[Digikam-devel] [Bug 144155] wrong(?) histogram y-axis when blown-out over/underexposure

[Marcel Wiesweg]

https://bugs.kde.org/show_bug.cgi?id=144155


Marcel Wiesweg <marcel.wiesweg at gmx.de> changed:

           What    |Removed                     |Added
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                 CC|                            |marcel.wiesweg at gmx.de




--- Comment #23 from Marcel Wiesweg <marcel wiesweg gmx de>  2009-08-03 23:32:05 ---
Activity here ceased long ago but it's still open and valid, as I found working
with some images showing the problem.
It seems the approach taken by the patch cuts off too much if there is a high,
but real peak (because it cuts off 10% in any case) and cuts off not enough if
the image is severly overexposed (more than 10% overexposed).

If we can't find the perfect technique we should not apply it to the (strictly)
linear histogram mode but maybe introduce a third drawing mode?

Some thought on this:
- this is mostly about under- or overexposure creating artificial single-line
peaks because the natural distribution is cut. Is there a natural image
situation where a peak in the middle of the scale need to be cut?
- restricting to the upper and lower limit, it would be possible to first find
the minimum and maximum (usually at 0 and 255), see if one of these takes more
than e.g. 5% (*) of the pixels. We could additionally check if the mean is
greater (overexposed) or less than (underexposed) the median (*). The amount of
cutting depends on the maximum value of the remaining segments. I am currently
not sure how far the following segments (255: 254, 253,...) need to be examined
as well.

(*) needs empirical investigation

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