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

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           Summary: wrong(?) histogram y-axis when blown-out
                    over/underexposure
           Product: digikam
           Version: unspecified
          Platform: Compiled Sources
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: NOR
         Component: general
        AssignedTo: digikam-devel kde org
        ReportedBy: lparrab gmx net


Version:           9.2 (svn 2007-04-17) (using KDE KDE 3.5.6)
Installed from:    Compiled From Sources
Compiler:          Target: i486-linux-gnu
OS:                Linux (i686) release 2.6.20-6-generic

for images with a certain degree of over/underexposure (clipping/blown-out) the histogram in digikam becomes useless because the y-axis seems to be scaled to accommodate the blown-out highlights or shadows, making the scale for the rest of the picture way to big.

the y-axis should be scaled so that (example) 50% of "Y" represents the highest peak of the non-clipped pixels, effectively cutting the top of the y-axis.
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------- Additional Comments From lparrab gmx net  2007-04-12 21:24 -------
Created an attachment (id=20254)
 --> (http://bugs.kde.org/attachment.cgi?id=20254&action=view)
histogram for a picture with blown-out highlights

take a look at the histogram: due to the blown-out parts, the y-scale is waaaay
to big for the rest of the histogram, effectively making it useless.
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------- Additional Comments From lparrab gmx net  2007-04-12 21:29 -------
Created an attachment (id=20255)
 --> (http://bugs.kde.org/attachment.cgi?id=20255&action=view)
LIGHTROOM: histogram for a picture with blown-out highlights

this is lightroom showing the same picture as digikam in the first attachment.
you can still see that the highlights are being clipped, but the rest of the
histogram still has a recognizable form.

in digikam you would have to use logarithmic scale to see anything --but that's
not intuitive for photography work.
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[Gilles Caulier-4]

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------- Additional Comments From caulier.gilles gmail com  2007-04-12 21:32 -------
>in digikam you would have to use logarithmic scale to see anything --but that's
>not intuitive for photography work.

Why it's not intuitive ? It work perfectly...

Gilles caulier
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[Arnd Baecker]

> >in digikam you would have to use logarithmic scale to see anything --but that's
> >not intuitive for photography work.
>
> Why it's not intuitive ? It work perfectly...

Well, while it works perfectly, there are many people
who are not familiar with logarithmic representations ;-)

So the histogram y-axis is surely not wrong
for over/underexposure, cutting off the upper part
of the histogramm looks like a good solution.

But how to determine the cut-off value?
Maybe
  cut_off = min( maximal_value_of_histogram, 2*vertical_variance)
or one determines the cut-off as
the largest maximum on the interval [0.1, 0.9] ?
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[Arnd Baecker]

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------- Additional Comments From arnd.baecker web de  2007-04-13 00:50 -------
> >in digikam you would have to use logarithmic scale to see anything --but that's
> >not intuitive for photography work.
>
> Why it's not intuitive ? It work perfectly...


Well, while it works perfectly, there are many people
who are not familiar with logarithmic representations ;-)

So the histogram y-axis is surely not wrong
for over/underexposure, cutting off the upper part
of the histogramm looks like a good solution.

But how to determine the cut-off value?
Maybe
  cut_off = min( maximal_value_of_histogram, 2*vertical_variance)
or one determines the cut-off as
the largest maximum on the interval [0.1, 0.9] ?
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------- Additional Comments From lparrab gmx net  2007-04-13 03:47 -------
yes, the logarithmic histogram does work perfectly: the calculation and the curve are "correct"...

however, I think that logarithmic scale is great for some physics or math graphs.. but in photography, a logarithmic histogram does not tell (at least not to me, specially not at the first blick) the proportion of the color distribution in the image

Since the problem is normally on the extrem left or right of the histogram, you could simply cut off the 1% of either side (not really cut, but don't take into account for the Y-axis)

Other approach would be to use a given percentil to set the limit.
I'm attaching an example-patch to the current SVN version (anonsvs) which ensures that 90% of the pixels take at least some 33% of the Y-scale.

The 90% and 33% are arbitrary values and you could play a bit with them, but they seem to give a nice result.
I'm also attaching a screenshot of digikam with(svn) and without the patch(0.9)

regards. Luis
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------- Additional Comments From lparrab gmx net  2007-04-13 03:50 -------
Created an attachment (id=20257)
 --> (http://bugs.kde.org/attachment.cgi?id=20257&action=view)
PATCH: limit the Y-axis of image histogram to 3*(90-percentil)

like I said, the limits are really arbitrary, but work for me.

