Auto Brightness, Auto Contrast, Auto Levels



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Auto Brightness, Auto Contrast, Auto Levels

Postby Bill Miller » Tue, 27 Apr 2004 00:21:09 GMT

Does anyone know how to set an image's Auto Brightness, Auto Contrast and
Auto Levels...
or what the concepts are to accomplish this assuming I am using an image
component that can set brightness and contrast to a specific value?  I guess
the key here is how to get the auto value?


Bill Miller

Re: Auto Brightness, Auto Contrast, Auto Levels

Postby Francesco Savastano » Tue, 27 Apr 2004 01:08:26 GMT

Hello Bill,
AutoLevels means that you stretch the histograms of the 3 channels in a
smart way: it's assumed that an histogram is well stretched when it covers
most of the tones (from 0..255). To know more about histogram stretching you
can google, i'm sure you will find a lot of information. About how to
automatically stretch, there is some trick, i think you can have a look to
EFG site where i saw an example of autostretch. My personal contribute to
your search is this: you should find the best optimal minimum and maximum
threshold to which perform the stretch: this can be obtained analyzing the
density of histogram.

About Auto Contrast, often it's related to autostretch of the only
Luminosity channel.

About Auto Brightness there is no way to exactly know  how much to brighten
or darken a picture (for example if it's a night picture it should be dark,
if it's a morning picture it should be bright) so i think the user should
choose brightness himself and that is the smartest thing that could be done.

New World Software
Creative Tools for Creative People
"Bill Miller" <w2matnetheavendotcom> ha scritto nel messaggio


Re: Auto Brightness, Auto Contrast, Auto Levels

Postby Bill Miller » Tue, 27 Apr 2004 22:52:16 GMT


Thanks for your reply...  but there must be ways to get an auto setting for
brightness and contrast because many commercial products have automatic
settings.  Paint Shop Pro, Adobe Elements, Microsoft Digital Image and so
on... all have auto levels, brightness and contrast among others including
one step "picture fix".

I'll check out EFG's site to see if I can come up with more.


Bill Miller

Re: Auto Brightness, Auto Contrast, Auto Levels

Postby Lord Crc » Tue, 27 Apr 2004 23:25:37 GMT

On Mon, 26 Apr 2004 09:52:16 -0400, "Bill Miller"

They all modify the histogram one way or another.

In PS:
- Auto Levels stretches the histogram of each individual channel
- Auto Contras stretches the luminosity channel

Both of them will change the brightness of an image.

- Asbjn

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