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Re: Fractional Pixels Origin? [message #47629 is a reply to message #47549]

Fri, 17 February 2006 13:25

JD Smith
Messages: 850
Registered: December 1999

Senior Member

On Thu, 16 Feb 2006 17:02:20 -0700, David Fanning wrote:

> Wayne Landsman writes:
>
>> I'll say that the FITS convention is that [0,0] locates the center of
>> the pixel. (This differs from most other image processing standards
>> where [0.,0.] defines the lower lefthand corner.)
>
>
> Then, JD Smith writes:
>
>> FITS does indeed use [0.0,0.0]. I'd urge those of you making the choice
>> for your programs to save the world confusion, and adopt the "natural"
>> choice: pixels centered on [a.5,b.5].
>
> I'm confused. :-(

Sorry, I had it backwards, FITS centers the first pixel at [1,1], and
the Nasa library uses [0,0] (which is called "IDL convention"). If
you have a choice, don't choose the FITS standard ([1,1]), or the "IDL
convention" ([0,0]), but the natural "I'm a tiny ant living on the
surface of your detector and measuring pixel positions from the edge
with my tiny little ruler": [0.5,0.5]. The only disadvantage is all
pixel centers are now fractional.

Wayne is very careful to document the convention in all of the NasaLib
routines, so if confused be sure to read the useful doc headers (as I
just did to remedy my confusion!).

JD

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[Message index]

		Re: Fractional Pixels Origin? By: David Fanning on Thu, 16 February 2006 21:15
		Re: Fractional Pixels Origin? By: Greg Hennessy on Thu, 16 February 2006 20:20
		Re: Fractional Pixels Origin? By: David Fanning on Thu, 16 February 2006 16:02
		Re: Fractional Pixels Origin? By: JD Smith on Thu, 16 February 2006 14:48
		Re: Fractional Pixels Origin? By: rm on Thu, 16 February 2006 06:24
		Re: Fractional Pixels Origin? By: Craig Markwardt on Wed, 15 February 2006 14:28
		Re: Fractional Pixels Origin? By: Wayne Landsman on Wed, 15 February 2006 13:31
		Re: Fractional Pixels Origin? By: David Fanning on Wed, 15 February 2006 13:03
		Re: Fractional Pixels Origin? By: JD Smith on Fri, 17 February 2006 13:25
		Re: Fractional Pixels Origin? By: mmiller3 on Fri, 17 February 2006 05:36
		Re: Fractional Pixels Origin? By: Greg Hennessy on Fri, 17 February 2006 05:06
		Re: Fractional Pixels Origin? By: Wayne Landsman on Thu, 16 February 2006 23:50

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