[ale] Processing color negative scans with ALE

[David Hilvert]

On Tue, 24 Mar 2009 19:15:16 +0200
Jari Juslin <zds at iki.fi> wrote:

> The reason for equalizing the exposure is that ALE is bad at aligning images
> that have different exposure, say 0 and +2 EV. As this is the case always
> with HDR images I use convert to brighten up the darker image. After that
> treatment ALE does the aligning very nicely.

There is an --ev option for specifying exposure (which the wrapper script tries
to set automatically from EXIF when exiftool is present), but it's currently
necessary to also specify --exp-meta-only for --ev to have much effect.  (I've
added a ditz issue about this (ale-14) to the git master branch, so that it
might be addressed in future versions.)

> Another way to make HDR with ALE is to shoot lots of images that are all
> underexposed enough so that no spot is burned and then use ALE to combine
> these. This is something I am experimenting as we speak. 

If possible, ensure that --bayer and --black are set correctly for this case,
and that the output format is suitable for HDR.  Ubuntu, for example, ships
with a build of libGraphicsMagick that is 8-bit, which is unsuitable.  To be
safe, specify --16bpc manually, as some versions of ALE may have a bug that
causes this to not be set properly by default.

If having blown out regions in supplemental frames produces artifacts, you
might try some of the following (but note that this case is not extensively
tested):-

- For 0.9.x branch runs, use '--multi none'.

- Ensure that --bayer is set correctly.

- If blown out regions are large, try --ev (with --exp-meta-only).

- For small input sets, try '--ips 0', and maybe also '--dchain triangle:2'.

- If possible, ensure that --black is set correctly for all frames.