[Digikam-devel] [Bug 128377] Adjust levels: histograms don't match, bad "auto level" function

[Mikolaj Machowski]

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http://bugs.kde.org/show_bug.cgi?id=128377         




------- Additional Comments From mikmach wp pl  2008-03-25 23:32 -------
> > Definitely don't agree. Current behaviour is standard. Auto-level is
> > widely understood as "stretching of each RGB channel separately"
>
> Fine, but at least this tiny "auto-button" should get a label so that
> users know what this button does.

If you think about gear button it has ToolTip and WhatsThis help. Worded
label wouldn't bring much. For full auto there is separate plugin.

> This new function (defined as in comment #8) could be named "stretch
> contrast" or "auto contrast" (e.g. as in Picasa). Then it is also clear
> what it will do: Stretch the contrast in the same way as if the user
> adjusted the luminance slider manually.


Auto contrast is the name. Just quick Google bring the name with good
description what is should do.

> > but don't change essence of auto-level tool.
>
> OK, but does it really have a use case? Changing color temperature is
> probably better done in the white balance tool. Adjusting a too dark or
> too light photo can be done with a auto-stretch-contrast function but
> not with the current auto-function as this function often creates wrong
> colors. So what's the typical use case of the current
> auto-stretch-rgb-function?


Remove haze from photographs, much simpler changing color temperature
than with white balance (and often with better effects). Yes, in many
cases it can be disastrous but in many it works. Interesting article
about another PS function:

http://www.creativepro.com/article/out-of-gamut-don-t-underestimate-photoshop-s-auto-color

Possibility to fine control behaviour of auto tools.
BTW - even if you don't find auto-levels very useful it is important to
change it into something other.

> > The only acceptable correction is removal of
> > shadow/highlight clipping
>
> From an photographers point of view that's not only acceptable but very
> important. It's not nice if you first try hard to avoid highlight
> clipping and then an auto-function deliberately creates highlight/shadow
> clipping.


Perfect would be possibility to control amount of clipping. Sometimes it
is needed to get better results, sometimes not (see article from link).

> > although this action is advised to make colors
> > more vivid.
>
> There are much better ways to make colors more vivid than
> highlight/shadow clipping. And making colors more vivid seems to fit
> better in the "hue/saturation/lightness" tool than in the basic "levels
> adjust" tool.
>
> I think it's important that users understand what an automatic function
> does - otherwise nobody will use it. Therefore it would be much better
> if the auto (and the new auto-contrast) function would change the
> sliders, not the data of the source image (and the source-histogram).
> Then everybody could see clearly that the current auto-color function
> adjusts the sliders of the three colour channels while the (new)
> auto-contrast function only changes the slider of the luminance channel.
> And if someone likes some small degree of highlight clipping then he
> could just move the slider a little bit further.


This works in Levels tools. Click Auto button and later use sliders to
correct effect. Maybe replace button with menu to choose type of Auto
correction?