaped (or similar) reduction of the filter values from the edge of the circle to the edge of the data.
> I was wondering if I applied some sort of smoothing to circleMask, it would blur the edge of the circle and achieve the smooth decent at the circle edge:
>
> PRO apod_filter
> image = dist(512)
> WINDOW, 0, TITLE = 'Test data before filtering'
> SHADE_SURF, image
> s = Size(image, /Dimensions)
> maxRadius = Min(s)/2;
> WINDOW, 1, TITLE = 'temporary window', xsize=s[0], ysize=s[1]
> ; Use of data coords for "circle" ok since data extent may not be square
> POLYFILL, CIRCLE(s[0]/2-1, s[1]/2-1, 0.8*maxRadius), color=1
> circleMask = float(TVRD())
> WDELETE, 1
> help, circlemask
> circleMask = SMOOTH(circleMask, 100, /EDGE_TRUNCATE, MISSING=0.0)
> indices = Where(circleMask GT 0.01)
> filter = fltarr(s[0], s[1])
> filter[indices] = circleMask[indices]
> WINDOW, 2, TITLE = 'Filter to be applied'
> SHADE_SURF, filter
> WINDOW, 3, TITLE = 'Apodized image'
> SHADE_SURF, filter*image
> WDELETE, 0, 3
> END
>
> ...but this smoothing eats back into my leave-it-alone circle of data; I'd like to start the descent to zero at the edge of the originally defined circleMask.
>
> Thanks for any further help.
Aren't you smoothing the wrong thing? I think you want this:
PRO apod_filter
image = dist(512)
WINDOW, 0, TITLE = 'Test data before filtering'
SHADE_SURF, image
s = Size(image, /Dimensions)
hf = Hanning(s[0], s[1], ALPHA=0.5)
maxRadius = Min(s)/2;
WINDOW, 1, TITLE = 'temporary window', xsize=s[0], ysize=s[1]
POLYFILL, CIRCLE(s[0]/2-1, s[1]/2-1, 0.8*maxRadius), color=1
circleMask = TVRD()
WDELETE, 1
indices = Where(circleMask EQ 1)
hf[indices] = 1
WINDOW, 2, TITLE = 'Filter to be applied'
SHADE_SURF, Smooth(hf, 50, /edge_truncate)
WDELETE, 0
END
Also, TVCircle (from the Astronomy Library) will produce
a real circle no matter what the aspect ratio of the window.
One of the reasons it's one of my favorite programs. :-)
Cheers,
David
--
David Fanning, Ph.D.
Fanning Software Consulting, Inc.
Coyote's Guide to IDL Programming: http://www.idlcoyote.com/
Sepore ma de ni thui. ("Perhaps thou speakest truth.")
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