| Re: Drizzling Algorithms [message #36558] |
Fri, 26 September 2003 10:36  |
JD Smith
Messages: 850
Registered: December 1999
|
Senior Member |
|
|
On Fri, 26 Sep 2003 08:26:48 -0700, Wayne Landsman wrote:
>> The second article is another dazzling one anchored by JD about
>> "drizzling" algorithms. No, we are not talking about chocolate
>> sprinkles on ice-cream sundaes. Drizzling algorithms are used in the
>> reconstruction or warping of images from undersampled or dithered data.
>> (I don't know, you'll have to read the article!) It's another can't
>> miss hit for you Histogram fans.
>>
>> http://www.dfanning.com/code_tips/drizzling.html
>>
>>
> Well, we are all thankful for David's late night inspirations (or sleep
> difficulties), and I feel very curmudgeonly to have to bring up a
> caveat. But the above article might lead people to think that there are
> drizzle or other flux-conserving algorithms available in IDL, and I am
> not aware that there are any. David's article is really about array
> decimation, which would be just one step in writing a vectorized drizzle
> code (as described in
> http://www-int.stsci.edu/~fruchter/dither/drizzle.html ) My own attempts
> at writing a vectorized drizzle code floundered on determining partial
> pixel weights.
>
> I'm pretty sure now that drizzle code is best written in C and linked to
> IDL. Or perhaps I need to try out the V6.0 IDL-Java bridge with Tom
> McGlynn's software at
> http://skyview.gsfc.nasa.gov/polysamp/PolySamp.java
I tend to agree with Wayne, but I do have a Sutherland-Hodgeman
polygon clipper written in pure IDL which, together with any one of
the aggregation techniques described in the article, could constitute
a full drizzle algorithm. I'm using it for a different, but similar,
purpose. If enough people bug me I'll polish it up and put it out for
the enjoyment of all.
I did find that it was fairly slow for even my (densely dithered)
128x128 arrays, and ended up writing a replacement clipper in C. The
most fun thing about it: it uses MAKE_DLL to compile the C version of
the code to a shared library auto-magically, and, if this fails, just
falls back on the slow, but equivalent, IDL version, with no one the
wiser. It's still not nearly as fast as coding the whole problem in
C, thanks in no small part to the CALL_EXTERNAL overhead, and the need
to make a round trip to the clipper so frequently, but it's suitable
for my needs. I suspect drizzling 10's of 2kx2k images would get a
bit ugly though.
By the way, this is one problem which cannot, at least insofar as
about 2 concerted weeks of deep thought on the problem revealed, be
vectorized using the many IDL tricks we discuss here. It's surprising
when you run up against these problems, but they do exist. If RSI is
listening, they should bone up on Sutherland-Hodgeman or other,
fancier clippers, and implement a vectorized version in IDL. I should
be able to give a large list of polygons, and have them all clipped to
a grid of a specified size, with optional return of the grid pixel(s)
and area(s) each polygon clipped to. Somebody would no doubt write a
nice, fast DRIZZLE algorithm, maybe with a GUI interface, and that
would be one less reason to fire up IRAF. Another very useful
routine: given a (long) input list of polygons, compute the internal
overlap with areas.
JD
|
|
|
|
comp.lang.idl-pvwave archive
Members
Search
Help
Login
Home
