| Re: Fractional Pixels Origin? [message #47599 is a reply to message #47583] |
Tue, 21 February 2006 13:44   |
Mark Hadfield
Messages: 783
Registered: May 1995
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Senior Member |
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JD Smith wrote:
> [snip]
>
> I don't see your argument. Fractional pixels are useful only when
> calculating things which relate pixel coordinates to some other
> coordinate (like celestial coordinates on the sky, etc), or when
> computing other derived fractional pixel positions (e.g. clip two
> polygons). Obviously, the computer has no understanding of a
> fractional pixel, but only the memory indexed offset [0,0]. But the
> latter does not have to drive the former. In fact I'd say it's rather
> strange to let the layout in memory dictate a physical coordinate
> system. You don't need to make this distinction.
Hang on. I'm not letting layout in memory dictate a physical coordinate
system. I'm just adopting a convention for "image plots" that is the
same as for other plot types.
I have a data array, mydata, dimensioned [m,n]
When I type
contour, mydata
the contour routine implicitly locates the data points at x =
[0,...,m-1], y = [0,...,n-1]. Ditto when I type
surface, mydata
Similarly for plots of 1D arrays.
When I plot this data via a false-colour image using my own
image-plotting routine
mgh_image_plot, mydata
then I adopt the same convention. If the image is "blocky" (eg an
IDLgrImage with INTERPOLATE=0) then the cell centres represent the data
points; the image has to fill the space -0.5 <= x <= m-0.5 (similarly
for y) to get them in the right location. If the image is "smooth" (eg
an IDLgrImage with INTERPOLATE=1) then the outer row & column of data
points lie on the edge of the image and it fills the space 0 <= x <=
m-1, etc.
That's the convention I prefer, for reasons which seem good to me. If
that's not the subject under discussion in this thread (which was never
entirely clear to me) then please disregard it.
--
Mark Hadfield "Kei puwaha te tai nei, Hoea tahi tatou"
m.hadfield@niwa.co.nz
National Institute for Water and Atmospheric Research (NIWA)
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