I changed the "getMaximum" function in imagehistogram.cpp to cache the
results.. but that is really not relevant. only the "getPercentil" and the
changes in histogramwidget.cpp are interesting.
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------- Additional Comments From lparrab gmx net  2007-04-13 03:56 -------
Created an attachment (id=20258)
 --> (http://bugs.kde.org/attachment.cgi?id=20258&action=view)
screenshot of the patch for limiting the y-axis based on percentiles

Digikam standard and patched versions showing the same image...

I think you can read much more information from the histogram in the
patched-version.
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[Arnd Baecker]

> I think you can read much more information from the histogram in the
> patched-version.

Full agreement - very nice!
(Haven't tried the patch yet, but the result looks really good to me...)
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[Arnd Baecker]

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------- Additional Comments From arnd.baecker web de  2007-04-13 08:46 -------
> I think you can read much more information from the histogram in the
> patched-version.


Full agreement - very nice!
(Haven't tried the patch yet, but the result looks really good to me...)
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[Gerhard Kulzer]

Am Friday 13 April 2007 schrieb [hidden email]:

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> http://bugs.kde.org/show_bug.cgi?id=144155
>
>
>
>
> ------- Additional Comments From lparrab gmx net  2007-04-13 03:47 -------
> yes, the logarithmic histogram does work perfectly: the calculation and the
> curve are "correct"...
>
> however, I think that logarithmic scale is great for some physics or math
> graphs.. but in photography, a logarithmic histogram does not tell (at
> least not to me, specially not at the first blick) the proportion of the
> color distribution in the image

I think the contrary is true, in photography you __always__ use logarithmic
representation because it corresponds more to the subjective impression
called "Lightness". The actual subjective law is a 1/3 power law
(http://www.poynton.com/notes/colour_and_gamma/GammaFAQ.html#lightness).

So if we want to improve the histogram I think the best would be (because it
corresponds to our visual perception) to represent the histogram as a power
law, and neither log nor linear, cupped or otherwise.

My 2ç

Gerhard



--
Hakuna matata
http://www.gerhard.fr

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[Gerhard Kulzer]

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------- Additional Comments From gerhard kulzer net  2007-04-13 12:40 -------
Am Friday 13 April 2007 schrieb lparrab gmx net:
[bugs.kde.org quoted mail]

I think the contrary is true, in photography you __always__ use logarithmic
representation because it corresponds more to the subjective impression
called "Lightness". The actual subjective law is a 1/3 power law
(http://www.poynton.com/notes/colour_and_gamma/GammaFAQ.html#lightness).

So if we want to improve the histogram I think the best would be (because it
corresponds to our visual perception) to represent the histogram as a power
law, and neither log nor linear, cupped or otherwise.

My 2�

Gerhard

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------- Additional Comments From lparrab gmx net  2007-04-13 23:44 -------

human perception/sensitivity to light is roughly logarithmic, yes. but I still find it easier(faster) to read a linear histogram that a logarithmic one.

If you really want to use the perception, then maybe(??) a histogram-graph is not the right tool, and some other kind of representation would be better...

Luis
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lparrab gmx net changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
  Attachment #20257|0                           |1
        is obsolete|                            |



------- Additional Comments From lparrab gmx net  2007-04-13 23:46 -------
(From update of attachment 20257)
PATCH OBSOLETE
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------- Additional Comments From lparrab gmx net  2007-04-13 23:51 -------
Created an attachment (id=20267)
 --> (http://bugs.kde.org/attachment.cgi?id=20267&action=view)
PATCH: limit the Y-axis of image histogram to 3*(90-percentil)

The first patch had a bug: it broke logarithmic scale (not on purpose, really
=)

This patch cleans up the code a bit and implements my proposed
percentile-solution.
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[Arnd Baecker]

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------- Additional Comments From arnd.baecker web de  2007-04-18 18:20 -------
Created an attachment (id=20315)
 --> (http://bugs.kde.org/attachment.cgi?id=20315&action=view)
histogram example

When trying to test this patch I realized, that it has already been applied
to svn, right?

Attached is an example of an image
with a large blown-out region, for which
I am not sure, whether the algorithm works so well,
i.e., wouldn't a much smaller vertical range be better?

On the other hand, for cases where it works
well, it might be good to leave a little bit
of vertical space so that the largest peaks
don't touch the upper edge of the histogramm.
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------- Additional Comments From arnd.baecker web de  2007-06-12 21:32 -------
Apart from my last comment - shouldn't this one be closed?
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[Gilles Caulier-4]

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------- Additional Comments From caulier.gilles gmail com  2007-06-12 21:34 -------
Arnd,

Nothing have been applied on svn...

Gilles
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------- Additional Comments From arnd.baecker web de  2007-06-13 13:11 -------
Of course, Gilles, nothing has been applied to svn (somehow
I must have screwed that up last time I tried ...)
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