Re: [Gimp-developer] Purple fringing removal extension

[Robert Krawitz <rlk alum mit edu>]

On Sun, 15 Jul 2012 01:19:52 +0400, Alexandre Prokoudine wrote:
> On Sat, Jul 14, 2012 at 12:35 PM, Martin Jambon wrote:
>> Hi,
>>
>> I am totally new here but I wrote a standalone program that does a
>> decent job of removing purple fringing from photos, and I would like to
>> be able to implement the same functionality for Gimp.
>>
>> Here are some examples of what it does:
>>
>>   http://mjambon.com/purple-fringe/examples.html
>>
>> My algorithm needs to perform pixelwise operations (+, -, min, max
>> between two channels; scaling of one channel) but I also would prefer
>> something that is easy to install (for users) and easy to maintain (for me).
>>
>> The OCaml source code is here; it should give an idea of what's needed:
>>
>>   https://github.com/mjambon/purple-fringe/blob/master/src/unpurple.ml
>>
>> (see functions make_purple_blur and remove_purple_blur, lines 103-177)
>>
>> My questions are:
>>
>> 1. Is it possible to do that using a Gimp script?
>>
>> 2. If so, Scheme or Python?
>>
>> 3. If a plugin makes more sense, will average users be able to install
>> it for themselves or would I have to wait for the inclusion in some
>> standard precompiled distribution?
>
> Just in case, are you aware of http://kcd.sourceforge.net/fix-ca.php ?

That fixes lateral chromatic aberration (the focal length, or image
magnification, is different for different wavelengths but the focal
plane is the same).  Purple fringing is caused by longitudinal chromatic
aberration (the focal plane is different for different wavelengths).
It's a different lens defect and is much more difficult to fix (stopping
down reduces the problem, since it increases depth of field, but there
are often reasons why you don't want to do that).

Lateral chromatic aberration will be zero at the center of the image and
will increase toward the edges; longitudinal chromatic aberration will
affect the entire image field (including the very center) more or less
equally.

Martin's tool looks very impressive, particularly for those of us who
have the Canon 85 f/1.8 -- a truly great lens marred only by a fair bit
of purple fringing at large apertures.
-- 
Robert Krawitz                                     <rlk alum mit edu>

MIT VI-3 1987 - Congratulations MIT Engineers men's hoops Final Four!
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