[ptx] Enblend performance with varying exposure values

[JD Smith]

For those users of enblend out there: have we reached the point that
exposure lock is no longer necessary?  That is, can you fix the focus,
and possibly the aperture (for good depth of field and/or resolution),
but otherwise let the camera meter its own desired shutter speed?  I'd
be interested in seeing example panoramas created with varying exposure
values from image to image, and blended with enblend.  This obviously
has the capacity to yield panoramas with higher dynamic range (think of
including the sun along with deep shadows opposite the sun).  

I've also been interested in a hypothetical camera "panorama mode" which
doesn't lock the exposure value (read shutter speed) as is traditional,
but does remember the last value from the previous exposure.  It will
allow the EV to change, but will not let it change by more than, say,
1/3EV (about 25% more or less light admitted) from frame to frame.  This
would help the final blend look smooth, but would also extend the
dynamic range smoothly from bright to faint portions of the panorama.  

The result would obviously depend on the order in which you took the
panorama: starting out with the sun and moving off into the shadows, it
would take several frames to bring the EV up enough to compensate, but
the change will be smooth and gradual, especially for panoramas formed
with lots of images (e.g. 30 degree FOV lens).

Another possibility would be to divide the image data by the exposure
time (assuming the aperture is fixed) from the header, before stitching
and blending.  While this won't help with clipped highlights, it should
normalize the color/intensity of smooth regions like the sky.  I
suspect, however, that the place this scaling should take place is in
the RAW data, before the Bayer interpolation, and before any particular
color (tonal) model is applied.  I'd imagine a combination of these two
techniques with a superior blender like enblend should yield very high
dynamic range panoramas, without the need for multi-exposure blending
(unless of course there is a large amount of contrast in the field of
view of single images).  Anyone have thoughts on this?

JD