Using hugin / panotools to help blend exposures.

[Ed Halley]

At the Luminous Landscape site, there's a nice tutorial on how to blend
two images of bracketed light levels, so you can get a wider dynamic
range to your images.  This is a great technique to capture those
challenging too-bright, too-shadowy situations.

   http://www.luminous-landscape.com/tutorials/blended_exposures.shtml

However, to get the best results, you generally need to use a tripod to
make sure both exposures (or all exposures of a set) are capturing
exactly the same scene.

This morning, when digging back through archives for a nice panoramic
series to play with Hugin, I realized that Hugin and panotools could be
used to assist in aligning two images that were hand-held.

Here's the results of my first experiment in using panotools to help
mix/merge two exposures for dynamic range from handheld bracketing.

   http://www.halley.cc/pix/mixing.with.hugin.jpg
   http://www.halley.cc/pix/mixing.with.gimp.jpg
   http://www.halley.cc/pix/mixing.results.jpg
   (May be removed in a couple weeks.)
   Contact me if you want full-sized copies.

There are some issues.

One, there's still no good way to get masked layers for GIMP.  I knew
that the panotools writes the _PTStitcher_tmp_X files in TIFF RGBA
format, so I have learned to abort the stitching after all frames are
converted, and rename the orphaned temporary files.  Then I merge
those.  Someone on this list was also talking about this issue.  GIMP's
tiff loader can and should load multiple-image TIFFs, but it doesn't do
that today.  If someone has the panotools source code, they could add a
"save as separate .tiff RGBA files" (which would just name the
temporaries and quit cleanly).  Saving as separate .png RGBA files would
be a plus.

The goal of panotools is to align images along a fairly narrow edge,
where feathering would transition from one image to the other.  However,
even when both images were at the same lens, same focal length, same
perspective (within an inch or so), the image scales were hard for
panotools to rectify exactly.  I experimented with different control
pairs, but could never get the distortions quite exactly to match.  I
eventually ended up shrinking the underexposed frame by a factor of
0.9975, which got nearly pixel-perfect alignment over the whole image. 
Nearly.

Someone on this list had mentioned the idea of making a fuzzier
"accuracy mask" method to decide how much of each image to blend.  That
would work nicely, but I fear the alignment is not always good enough,
and you'll see a lot more ghosted displacements.  It's worth trying and
seeing the results!

-- 
[ e d @ h a l l e y . c c ]